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: