Wednesday, December 28, 2011

Chemical Shifts for the Unusual DNA Structure in Pf1 Bacteriophage from Dynamic-Nuclear-Polarization-Enhanced Solid-State NMR Spectroscopy

Sergeyev, I.V., et al., Chemical Shifts for the Unusual DNA Structure in Pf1 Bacteriophage from Dynamic-Nuclear-Polarization-Enhanced Solid-State NMR Spectroscopy. J. Am. Chem. Soc., 2011. 133(50): p. 20208-20217.

Solid-state NMR spectra, including dynamic nuclear polarization enhanced 400 MHz spectra acquired at 100 K, as well as non-DNP spectra at a variety of field strengths and at temperatures in the range 213-243 K, have allowed the assignment of the 13C and 15N resonances of the unusual DNA structure in the Pf1 virion.

Thursday, December 22, 2011

Characterization of Membrane Proteins in Isolated Native Cellular Membranes by Dynamic Nuclear Polarization Solid-State NMR Spectroscopy without Purification and Reconstitution

Jacso, T., et al., Characterization of Membrane Proteins in Isolated Native Cellular Membranes by Dynamic Nuclear Polarization Solid-State NMR Spectroscopy without Purification and Reconstitution. Angew. Chem. Int. Ed., 2011: p. n/a-n/a.

Membrane proteins in their native cellular membranes are accessible by dynamic nuclear polarization magic angle spinning solid-state NMR spectroscopy without the need of purification and reconstitution (see picture). Dynamic nuclear polarization is essential to achieve the required gain in sensitivity to observe the membrane protein of interest.

Wednesday, December 21, 2011

Correlation of the EPR properties of perchlorotriphenylmethyl radicals and their efficiency as DNP polarizers

Banerjee, D., et al., Correlation of the EPR properties of perchlorotriphenylmethyl radicals and their efficiency as DNP polarizers. Phys. Chem. Chem. Phys., 2011. 13(41): p. 18626-18637.

Water soluble perchlorinated trityl (PTM) radicals were found to be effective 95 GHz DNP (dynamic nuclear polarization) polarizers in ex situ (dissolution) 13C DNP (Gabellieri et al., Angew Chem., Int. Ed. 2010, 49, 3360). The degree of the nuclear polarization obtained was reported to be dependent on the position of the chlorine substituents on the trityl skeleton. In addition, on the basis of the DNP frequency sweeps it was suggested that the 13C NMR signal enhancement is mediated by the Cl nuclei. To understand the DNP mechanism of the PTM radicals we have explored the 95 GHz EPR characteristics of these radicals that are relevant to their performance as DNP polarizers. The EPR spectra of the radicals revealed axially symmetric g-tensors. A comparison of the spectra with the 13C DNP frequency sweeps showed that although the solid effect mechanism is operational the DNP frequency sweeps reveal some extra width suggesting that contributions from EPR forbidden transitions involving 35,37Cl nuclear flips are likely. This was substantiated experimentally by ELDOR (electron-electron double resonance) detected NMR measurements, which map the EPR forbidden transitions, and ELDOR experiments that follow the depolarization of the electron spin upon irradiation of the forbidden EPR transitions. DFT (density functional theory) calculations helped to assign the observed transitions and provided the relevant spin Hamiltonian parameters. These results show that the 35,37Cl hyperfine and nuclear quadrupolar interactions cause a considerable nuclear state mixing at 95 GHz thus facilitating the polarization of the Cl nuclei upon microwave irradiation. Overlap of Cl nuclear frequencies and the 13C Larmor frequency further facilitates the polarization of the 13C nuclei by spin diffusion. Calculation of the 13C DNP frequency sweep based on the Cl nuclear polarization showed that it does lead to an increase in the width of the spectra, improving the agreement with the experimental sweeps, thus supporting the existence of a new heteronuclear assisted DNP mechanism.

One hundred fold overall sensitivity enhancements for Silicon-29 NMR spectroscopy of surfaces by dynamic nuclear polarization with CPMG acquisition

Rossini, A.J., et al., One hundred fold overall sensitivity enhancements for Silicon-29 NMR spectroscopy of surfaces by dynamic nuclear polarization with CPMG acquisition. Chemical Science, 2011. 3: p. 108-115.

Dynamic nuclear polarization (DNP) 29Si solid-state NMR spectra of a hybrid mesoporous silica material impregnated with aqueous biradical solutions have been acquired with cross-polarization (CP) and cross-polarization Carr-Purcell Meiboom-Gill (CP/CPMG) pulse sequences. The integrated intensities (II) and signal to noise ratios (S/N) of the 29Si solid-state NMR spectra are monitored in order to measure the DNP enhancement factors ([varepsilon]Si[space]CP) as well as the overall sensitivity enhancement ([capital Sigma]Si[space]CP) available from the combination of DNP and CPMG acquisition. Here, , where [small theta]Si is a factor which quantifies reduction of the NMR signal by paramagnetic effects (quenching) and [small kappa] is the square root of the ratio of nuclear longitudinal relaxation times of the dry material and material impregnated with radical solution. It is found that [capital Sigma]Si[space]CP is always substantially lower than the measured value of [varepsilon]Si[space]CP due to paramagnetic effects which reduce the II of the 29Si CP solid-state NMR spectra at high biradical concentrations. In this system, it is observed that the sample preparation which provides optimal DNP signal enhancement does not provide optimal overall signal enhancement. Notably, optimal signal enhancements are obtained for CPMG acquisition of the 29Si solid-state NMR spectra when lower radical concentrations are employed due to slower transverse relaxation rates. To the best of our knowledge this is the first study which seeks to quantify the overall sensitivity enhancements available from DNP solid-state NMR experiments.

Dynamic Nuclear Polarization-Enhanced Solid-State NMR of a 13C-Labeled Signal Peptide Bound to Lipid-Reconstituted Sec Translocon

Reggie, L., et al., Dynamic Nuclear Polarization-Enhanced Solid-State NMR of a 13C-Labeled Signal Peptide Bound to Lipid-Reconstituted Sec Translocon. J. Am. Chem. Soc., 2011. 133(47): p. 19084-19086.

Dynamic nuclear polarization (DNP) has made it possible to record 2D double-quantum-filtered (DQF) solid-state NMR (ssNMR) spectra of a signal peptide bound to a lipid-reconstituted SecYEG translocon complex. The small quantity of peptide in the sample (?40 nmol) normally prohibits multidimensional ssNMR experiments. Such small amounts are not the exception, because for samples involving membrane proteins, most of the limited sample space is occupied by lipids. As a consequence, a conventional 2D DQF ssNMR spectrum with the sample used here would require many weeks if not months of measurement time. With the help of DNP, however, we were able to acquire such a 2D spectrum within 20 h. This development opens up new possibilities for membrane protein studies, particularly in the exploitation of high-resolution spectroscopy and the assignment of individual amino acid signals, in this case for a signal peptide bound to the translocon complex.

Neurotoxin II Bound to Acetylcholine Receptors in Native Membranes Studied by Dynamic Nuclear Polarization NMR

Linden, A.H., et al., Neurotoxin II Bound to Acetylcholine Receptors in Native Membranes Studied by Dynamic Nuclear Polarization NMR. J. Am. Chem. Soc., 2011. 133(48): p. 19266-19269.

Methods enabling structural studies of membrane-integrated receptor systems without the necessity of purification provide an attractive perspective in membrane protein structural and molecular biology. This has become feasible in principle since the advent of dynamic nuclear polarization (DNP) magic-angle-spinning NMR spectroscopy, which delivers the required sensitivity. In this pilot study, we observed well-resolved solid-state NMR spectra of extensively 13C-labeled neurotoxin II bound to the nicotinic acetylcholine receptor (nAChR) in native membranes. We show that TOTAPOL, a biradical required for DNP, is localized at membrane and protein surfaces. The concentration of active, membrane-attached biradical decreases with time, probably because of reactive components of the membrane preparation. An optimal distribution of active biradical has strong effects on the NMR data. The presence of inactive TOTAPOL in membrane-proximal situations but active biradical in the surrounding water/glycerol "glass" leads to well-resolved spectra, yet a considerable enhancement (e = 12) is observed. The resulting spectra of a protein ligand bound to its receptor are paving the way for further DNP investigations of proteins embedded in native membrane patches.

A 10‚000-fold Nuclear Hyperpolarization of a Membrane Protein in the Liquid Phase via a Solid-State Mechanism

Daviso, E., et al., A 10‚000-fold Nuclear Hyperpolarization of a Membrane Protein in the Liquid Phase via a Solid-State Mechanism. J. Am. Chem. Soc., 2011. 133(42): p. 16754-16757.

Several techniques rely on electron-nuclear interactions to boost the polarization of nuclear spins in the solid phase. Averaging out of anisotropic interactions as a result of molecular tumbling strongly reduces the applicability of such hyperpolarization approaches in liquids. Here we show for the first time that anisotropic electron-nuclear interactions in solution can survive sufficiently long to generate nuclear spin polarization by the solid-state photo-CIDNP mechanism. A 10,000-fold NMR signal increase in solution was observed for a giant biomolecular complex of a photosynthetic membrane protein with a tumbling correlation time in the submicrosecond regime, corresponding to a molecular weight close to 1 MDa.

Band-selective chemical exchange saturation transfer imaging with hyperpolarized xenon-based molecular sensors

Meldrum, T., et al., Band-selective chemical exchange saturation transfer imaging with hyperpolarized xenon-based molecular sensors. J. Magn. Reson., 2011. 213(1): p. 14-21.

Molecular imaging based on saturation transfer in exchanging systems is a tool for amplified and chemically specific magnetic resonance imaging. Xenon-based molecular sensors are a promising category of molecular imaging agents in which chemical exchange of dissolved xenon between its bulk and agent-bound phases has been use to achieve sub-picomolar detection sensitivity. Control over the saturation transfer dynamics, particularly when multiple exchanging resonances are present in the spectra, requires saturation fields of limited bandwidth and is generally accomplished by continuous wave irradiation. We demonstrate instead how band-selective saturation sequences based on multiple pulse inversion elements can yield saturation bandwidth tuneable over a wide range, while depositing less RF power in the sample. We show how these sequences can be used in imaging experiments that require spatial–spectral and multispectral saturation. The results should be applicable to all CEST experiments and, in particular, will provide the spectroscopic control required for applications of arrays of xenon chemical sensors in microfluidic chemical analysis devices.

Transfer of hyperpolarization from long T1 storage nuclei to short T1 neighbors using FLOPSY-8

Moreno, K.X., et al., Transfer of hyperpolarization from long T1 storage nuclei to short T1 neighbors using FLOPSY-8. J. Magn. Reson., 2011. 213(1): p. 187-191.

Nuclei with long T1s are optimal targets for dynamic nuclear polarization (DNP). Therefore, most of the agents used in metabolic imaging and spectroscopy studies are based on carboxylic acid moieties that lack protons, a strong source of dipolar relaxation. Metabolic flux information encoded into spectra of small molecule metabolites in the form of the 13C isotopomer data cannot be accessed using standard 13C hyperpolarization methods because protonated carbons relax too quickly through T1 dipolar relaxation. It is shown here that the longitudinal mixing sequence FLOPSY-8 can be used to transfer polarization from a long T1 storage nucleus to adjacent protonated carbons so that they may be detected with high sensitivity. We demonstrate that FLOPSY-8 allows a direct readout of isotopomer populations in butyrate and glutamate in vitro.

Thursday, December 15, 2011

Albert Overhauser passed away at the age of 86

From Dror Warschawski (NMR web database):

Some of the people who use NOESY or DNP on a daily basis know about the Overhauser effect without necessarily knowing that it bears the name of the person who discovered it. 

It was first mentionned in 1953: Overhauser A.W., Phys.Rev. 91, 476 (1953), immediately confirmed by Carver and Slichter in Phys.Rev. 92, 212-213 (1953) and better described in Overhauser A.W., Polarization of Nuclei in Materials, Phys.Rev. 92, 411-415 (1953), that was cited over 600 times. 

Well, this giant of the magnetic resonance community is no longer with us, Albert Overhauser passed away quietly on Saturday, at age 86. You can find his obituary and leave condolence messages here: 

Thursday, November 17, 2011

A spinning thermometer to monitor microwave heating and glass transitions in DNP

Miéville, P.; Vitzthum, V.; Caporini, M. A.; Jannin, S.; Gerber-Lemaire, S.; Bodenhausen, G. Magn. Reson. Chem. 2011, 49, 689.

As previously demonstrated by Thurber and Tycko, the peak position of 79Br in potassium bromide (KBr) allows one to determine the temperature of a spinning sample. We propose to adapt the original design by using a compact KBr tablet placed at the bottom of the magic angle spinning rotor, separated from the sample under investigation by a thin disk made of polytetrafluoroethylene (or ‘Teflon’®). This design allows spinning the sample up to at least 16 kHz. The KBr tablet can remain in the rotor when changing the sample under investigation. Calibration in the range of 98 < T < 320 K has been carried out in a static rotor by inserting a platinum thermometer. The accuracy is better than ± 0.9 K, even in the presence of microwave irradiation. Irradiation with 5 W microwaves at 263 GHz leads to a small temperature increase of 3.6 ± 1.4 K in either static or spinning samples. The dynamic nuclear polarization enhancement decreases with increasing temperature, in particular when a frozen glassy sample undergoes a glass transition.

Tuesday, November 15, 2011

Hot Topics in Spin-Hyperpolarization

Nuclear Magnetic Resonance (NMR) spectroscopy, microscopy and imaging techniques (MRI) play a crucial role in numerous fields of science ranging from physics, chemistry, material sciences, biology to medicine. The high information content of modern multi-dimensional NMR spectroscopy makes it possible to obtain structural and dynamical information with atomic resolution. In addition, owing to the low energy excitation in the MHz frequency region, the method is non-invasive, making it one of the most important imaging modalities in medical diagnostics. However, despite all its versatility, the key issue is frequently sensitivity, which limits the applicability of NMR spectroscopy and imaging techniques in the case of fast dynamical processes and detection of lowly concentrated molecules in both in vitro and in vivo applications.

Several strategies for spin-hyperpolarization are used to increase substantially NMR sensitivity. Two of these strategies are based on the transfer of the much bigger electron spin polarization onto the nuclear spin system either during a chemical reaction (Chemical Induced Dynamic Nuclear Polarization, CIDNP) or by using microwave fields (Dynamic Nuclear Polarization, DNP).Parahydrogen Induced Polarization (PHIP) is based on the correlation between a particular quantum mechanical spin state and a rotational state in diatomic hydrogen. In a chemical reaction the hydrogen spin state can be used to generate target molecules with high spin polarization. In spin exchange optical pumping (SEOP) the polarization of excited electrons is transferred onto noble gas atoms such as Helium, Xenon and Krypton to generate highly polarized gases for MRI application to lung studies, characterization of porous media and surfaces.

Although the hyperpolarization strategies differ in their underlying physico-chemical principles they have a number of problems in common. Recently, a network across Europe and associated states has been launched within the ESF COST programme (Action TD1103) to stimulate the communication between technology developers and users of the different hyperpolarisationtechniques. The Lorentz workshop will provide an overview of the current state-of-the-art in spinhyperpolarisation and aims to identify common problems and mutual points of interest to initiate communication and collaborative projects within the COST Action.

Tuesday, November 1, 2011

Non-aqueous solvents for DNP surface enhanced NMR spectroscopy

Zagdoun, A.; Rossini, A. J.; Gajan, D.; Bourdolle, A.; Ouari, O.; Rosay, M.; Maas, W. E.; Tordo, P.; Lelli, M.; Emsley, L.; Lesage, A.; Coperet, C. Chemical Communications 2011.

A series of non-aqueous solvents combined with the exogenous biradical bTbK are developed for DNP NMR that yield enhancements comparable to the best available water based systems. 1,1,2,2-tetrachloroethane appears to be one of the most promising organic solvents for DNP solid-state NMR. Here this results in a reduction in experimental times by a factor of 1000. These new solvents are demonstrated with the first DNP surface enhanced NMR characterization of an organometallic complex supported on a hydrophobic surface.

Factors Affecting DNP NMR in Polycrystalline Diamond Samples

Casabianca, L. B.; Shames, A. I.; Panich, A. M.; Shenderova, O.; Frydman, L. The Journal of Physical Chemistry C 2011, 115, 19041.

This work examines several polycrystalline diamond samples for their potential as polarizing agents for dynamic nuclear polarization (DNP) in NMR. Diamond samples of various origin and particle sizes ranging from a few nanometers to micrometers were examined by EPR, solid-state NMR and DNP techniques. A correlation was found between the size of the diamond particles and the electron spin-lattice relaxation time, the 13C nuclear spin-lattice relaxation times in room temperature magic-angle-spinning experiments, and the ability of the diamond carbons to be hyperpolarized by irradiating unpaired electrons of inherent defects by microwaves at cryogenic temperatures. As the size of the diamond particles approaches that of bulk diamond, both electron and nuclear relaxation times become longer. NMR signal enhancement through DNP was found to be very efficient only for these larger size diamond samples. The reasons and implications of these results are briefly discussed, in the light of these EPR, DNP, and NMR observations.

Analysis of Cancer Metabolism by Imaging Hyperpolarized Nuclei: Prospects for Translation to Clinical Research

Kurhanewicz, J.; Vigneron, D. B.; Brindle, K. M.; Chekmenev, E. Y.; Comment, A.; Cunningham, C.; DeBerardinis, R. J.; Green, G. G. R.; Leach, M. O.; Rajan, S. S.; Rizi, R. R.; Ross, B. D.; Warren, W.; Malloy, C. R. Neoplasia 2011, 13, 81.

A major challenge in cancer biology is to monitor and understand cancer metabolism in vivo with the goal of improved diagnosis and perhaps therapy. Because of the complexity of biochemical pathways, tracer methods are required for detecting specific enzyme-catalyzed reactions. Stable isotopes such as 13C or 15N with detection by nuclear magnetic resonance provide the necessary information about tissue biochemistry, but the crucial metabolites are present in low concentration and therefore are beyond the detection threshold of traditional magnetic resonance methods. A solution is to improve sensitivity by a factor of 10,000 or more by temporarily redistributing the populations of nuclear spins in a magnetic field, a process termed hyperpolarization. Although this effect is short-lived, hyperpolarized molecules can be generated in an aqueous solution and infused in vivo where metabolism generates products that can be imaged. This discovery lifts the primary constraint on magnetic resonance imaging for monitoring metabolism—poor sensitivity—while preserving the advantage of biochemical information. The purpose of this report was to briefly summarize the known abnormalities in cancer metabolism, the value and limitations of current imaging methods for metabolism, and the principles of hyperpolarization. Recent preclinical applications are described. Hyperpolarization technology is still in its infancy, and current polarizer equipment and methods are suboptimal. Nevertheless, there are no fundamental barriers to rapid translation of this exciting technology to clinical research and perhaps clinical care.

Thursday, October 6, 2011

Optimization of 1H spin density for dynamic nuclear polarization using photo-excited triplet electron spins

Kagawa, A.; Murokawa, Y.; Takeda, K.; Kitagawa, M. J. Magn. Reson. 2009, 197, 9.

In dynamic nuclear polarization (DNP) using photo-excited triplet electron spins, known as Microwave-Induced Optical Nuclear Polarization (MIONP), the attainable 1H polarization is determined by the ratio of the buildup rate and the spin-lattice relaxation rate, in turn depend on the 1H spin density. It is shown that the final 1H polarization can be enhanced by diluting the 1H spins with partial deuteration. The DNP experiments are demonstrated in 0.05 mol% pentacene-doped p-terphenyl for various 1H abundances. It is also shown that the 1H spin diffusion coefficient can be determined by examining the initial buildup rate of 1H polarization for various repetition rates of the DNP sequence.

Tensors and rotations in NMR

This article does not cover DNP. However, for those of us who still like to write their own simulation programs it is a very helpful article.

Mueller, L. J. Concepts in Magnetic Resonance Part A 2011, 38A, 221.

The transformation of second-rank Cartesian tensors under rotation plays a fundamental role in the theoretical description of nuclear magnetic resonance experiments, providing the framework for describing anisotropic phenomena such as single crystal rotation patterns, tensor powder patterns, sideband intensities under magic-angle sample spinning, and as input for relaxation theory. Here, two equivalent procedures for effecting this transformation—direct rotation in Cartesian space and the decomposition of the Cartesian tensor into irreducible spherical tensors that rotate in subgroups of rank 0, 1, and 2—are reviewed. In a departure from the standard formulation, the explicit use of the spherical tensor basis for the decomposition of a spatial Cartesian tensor is introduced, helping to delineate the rotational properties of the basis states from those of the matrix elements. The result is a uniform approach to the rotation of a physical system and the corresponding transformation of the spatial components of the NMR Hamiltonian, expressed as either Cartesian or spherical tensors. This clears up an apparent inconsistency in the NMR literature, where the rotation of a spatial tensor in spherical tensor form has typically been partnered with the inverse rotation in Cartesian form to produce equivalent transformations. © 2011 Wiley Periodicals, Inc. Concepts Magn Reson Part A 38: 221–235, 2011.

Dynamic Nuclear Polarization in NMR

Chandrakumar, N. Journal of the Indian Institure of Science 2010, 90, 133.

Dynamic nuclear polarization was first predicted — and, shortly thereafter, established experimentally — in 1953, the first demonstration being on Lithium metal. The basic approach involves the saturation of the ESR of a paramagnetic species in the system, while the NMR is observed. Initial applications of DNP involved low and moderate field studies that focused especially on investigations of molecular hydrodynamics. Applications to MRI provided a subsequent fillip to the technique. In the meanwhile, the closely related nuclear Overhauser effect (NOE) — which involves saturation, as well as observation of different NMR signals — had become an essential technique for the structure elucidation of both small molecules, as well as biomolecules. Most recently, DNP is witnessing rejuvenation, with high field applications to sensitivity enhancement in NMR. We present in the following an overview of Dynamic nuclear polarization (DNP). The elementary general theory of the phenomenon is discussed. Four different DNP mechanisms that are currently recognized are briefly introduced and different modes of the experiment — involving either cw ESR irradiation, or pulsed ESR excitation — are pointed out. A brief run down of various possible implementations is presented, including our own early work at moderate fields in cw mode, as well as hardware configurations and requirements for high field DNP. Different current implementations of DNP experiments are summarized, including solid state, as well as in situ and ex situ dissolution DNP variants. Typical results of DNP enhanced high resolution NMR are then briefly discussed, including the results of our own early work on differential 19F enhancements at moderate fields. Design of free radicals that satisfy the requirements to establish an efficient cross effect DNP is discussed. Recent experiments that have succeeded in detecting an intermediate in the photocycle of bacteriorhodopsin are alluded to. Finally, the implementation of ultrafast multi-dimensional NMR techniques under DNP conditions is briefly discussed, as an approach to further exploitation of the prospects that are on offer.

Wednesday, October 5, 2011

Stacked rings for terahertz wave-guiding

de Rijk, E.; Macor, A.; Hogge, J.; Alberti, S.; Ansermet, J. Review of Scientific Instruments 2011, 82, 066102.

We demonstrate the construction of corrugated waveguides using stacked rings to propagate terahertz frequencies. The waveguide allows propagation of the same fundamental mode as an optical-fiber, namely, the HE11 mode. This simple concept opens the way for corrugated wave-guides up to several terahertz, maintaining beam characteristics as for terahertz applications.

Monday, October 3, 2011

Polarizing agents and mechanisms for high-field dynamic nuclear polarization of frozen dielectric solids

Hu, K.-N., Polarizing agents and mechanisms for high-field dynamic nuclear polarization of frozen dielectric solids. Solid State Nuclear Magnetic Resonance, 2011. 40(2): p. 31-41.

This article provides an overview of polarizing mechanisms involved in high-frequency dynamic nuclear polarization (DNP) of frozen biological samples at temperatures maintained using liquid nitrogen, compatible with contemporary magic-angle spinning (MAS) nuclear magnetic resonance (NMR). Typical DNP experiments require unpaired electrons that are usually exogenous in samples via paramagnetic doping with polarizing agents. Thus, the resulting nuclear polarization mechanism depends on the electron and nuclear spin interactions induced by the paramagnetic species. The Overhauser Effect (OE) DNP, which relies on time-dependent spin-spin interactions, is excluded from our discussion due the lack of conducting electrons in frozen aqueous solutions containing biological entities. DNP of particular interest to us relies primarily on time-independent, spin-spin interactions for significant electron-nucleus polarization transfer through mechanisms such as the Solid Effect (SE), the Cross Effect (CE) or Thermal Mixing (TM), involving one, two or multiple electron spins, respectively. Derived from monomeric radicals initially used in high-field DNP experiments, bi- or multiple-radical polarizing agents facilitate CE/TM to generate significant NMR signal enhancements in dielectric solids at low temperatures (< 100 K). For example, large DNP enhancements (~ 300 times at 5 T) from a biologically compatible biradical, 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-ol (TOTAPOL), have enabled high-resolution MAS NMR in sample systems existing in submicron domains or embedded in larger biomolecular complexes. The scope of this review is focused on recently developed DNP polarizing agents for high-field applications and leads up to future developments per the CE DNP mechanism. Because DNP experiments are feasible with a solid-state microwave source when performed at < 20 K, nuclear polarization using lower microwave power (< 100 mW) is possible by forcing a high proportion of biradicals to fulfill the frequency matching condition of CE (two EPR frequencies separated by the NMR frequency) using the strategies involving hetero-radical moieties and/or molecular alignment. In addition, the combination of an excited triplet and a stable radical might provide alternative DNP mechanisms without the microwave requirement.

Monday, August 22, 2011

Meet the Bridge12 Team

The Bridge12 team is looking forward to attending the following meetings this fall:

You can meet our team to discuss our lates innovations in gyrotron technology for DNP-enhanced NMR spectroscopy (DNP-NMR).

Operation of a Continuously Frequency-Tunable Second-Harmonic CW 330-GHz Gyrotron for Dynamic Nuclear Polarization

Torrezan, A.C., et al., Operation of a Continuously Frequency-Tunable Second-Harmonic CW 330-GHz Gyrotron for Dynamic Nuclear Polarization. IEEE Transactions on Electron Devices, 2011. 58(8): p. 2777-83.

The design and the operation of a frequency-tunable continuous-wave (CW) 330-GHz gyrotron oscillator operating at the second harmonic of the electron cyclotron frequency are reported. The gyrotron has generated 18 W of power from a 10.1 kV 190 mA electron beam working in a TE-4,3 cylindrical mode, corresponding to an efficiency of 0.9 %. The measured start oscillation current over a range of magnetic field values is in good agreement with theoretical start currents obtained from linear theory for successive high-order axial modes TE-4,3,q, where q = 1-6. Moreover, the observed frequency range in the start current measurement is in reasonable agreement with the frequency range obtained from numerical simulations. The minimum start current was measured to be 33 mA. A continuous tuning range of 1.2 GHz was experimentally observed via a combination of magnetic, voltage, and thermal tuning. The gyrotron output power and frequency stabilities were assessed to be 0.4% and 3 ppm, respectively, during a 110-h uninterrupted CW run. Evaluation of the gyrotron output microwave beam pattern using a pyroelectric camera indicated a Gaussian-like mode content of 92% with an ellipticity of 28%. The gyrotron will be used for 500-MHz nuclear magnetic resonance experiments with sensitivity enhanced by dynamic nuclear polarization.

Tuesday, August 16, 2011

Intermolecular Structure Determination of Amyloid Fibrils with Magic-Angle Spinning and Dynamic Nuclear Polarization NMR

Bayro, M. J., Debelouchina, G. T., Eddy, M. T., Birkett, N. R., MacPhee, C. E., Rosay, M., Maas, W. E., Dobson, C. M., and Griffin, R. G. (2011) Intermolecular Structure Determination of Amyloid Fibrils with Magic-Angle Spinning and Dynamic Nuclear Polarization NMR, J. Am. Chem. Soc.

We describe magic-angle spinning NMR experiments designed to elucidate the interstrand architecture of amyloid fibrils. Three methods are introduced for this purpose, two being based on the analysis of long-range 13C-13C correlation spectra and the third based on the identification of intermolecular interactions in 13C-15N spectra. We show, in studies of fibrils formed by the 86-residue SH3 domain of PI3 kinase (PI3-SH3 or PI3K-SH3), that efficient 13C-13C correlation spectra display a resonance degeneracy that establishes a parallel, in-register alignment of the proteins in the amyloid fibrils. In addition, this degeneracy can be circumvented to yield direct intermolecular constraints. The 13C-13C experiments are corroborated by 15N-13C correlation spectra obtained from a mixed [15N,12C]/[14N,13C] sample which directly quantify interstrand distances. Furthermore, when the spectra are recorded with signal enhancement provided by dynamic nuclear polarization (DNP) at 100 K, we demonstrate a dramatic increase (from 23 to 52) in the number of intermolecular 15N-13C constraints detectable in the spectra. The increase in the information content is due to the enhanced signal intensities and to the fact that dynamic processes, leading to spectral intensity losses, are quenched at low temperatures. Thus, acquisition of low temperature spectra addresses a problem that is frequently encountered in MAS spectra of proteins. In total, the experiments provide 111 intermolecular 13C-13C and 15N-13C constraints that establish that the PI3-SH3 protein strands are aligned in a parallel, in-register arrangement within the amyloid fibril.

Para-hydrogen induced polarization of amino acids, peptides and deuterium-hydrogen gas

This article is about para-hydrogen induced polarization, a technique related to microwave/terahertz driven polarization enhancement.

Signal Amplification by Reversible-Exchange (SABRE) is a method of hyperpolarizing substrates by polarization transfer from para-hydrogen without hydrogenation. Here, we demonstrate that this method can be applied to hyperpolarize small amounts of all proteinogenic amino acids and some chosen peptides down to the nanomole regime and can be detected in a single scan in low-magnetic fields down to 0.25 mT (10 kHz proton frequency). An outstanding feature is that depending on the chemical state of the used catalyst and the investigated amino acid or peptide, hyperpolarized hydrogen-deuterium gas is formed, which was detected with 1H and 2H NMR spectroscopy at low magnetic fields of B0 = 3.9 mT (166 kHz proton frequency) and 3.2 mT (20 kHz deuterium frequency).

Hyperpolarized [1-13C]-Ascorbic and Dehydroascorbic Acid: Vitamin C as a Probe for Imaging Redox Status in Vivo

Bohndiek, S. E., Kettunen, M. I., Hu, D.-e., Kennedy, B. W. C., Boren, J., Gallagher, F. A., and Brindle, K. M. (2011) Hyperpolarized [1-13C]-Ascorbic and Dehydroascorbic Acid: Vitamin C as a Probe for Imaging Redox Status in Vivo, J. Am. Chem. Soc. 133, 11795-11801.

Dynamic nuclear polarization (DNP) of 13C-labeled metabolic substrates in vitro and their subsequent intravenous administration allow both the location of the hyperpolarized substrate and the dynamics of its subsequent conversion into other metabolic products to be detected in vivo. We report here the hyperpolarization of [1-13C]-ascorbic acid (AA) and [1-13C]-dehydroascorbic acid (DHA), the reduced and oxidized forms of vitamin C, respectively, and evaluate their performance as probes of tumor redox state. Solution-state polarization of 10.5 ± 1.3% was achieved for both forms at pH 3.2, whereas at pH 7.0, [1-13C]-AA retained polarization of 5.1 ± 0.6% and [1-13C]-DHA retained 8.2 ± 1.1%. The spin‚Äìlattice relaxation times (T1's) for these labeled nuclei are long at 9.4 T: 15.9 ± 0.7 s for AA and 20.5 ± 0.9 s for DHA. Extracellular oxidation of [1-13C]-AA and intracellular reduction of [1-13C]-DHA were observed in suspensions of murine lymphoma cells. The spontaneous reaction of DHA with the cellular antioxidant glutathione was monitored in vitro and was approximately 100-fold lower than the rate observed in cell suspensions, indicating enzymatic involvement in the intracellular reduction. [1-13C]-DHA reduction was also detected in lymphoma tumors in vivo. In contrast, no detectable oxidation of [1-13C]-AA was measured in the same tumors, consistent with the notion that tumors maintain a reduced microenvironment. This study demonstrates that hyperpolarized 13C-labeled vitamin C could be used as a noninvasive biomarker of redox status in vivo, which has the potential to translate to the clinic.

Beyond the Silica Surface by Direct Silicon-29 Dynamic Nuclear Polarization

Lafon, O., Rosay, M., Aussenac, F., Lu, X., Trébosc, J., Cristini, O., Kinowski, C., Touati, N., Vezin, H., and Amoureux, J.-P. (2011) Beyond the Silica Surface by Direct Silicon-29 Dynamic Nuclear Polarization, Angew. Chem. Int. Ed., n/a-n/a.

Buried truth: High-field magic angle spinning dynamic nuclear polarization (MAS DNP) enhances the sensitivity of solid-state NMR spectroscopy, but only for protonated surfaces. Direct 29Si DNP using the biradical TOTAPOL circumvents this limitation by producing a 30-fold enhancement of subsurface 29Si NMR signals in mesoporous silica, a material with applications in photonics, nanotechnology and catalysis.

Friday, July 22, 2011

Final Program for 3rd International DNP symposium in Lausanne

The final program for the 3rd International Symposium on DNP which will be held in Lausanne, 7-10 September 2011, including many 'promoted poster talks' that will reveal recent and unexpected advances in DNP, is now available on line: 

The meeting is not fully booked up yet and you can still register at: 

Further information can be found on the home-page: 

Friday, June 24, 2011

DNP by Thermal Mixing under Optimized Conditions Yields >60‚000-fold Enhancement of 89Y NMR Signal

Lumata, L., et al., DNP by Thermal Mixing under Optimized Conditions Yields >60‚000-fold Enhancement of 89Y NMR Signal. J. Am. Chem. Soc., 2011. 133(22): p. 8673-8680

Hyperpolarized 89Y complexes are attractive NMR spectroscopy and MR imaging probes due to the exceptionally long spin–lattice relaxation time (T1 ~ 10 min) of the 89Y nucleus. However, in vivo imaging of 89Y has not yet been realized because of the low NMR signal enhancement levels previously achieved for this ultra low-gamma nucleus. Here, we report liquid-state 89Y NMR signal enhancements over 60 000 times the thermal signal at 298 K in a 9.4 T magnet, achieved after the dynamic nuclear polarization (DNP) of Y(III) complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) samples at 3.35 T and 1.4 K. The 89Y DNP was shown to proceed by thermal mixing and the liquid state 89Y NMR signal enhancement was maximized by (i) establishing the optimal microwave irradiation frequency, (ii) optimizing the glassing matrix, (iii) choosing a radical with negligible inhomogeneous line broadening contribution to the ESR linewidth, and (iv) addition of an electron T1e relaxation agent. The highest enhancements were achieved using a trityl OX063 radical combined with a gadolinium relaxation agent in water-glycerol matrix. Co-polarization of 89YDOTA and sodium [1-13C]pyruvate showed that both 89Y and 13C nuclear species acquired the same spin temperature, consistent with thermal mixing theory of DNP. This methodology may be applicable for the optimization of DNP of other low-gamma nuclei.

Dissolution DNP NMR with solvent mixtures: Substrate concentration and radical extraction

Harris, T., C. Bretschneider, and L. Frydman, Dissolution DNP NMR with solvent mixtures: Substrate concentration and radical extraction. J. Magn. Reson., 2011. 211(1): p. 96-100

Dynamic nuclear polarization (DNP) followed by sudden sample dissolution, is a topic of active investigation owing to the method's unique prospects for the delivery of NMR spectra and images with unprecedented sensitivity. This experiment achieves hyperpolarization by the combined effects of electron-nuclear irradiation and cryogenic operation; the exploitation of these states occurs following a sudden melting and flushing of the resulting pellet from its original environment into a conventional, liquid-state setting. This melting and flushing usually demands using the equivalent of a few milliliters of hot solvent, a procedure which although well suited for in vivo studies leads to an excessive sample volume when considering typical analytical settings. The present study explores a way of reducing the ensuing dilution of the hyperpolarized analytes, by employing a combination of immiscible liquids for performing the melting and flushing. It is shown that suitable combinations of immiscible solvents - both in terms of their heat capacities and densities - allow one to melt the targeted cryogenic pellet and dissolve the hyperpolarized analytes in a fraction of the solvent hitherto required. By tailoring the resulting volume to the needs of a conventional 5 mm NMR probe, a substantial sensitivity enhancement can be added to the hyperpolarization process. An extra benefit may arise from using radicals that preferentially dissolve in the immiscible organic phase, by way of a lengthening of the relaxation time of the investigated analytes. Examples of these principles are given, and further potential extensions of this approach are discussed.

Wednesday, June 22, 2011

Low-temperature dynamic nuclear polarization at 9.4 T with a 30 mW microwave source

Thurber, K.R., W.-M. Yau, and R. Tycko, Low-temperature dynamic nuclear polarization at 9.4 T with a 30 mW microwave source. J. Magn. Reson., 2010. 204(2): p. 303-313

Dynamic nuclear polarization (DNP) can provide large signal enhancements in nuclear magnetic resonance (NMR) by transfer of polarization from electron spins to nuclear spins. We discuss several aspects of DNP experiments at 9.4 T (400 MHz resonant frequency for 1H, 264 GHz for electron spins in organic radicals) in the 7-80 K temperature range, using a 30 mW, frequency-tunable microwave source and a quasi-optical microwave bridge for polarization control and low-loss microwave transmission. In experiments on frozen glycerol/water doped with nitroxide radicals, DNP signal enhancements up to a factor of 80 are observed (relative to 1H NMR signals with thermal equilibrium spin polarization). The largest sensitivity enhancements are observed with a new triradical dopant, DOTOPA-TEMPO. Field modulation with a 10 G root-mean-squared amplitude during DNP increases the nuclear spin polarizations by up to 135%. Dependencies of 1H NMR signal amplitudes, nuclear spin relaxation times, and DNP build-up times on the dopant and its concentration, temperature, microwave power, and modulation frequency are reported and discussed. The benefits of low-temperature DNP can be dramatic: the 1H spin polarization is increased approximately 1000-fold at 7 K with DNP, relative to thermal polarization at 80 K.

Tuesday, June 21, 2011

Simulating spin dynamics in NMR with a new computer program intended for education: Insensitive

This article is not directly related to Dynamic Nuclear Polarization, however, it describes a simulation program for NMR spectroscopy that can even be used on an iPhone.

Boldt, K., Simulating spin dynamics in NMR with a new computer program intended for education: Insensitive. Concepts in Magnetic Resonance Part A, 2011. 38A(2): p. 17-24

Abstract The NMR experiment is usually described by a choice of three models that operate on different levels of abstraction: the vector model, the product operator formalism and the density matrix approach. The transition between these models poses a didactic challenge for teacher and student alike. A new computer program is presented, which simulates a spin system on the textbook level and compares the three approaches, with the possibility to manipulate the system at every step. It closes a gap between NMR education and professional simulation tools. Some algorithms are explained, which are used in the simulation to extract information from the density matrix. © 2011 Wiley Periodicals, Inc. Concepts Magn Reson Part A 38A: 17–24, 2011.

Water 1H relaxation dispersion analysis on a nitroxide radical provides information on the maximal signal enhancement in Overhauser dynamic nuclear polarization experiments

Bennati, M., et al., Water 1H relaxation dispersion analysis on a nitroxide radical provides information on the maximal signal enhancement in Overhauser dynamic nuclear polarization experiments. Phys. Chem. Chem. Phys., 2010. 12(22): p. 5902-5910

Water 1H relaxation rate measurements of 15N-2H-TEMPONE solutions at temperatures ranging from 298 to 328 K have been performed as a function of magnetic field from 0.00023 to 9.4 T, corresponding to 1H Larmor frequencies of 0.01 to 400 MHz. The relaxation profiles were analyzed according to the full theory for dipolar and contact relaxation, and used to estimate the coupling factor responsible for observed solution DNP effects. The experimental DNP enhancement at 1H Larmor frequency of 15 MHz obtained by saturating one of the lines of the 15N doublet is only ca. 20% lower than the limiting value predicted from the relaxation data, indicating that the experimental DNP setup is nearly optimal, the residual discrepancy arising from incomplete saturation of the other line.

Continuous flow Overhauser dynamic nuclear polarization of water in the fringe field of a clinical magnetic resonance imaging system for authentic image contrast

Lingwood, M.D., et al., Continuous flow Overhauser dynamic nuclear polarization of water in the fringe field of a clinical magnetic resonance imaging system for authentic image contrast. J. Magn. Reson., 2010. 205(2): p. 247-254

We describe and demonstrate a system to generate hyperpolarized water in the 0.35 T fringe field of a clinical 1.5 T whole-body magnetic resonance imaging (MRI) magnet. Once generated, the hyperpolarized water is quickly and continuously transferred from the 0.35 T fringe to the 1.5 T center field of the same magnet for image acquisition using standard MRI equipment. The hyperpolarization is based on Overhauser dynamic nuclear polarization (DNP), which effectively and quickly transfers the higher spin polarization of free radicals to nuclear spins at ambient temperatures. We visualize the dispersion of hyperpolarized water as it flows through water-saturated systems by utilizing an observed -15-fold DNP signal enhancement with respect to the unenhanced 1H MRI signal of water at 1.5 T. The experimental DNP apparatus presented here is readily portable and can be brought to and used with any conventional unshielded MRI system. A new method of immobilizing radicals to gel beads via polyelectrolyte linker arms is described, which led to superior flow Overhauser DNP performance compared to previously presented gels. We discuss the general applicability of Overhauser DNP of water and aqueous solutions in the fringe field of commercially available magnets with central fields up to 4.7 T.

Wednesday, June 1, 2011

2H-decoupling-accelerated 1H spin diffusion in dynamic nuclear polarization with photoexcited triplet electrons

Negoro, M., et al., [sup 2]H-decoupling-accelerated [sup 1]H spin diffusion in dynamic nuclear polarization with photoexcited triplet electrons. J. Chem. Phys., 2010. 133(15): p. 154504-6

In dynamic nuclear polarization (DNP) experiments applied to organic solids for creating nonequilibrium, high 1H spin polarization, an efficient buildup of 1H polarization is attained by partially deuterating the material of interest with an appropriate 1H concentration. In such a dilute 1H spin system, it is shown that the 1H spin diffusion rate and thereby the buildup efficiency of 1H polarization can further be enhanced by continually applying radiofrequency irradiation for deuterium decoupling during the DNP process. As experimentally confirmed in this work, the electron spin polarization of the photoexcited triplet state is mainly transferred only to those 1H spins, which are in the vicinity of the electron spins, and 1H spin diffusion transports the localized 1H polarization over the whole sample volume. The 1H spin diffusion coefficients are estimated from DNP repetition interval dependence of the initial buildup rate of 1H polarization, and the result indicates that the spin diffusion coefficient is enhanced by a factor of 2 compared to that without 2H decoupling.

Quantum mechanical theory of dynamic nuclear polarization in solid dielectrics

Hu, K.-N., et al., Quantum mechanical theory of dynamic nuclear polarization in solid dielectrics. J. Chem. Phys., 2011. 134(12): p. 125105-19

Microwave driven dynamic nuclear polarization (DNP) is a process in which the large polarization present in an electron spin reservoir is transferred to nuclei, thereby enhancing NMR signal intensities. In solid dielectrics there are three mechanisms that mediate this transfer—the solid effect (SE), the cross effect (CE), and thermal mixing (TM). Historically these mechanisms have been discussed theoretically using thermodynamic parameters and average spin interactions. However, the SE and the CE can also be modeled quantum mechanically with a system consisting of a small number of spins and the results provide a foundation for the calculations involving TM. In the case of the SE, a single electron–nuclear spin pair is sufficient to explain the polarization mechanism, while the CE requires participation of two electrons and a nuclear spin, and can be used to understand the improved DNP enhancements observed using biradical polarizing agents. Calculations establish the relations among the electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) frequencies and the microwave irradiation frequency that must be satisfied for polarization transfer via the SE or the CE. In particular, if δ, Δ < ω0I, where δ and Δ are the homogeneous linewidth and inhomogeneous breadth of the EPR spectrum, respectively, we verify that the SE occurs when ωM = ω0S ± ω0I, where ωM, ω0S and ω0I are, respectively, the microwave, and the EPR and NMR frequencies. Alternatively, when Δ > ω0I > δ, the CE dominates the polarization transfer. This two-electron process is optimized when ω0S1−ω0S2 = ω0I and ωM ∼ ω0S1 or ω0S2, where ω0S1 and ω0S2 are the EPR Larmor frequencies of the two electrons. Using these matching conditions, we calculate the evolution of the density operator from electron Zeeman order to nuclear Zeeman order for both the SE and the CE. The results provide insights into the influence of the microwave irradiation field, the external magnetic field, and the electron−electron and electron−nuclear interactions on DNP enhancements.

Saturation factor of nitroxide radicals in liquid DNP by pulsed ELDOR experiments

Turke, M.-T. and M. Bennati, Saturation factor of nitroxide radicals in liquid DNP by pulsed ELDOR experiments. Phys. Chem. Chem. Phys., 2011. 13(9): p. 3630-3633

We propose the use of the pulse electron double resonance (ELDOR) method to determine the effective saturation factor of nitroxide radicals for dynamic nuclear polarization (DNP) experiments in liquids. The obtained values for the nitroxide radical TEMPONE-D,15N at different concentrations are rationalized in terms of spin relaxation and are shown to fulfil the Overhauser theory.

Dynamics and state of lipid bilayer-internal water unraveled with solution state 1H dynamic nuclear polarization

Kausik, R. and S. Han, Dynamics and state of lipid bilayer-internal water unraveled with solution state 1H dynamic nuclear polarization. Phys. Chem. Chem. Phys., 2011. 13(17): p. 7732-7746

The dynamics and state of lipid bilayer-internal hydration water of unilamellar lipid vesicles dispersed in solutions is characterized. This study was enabled by a recently developed technique based on Overhauser dynamic nuclear polarization (DNP)-driven amplification of 1H nuclear magnetic resonance (NMR) signal of hydration water. This technique can, in the full presence of bulk water, selectively quantify the translational dynamics of hydration water within [similar]10 A around spin labels that are specifically introduced to the local volume of interest within the lipid bilayer. With this approach, the local apparent diffusion coefficients of internal water at different depths of the lipid bilayer were determined. The modulation of these values as a response to external stimuli, such as the addition of sodium chloride or ethanol and the lipid phase transitions, that alter the fluctuations of bilayer interfaces together with the activation energy values of water diffusivity shows that water is not individually and homogeneously solvating lipid's hydrocarbon tails in the lipid bilayer. We provide experimental evidence that instead, water and the lipid membrane comprise a heterogeneous system whose constituents include transient hydrophobic water pores or water structures traversing the lipid bilayer. We show how these transient pore structures, as key vehicles for passive water transport can better reconcile our experimental data with existing literature data on lipid bilayer hydration and dynamics.

Microwave field distribution in a magic angle spinning dynamic nuclear polarization NMR probe

Nanni, E.A., et al., Microwave field distribution in a magic angle spinning dynamic nuclear polarization NMR probe. J. Magn. Reson., 2011. 210(1): p. 16-23

We present a calculation of the microwave field distribution in a magic angle spinning (MAS) probe utilized in dynamic nuclear polarization (DNP) experiments. The microwave magnetic field (B1S) profile was obtained from simulations performed with the High Frequency Structure Simulator (HFSS) software suite, using a model that includes the launching antenna, the outer Kel-F stator housing coated with Ag, the RF coil, and the 4 mm diameter sapphire rotor containing the sample. The predicted average B1S field is 13 microT/W^1/2, where S denotes the electron spin. For a routinely achievable input power of 5 W the corresponding value is 0.84 MHz. The calculations provide insights into the coupling of the microwave power to the sample, including reflections from the RF coil and diffraction of the power transmitted through the coil. The variation of enhancement with rotor wall thickness was also successfully simulated. A second, simplified calculation was performed using a single pass model based on Gaussian beam propagation and Fresnel diffraction. This model provided additional physical insight and was in good agreement with the full HFSS simulation. These calculations indicate approaches to increasing the coupling of the microwave power to the sample, including the use of a converging lens and fine adjustment of the spacing of the windings of the RF coil. The present results should prove useful in optimizing the coupling of microwave power to the sample in future DNP experiments. Finally, the results of the simulation were used to predict the cross effect DNP enhancement ([epsilon]) vs. [omega]1S/(2[pi]) for a sample of 13C-urea dissolved in a 60:40 glycerol/water mixture containing the polarizing agent TOTAPOL; very good agreement was obtained between theory and experiment.

Relaxometry of insensitive nuclei: Optimizing dissolution dynamic nuclear polarization

MiEville, P., S. Jannin, and G. Bodenhausen, Relaxometry of insensitive nuclei: Optimizing dissolution dynamic nuclear polarization. J. Magn. Reson., 2011. 210(1): p. 137-140

We report measurements of spin-lattice relaxation of carbon-13 as a function of the magnetic field ('relaxometry') in view of optimizing dissolution-DNP. The sample is temporarily lifted into the stray field above a high-resolution magnet using a simple and inexpensive 'shuttle'. The signals of arbitrary molecules can be observed at high field with high-resolution and sensitivity. During the dissolution process and subsequent 'voyage' from the polarizer to the NMR magnet, relaxation is accelerated by paramagnetic polarizing agents, but it can be quenched by using scavengers.

Application of double spin echo spiral chemical shift imaging to rapid metabolic mapping of hyperpolarized [1-13C]-pyruvate

Josan, S., et al., Application of double spin echo spiral chemical shift imaging to rapid metabolic mapping of hyperpolarized [1-13C]-pyruvate. J. Magn. Reson., 2011. 209(2): p. 332-336

Undersampled spiral CSI (spCSI) using a free induction decay (FID) acquisition allows sub-second metabolic imaging of hyperpolarized 13C. Phase correction of the FID acquisition can be difficult, especially with contributions from aliased out-of-phase peaks. This work extends the spCSI sequence by incorporating double spin echo radiofrequency (RF) pulses to eliminate the need for phase correction and obtain high quality spectra in magnitude mode. The sequence also provides an added benefit of attenuating signal from flowing spins, which can otherwise contaminate signal in the organ of interest. The refocusing pulses can potentially lead to a loss of hyperpolarized magnetization in dynamic imaging due to flow of spins through the fringe field of the RF coil, where the refocusing pulses fail to provide complete refocusing. Care must be taken for dynamic imaging to ensure that the spins remain within the B1-homogeneous sensitive volume of the RF coil.

A Dynamic Nuclear Polarization spectrometer at 95 GHz/144 MHz with EPR and NMR excitation and detection capabilities

Feintuch, A., et al., A Dynamic Nuclear Polarization spectrometer at 95†GHz/144†MHz with EPR and NMR excitation and detection capabilities. J. Magn. Reson., 2011. 209(2): p. 136-141

A spectrometer specifically designed for systematic studies of the spin dynamics underlying Dynamic Nuclear Polarization (DNP) in solids at low temperatures is described. The spectrometer functions as a fully operational NMR spectrometer (144 MHz) and pulse EPR spectrometer (95 GHz) with a microwave (MW) power of up to 300 mW at the sample position, generating a MW B1 field as high as 800 KHz. The combined NMR/EPR probe comprises of an open-structure horn-reflector configuration that functions as a low Q EPR cavity and an RF coil that can accommodate a 30-50 micro liter sample tube. The performance of the spectrometer is demonstrated through some basic pulsed EPR experiments, such as echo-detected EPR, saturation recovery and nutation measurements, that enable quantification of the actual intensity of MW irradiation at the position of the sample. In addition, DNP enhanced NMR signals of samples containing TEMPO and trityl are followed as a function of the MW frequency. Buildup curves of the nuclear polarization are recorded as a function of the microwave irradiation time period at different temperatures and for different MW powers.

Tuesday, May 31, 2011

The Effect of Sample Age on Dynamic Nuclear Polarization Parameters in Asphalt Suspensions

Kirimli, H.E., et al., The Effect of Sample Age on Dynamic Nuclear Polarization Parameters in Asphalt Suspensions. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2011. 33(7): p. 604 - 611

The Overhauser effect type dynamic nuclear polarization was used to study some colloidal suspensions with benzene, toluene, ethylbenzene, mesitylene, tetralin, and chlorobenzene solvent media of asphaltene obtained from MC-30 liquid asphalt in a weak magnetic field of 1.437 mT at room temperature. For each sample of solvent medium, the samples were prepared in three different asphaltene concentrations. The samples were held for ten years and then dynamic nuclear polarization parameters were found via dynamic nuclear polarization experiments that were performed again. Thus, the effect of sample age to dynamic nuclear polarization parameters was investigated. The results were discussed.

Friday, May 20, 2011

High-field Overhauser dynamic nuclear polarization in silicon below the metal--insulator transition

Dementyev, A.E., D.G. Cory, and C. Ramanathan, High-field Overhauser dynamic nuclear polarization in silicon below the metal--insulator transition. J. Chem. Phys., 2011. 134(15): p. 154511-9

Single crystal silicon is an excellent system to explore dynamic nuclear polarization (DNP), as it exhibits a continuum of properties from metallic to insulating as a function of doping concentration and temperature. At low doping concentrations DNP has been observed to occur via the solid effect, while at very high-doping concentrations an Overhauser mechanism is responsible. Here we report the hyperpolarization of 29Si in n-doped silicon crystals, with doping concentrations in the range of (1–3)×1017 cm−3. In this regime exchange interactions between donors become extremely important. The sign of the enhancement in our experiments and its frequency dependence suggest that the 29Si spins are directly polarized by donor electrons via an Overhauser mechanism within exchange-coupled donor clusters. The exchange interaction between donors only needs to be larger than the silicon hyperfine interaction (typically much smaller than the donor hyperfine coupling) to enable this Overhauser mechanism. Nuclear polarization enhancement is observed for a range of donor clusters in which the exchange energy is comparable to the donor hyperfine interaction. The DNP dynamics are characterized by a single exponential time constant that depends on the microwave power, indicating that the Overhauser mechanism is a rate-limiting step. Since only about 2% of the silicon nuclei are located within 1 Bohr radius of the donor electron, nuclear spin diffusion is important in transferring the polarization to all the spins. However, the spin-diffusion time is much shorter than the Overhauser time due to the relatively weak silicon hyperfine coupling strength. In a 2.35 T magnetic field at 1.1 K, we observed a DNP enhancement of 244 ± 84 resulting in a silicon polarization of 10.4 ± 3.4% following 2 h of microwave irradiation.

Quantifying the transfer and settling in NMR experiments with sample shuttling

Granwehr, J., et al., Quantifying the transfer and settling in NMR experiments with sample shuttling. J. Chem. Phys., 2010. 132(24): p. 244507-13

Nuclear magnetic resonance NMR in combination with pulsed magnetic field gradients has proven very successful for measuring molecular diffusion, where the correlation time of the motion is much shorter than the timescale of the experiment. In this article, it is demonstrated that a single-scan NMR technique to measure molecular diffusion can be employed to also study incoherent random motions over macroscopic length scales that show correlation times similar to the timescale of the experiment. Such motions are observed, for example, after the mixing of two components or after transferring a sample from one container into another. To measure the fluid settling, a series of magnetization helices were encoded onto a sample. Stimulated gradient echo trains were then generated after different mixing times, which enabled the determination of an effective dispersion coefficient for the fluid. This technique was used to optimize the timing of NMR experiments combined with dissolution dynamic nuclear polarization, where a sample was shuttled between two magnets. In addition to the decay of fluid turbulences, the presence of microbubbles in the sample tube at the end of the shuttling step was identified as another contribution to the NMR linewidth. Microbubbles could be indirectly observed through the line broadening effect on the NMR signal due to their different susceptibility compared to the solvent, which induced field gradients near the interfaces. Using these data, the signal attenuation caused by sample motion in single-scan two-dimensional correlation spectroscopy NMR experiments could be predicted with reasonable accuracy.

Quantitative dynamic nuclear polarization-NMR on blood plasma for assays of drug metabolism

Lerche, M.H., et al., Quantitative dynamic nuclear polarization-NMR on blood plasma for assays of drug metabolism. NMR in Biomedicine, 2011. 24(1): p. 96-103

Analytical platforms for the fast detection, identification and quantification of circulating drugs with a narrow therapeutic range are vital in clinical pharmacology. As a result of low drug concentrations, analytical tools need to provide high sensitivity and specificity. Dynamic nuclear polarization-NMR (DNP-NMR) in the form of the hyperpolarization–dissolution method should afford the sensitivity and spectral resolution for the direct detection and quantification of numerous isotopically labeled circulating drugs and their metabolites in single liquid-state NMR transients. This study explores the capability of quantitative in vitro DNP-NMR to assay drug metabolites in blood plasma. The lower limit of detection for the anti-epileptic drug 13C-carbamazepine and its pharmacologically active metabolite 13C-carbamazepine-10,11-epoxide is 0.08 µg/mL in rabbit blood plasma analyzed by single-scan 13C DNP-NMR. An internal standard is used for the accurate quantification of drug and metabolite. Comparison of quantitative DNP-NMR data with an established analytical method (liquid chromatography-mass spectrometry) yields a Pearson correlation coefficient r of 0.99. Notably, all DNP-NMR determinations were performed without analyte derivatization or sample purification other than plasma protein precipitation. Quantitative DNP-NMR is an emerging methodology which requires little sample preparation and yields quantitative data with high sensitivity for therapeutic drug monitoring

Spin-Labeled Heparins as Polarizing Agents for Dynamic Nuclear Polarization

Dollmann, B.C., et al., Spin-Labeled Heparins as Polarizing Agents for Dynamic Nuclear Polarization. ChemPhysChem, 2010. 11(17): p. 3656-3663

A potentially biocompatible class of spin-labeled macromolecules, spin-labeled (SL) heparins, and their use as nuclear magnetic resonance (NMR) signal enhancers are introduced. The signal enhancement is achieved through Overhauser-type dynamic nuclear polarization (DNP). All presented SL-heparins show high 1H DNP enhancement factors up to E=−110, which validates that effectively more than one hyperfine line can be saturated even for spin-labeled polarizing agents. The parameters for the Overhauser-type DNP are determined and discussed. A striking result is that for spin-labeled heparins, the off-resonant electron paramagnetic resonance (EPR) hyperfine lines contribute a non-negligible part to the total saturation, even in the absence of Heisenberg spin exchange (HSE) and electron spin-nuclear spin relaxation (T1ne). As a result, we conclude that one can optimize the use of, for example, biomacromolecules for DNP, for which only small sample amounts are available, by using heterogeneously distributed radicals attached to the molecule.

Kinetics from Indirectly Detected Hyperpolarized NMR Spectroscopy by Using Spatially Selective Coherence Transfers

T. Harris et al., Kinetics from Indirectly Detected Hyperpolarized NMR Spectroscopy by Using Spatially Selective Coherence Transfers, Chemistry – A European Journal, 2011, 17, 697-703.

An important recent development in NMR spectroscopy is the advent of ex situ dynamic nuclear polarization (DNP) approaches, which are capable of yielding liquid-state sensitivities that exceed considerably those afforded by the highest-field spectrometers. This increase in sensitivity has triggered new research avenues, particularly concerning the in vivo monitoring of metabolism and disease by NMR spectroscopy. So far such gains have mainly materialized for experiments that focus on nonprotonated, low-γ nuclei; targets favored by relatively long relaxation times T1, which enable them to withstand the transfer from the cryogenic hyperpolarizer to the reacting centers of interest. Recent studies have also shown that transferring this hyperpolarization to protons by indirectly detected methods could successfully give rise to 1H NMR spectra of hyperpolarized compounds with a high sensitivity. The present study demonstrates that, when merged with spatially encoded methods, indirectly detected 1H NMR spectroscopy can also be exploited as time-resolved hyperpolarized spectroscopy. A methodology is thus introduced that can successfully deliver a series of hyperpolarized 1H NMR spectra over a minutes-long timescale. The principles and opportunities presented by this approach are exemplified by following the in vitro phosphorylation of choline by choline kinase, a potential metabolic marker of cancer; and by tracking acetylcholine’s hydrolysis by acetylcholine esterase, an important enzyme partaking in synaptic transmission and neuronal degradation.

Tuesday, April 12, 2011

Bridge12 Collaborates with Agilent 
on THz Technology for DNP-NMR

Click here to read the press release by Agilent Technologies

Cambridge, Mass. – April 11, 2011 – Bridge12 Technologies, a leading provider of terahertz technology for applications in science, medicine, security and defense, today announces it has entered into an agreement with Agilent to collaborate on technology for dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) spectroscopy. As part of the collaboration, Bridge12 will provide gyrotrons, a critical component for an emerging application known as DNP, which accelerates NMR experiments that typically require weeks to be performed in minutes.
Under the agreement, Bridge12 will provide critical terahertz components such as gyrotrons and terahertz transmission lines for DNP-NMR to be integrated with new and existing Agilent spectrometers. Where other solutions require expensive proprietary equipment, Bridge12's gyrotron can be retrofitted to existing spectrometers and requires no changes to the facility's layout.
Applying DNP to NMR experiments significantly improves measurement sensitivity for solid-state samples, reducing the duration of experiments by several orders of magnitude. Structure determination of proteins at atomic resolution by solid-state NMR (SSNMR) spectroscopy is a time-consuming, iterative process. Signal intensities are typically very small due to the very small gyromagnetic ratio of the nuclei under study, such as 1H, 2H, 13C, and 15N. However, free electrons possess a magnetic moment that is 660 times larger than that of protons. In a DNP experiment this polarization can be transferred to surrounding nuclei by radiating the sample with terahertz radiation to boost NMR signal intensities. With DNP, experiments that typically require several weeks of signal averaging can be performed in minutes. This acceleration is of great value in structural biology, pharmaceutical research and material science
“Bridge12 gyrotrons for Agilent's spectrometers are turn-key, low-maintenance, and designed specifically for the requirements of the NMR community,” says Dr. Thorsten Maly, a Bridge12 co-founder. “The joint system enables research groups to run DNP NMR experiments without requiring terahertz specialists on staff. Thus far, the scientific community has not been able to take advantage of DNP due to the lack of affordable, reliable terahertz sources. As a result of our collaboration with Agilent, we will be offering a turn-key solution to the NMR community that has far lower maintenance costs than current home-grown systems.”
Bridge12 staff will be available to answer questions at the Agilent booth at the 52nd Experimental Nuclear Magnetic Resonance Conference (ENC) held at the Asilomar Conference Center (Pacific Grove, California) from April 10th to 15th, 2011.

Monday, April 4, 2011

Solution-State Dynamic Nuclear Polarization

M. D. Lingwood et al., Solution-State Dynamic Nuclear Polarization, Annu. Rev. NMR Spec., 2011, 73, 83-126


Solution-state dynamic nuclear polarization (DNP) is an increasingly popular method of enhancing nuclear spin polarization that has many applications in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). The theory, methods and applications of DNP in the solution state using the Overhauser effect are distinctly different from those of solid-state DNP or what is known as dissolution DNP. This review discusses the theory and recent experimental advances of Overhauser DNP techniques in the solution state at various field strengths ranging from the earth's magnetic field to 9.2 T, covering the literature from 1986 to late 2010. Most of the focus in this review is on spectroscopy applications of DNP, although proton–electron double resonance imaging (PEDRI) and remotely enhanced liquids for imaging contrast (RELIC) applications are briefly covered.

Saturday, April 2, 2011

High-Field Dynamic Nuclear Polarization with High-Spin Transition Metal Ions

B. Corzilius et al., High-Field Dynamic Nuclear Polarization with High-Spin Transition  Metal Ions, 2011, JACS

We report the dynamic nuclear polarization of  1H spins in magic-angle-spinning spectra recorded at 5 T and 84 K via the solid effect using Mn2+ and Gd3+ complexes as polarizing agents. We show that the magnitude of the enhancements can be directly related to the effective line width of the central (mS = -1/2, 1/2) EPR transition. Using a Gd3+ complex with a narrow central transition EPR line width of 29 MHz, we observed amaximum enhancement of ∼13, which is comparable to previous results on the narrow-line-width trityl radical.

Dynamical nuclear polarization and confinement effects in ZnO quantum dots

P. G. Baranov et al., Dynamical nuclear polarization and confinement effects in ZnO quantum dots, Phys. Status Solidi B., 2010, 247, 1476-1479

The spatial distribution of the electronic wave function of a  shallow donor (SD) in a ZnO semiconductor quantum dots (QD’s) has been determined in the regime of quantum confinement by using the nuclear spins as probes. Hyperfine (HF) interactions as monitored by electron nuclear double resonance spectroscopy quantitatively reveal the transition from semiconductor to molecular properties upon reduction of the size of the nanoparticles. Influence of confinement effect on g-factor value of SD’s in ZnOand CdS QD’s was displayed. The almost complete dynamic nuclear polarization (DNP) of nuclear spins has been demonstrated can be achieved in ZnO QD’s by saturating the EPR transition of the SD present in the QD’s with using high-frequency at low temperatures. Polarization of 67Zn nuclear spins inZnO core and of 1Hnuclear spins in the Zn(OH)2 capping layer have been obtained which manifests itself via the creation of a hole and an antihole in the EPR absorption line of the SD in QD’s and a shift of the hole (antihole). The enhancement of the nuclear polarization opens the possibility to study semiconductor nanostructures with NMR techniques

Dynamic Nuclear Polarization Polarizer for Sterile Use Intent

J. H. Ardenkjaer-Larsen et al., Dynamic Nuclear Polarization Polarizer for Sterile Use Intent, NMR Biomed., 2011, published online.

A novel polarizer based on the dissolution-dynamic nuclear polarization (DNP) method has been designed, built and tested. The polarizer differs from those previously described by being designed with sterile use intent and being compatible with clinical use. The main features are: (1) an integral, disposable fluid path containing all pharmaceuticals constituting a sterile barrier, (2) a closed-cycle cryogenic system designed to eliminate consumption of liquid cryogens and (3) multi-sample polarization to increase throughput. The fluid path consists of a vial with the agent to be polarized, a pair of concentric inlet and outlet tubes connected to a syringe with dissolution medium and a receiver, respectively. The fluid path can operate at up to 400 K and 2.0MPa and generates volumes as high as 100 mL. An inline filter removes the amount of electron paramagnetic agent in the final product by more than 100-fold in the case of [1-13C]pyruvate. The system uses a sorption pump in conjunction with a conventional cryocooler. The system operates through cycles of pumping to low temperature and regeneration of the sorption pump. The magnet accommodates four samples at the same time. A temperature of less than 1 K was achieved for 68 h (no sample heat loads) with a liquid helium volume of 2.4 L. The regeneration of the liquid helium could be achieved in less than 10 h, and the transition to cold (< 1.2 K) was achieved in less than 90 min. A solid state polarization of 36W4% for [1-13C]pyruvic acid was obtained with only 10 mW of microwave power. The loading of a sample adds less than 50 J of heat to the helium bath by introducing the sample over 15 min. The heat load imposed on the helium bath during dissolution was less than 70 J. The measured liquid state polarization was 18W2%.

Dynamic Nuclear Polarization with Polychlorotriphenylmethyl Radicals: Supramolecular Polarization-Transfer Effects

C. Gaellieri et al., Dynamic Nuclear Polarization with Polychlorotriphenylmethyl Radicals: Supramolecular Polarization-Transfer Effects, Angew. Chem. Int. Ed., 2010, 49, 3360-3362

Polychlorinated trityl radicals (see structure: C black, Cl green, Na gray, O red) used for dynamic nuclear polarization (DNP) showed a new transfer mechanism involving quadrupolar chlorine nuclei. The observation of positive or negative enhancements, depending on the substrate, highlights the supramolecular character of the initial polarization-transfer process.

Wednesday, March 16, 2011

Meet Bridge12 at ENC

The Bridge12 team is looking forward to attending this year’s 52nd Experimental Nuclear Magnetic Resonance Conference (ENC) held at the Asilomar conference center (Pacific Grove, California) from April 10th to 15th, 2011.

You can meet our core team members Jagadishwar Sirigiri and Thorsten Maly to discuss our latest innovations in gyrotron technology for DNP-enhanced NMR spectroscopy (DNP-NMR). Please let us know in advance if you’d like to meet up, or simply pull us aside at the conference. We are looking forward to this exciting event to discuss how we can help you push the boundaries of nuclear magnetic research.

Sunday, March 6, 2011

Update: 3rd DNP Meeting in Lausanne (CH) - Registration Is Open

Registration is now open for the 3rd International Meeting on Dynamic Nuclear Polarization enhanced NMR Spectroscopy (DNP-NMR). The meeting will be held from September 7th to 10th, 2011, at EPFL Lausanne, Switzerland, following the highly successful meetings in Nottingham (2007) and Koenigstein (2009).

An array of key players of this emerging field will assess the present state of the art while participants will be encouraged to reveal and discuss the current issues and new trends in DNP, including the developments in Overhauser effect, dissolution DNP, gyrotrons, magic angle spinning, low temperature DNP, radical chemistry, and hyperpolarized imaging.

The preliminary program comprises 24 invited speakers. Additional speakers will be selected from the abstracts submitted.

Abstract submission deadline: June 1st, 2011
Registration deadline: July 15th, 2011
Post deadline abstract submission (poster only): August 1st, 2011