Feb 26, 2012

High resolution NMR study of T1 magnetic relaxation dispersion. II. Influence of spin-spin couplings on the longitudinal spin relaxation dispersion in multispin systems

Korchak, S., et al., High resolution NMR study of T1 magnetic relaxation dispersion. II. Influence of spin-spin couplings on the longitudinal spin relaxation dispersion in multispin systems. J. Chem. Phys., 2010. 133(19): p. 194502-11.

Effects of scalar spin-spin interactions on the nuclear magnetic relaxation dispersion NMRD of coupled multispin systems were analyzed. Taking spin systems of increasing complexity we demonstrated pronounced influence of the intramolecular spin-spin couplings on the NMRD of protons. First, at low magnetic fields where there is strong coupling of spins the apparent relaxation times of the coupled spins become equal. Second, there are new features, which appear at the positions of the nuclear spin level anticrossings. Finally, in coupled spin systems there can be a coherent contribution to the relaxation kinetics present at low magnetic fields. All these peculiarities caused by spin-spin interactions are superimposed on the features in NMRD, which are conditioned by changes of the motional regime. Neglecting the effects of couplings may lead to misinterpretation of the NMRD curves and significant errors in determining the correlation times of molecular motion. Experimental results presented are in good agreement with theoretical calculations.

High resolution NMR study of T1 magnetic relaxation dispersion. I. Theoretical considerations of relaxation of scalar coupled spins at arbitrary magnetic field

Ivanov, K., A. Yurkovskaya, and H.-M. Vieth, High resolution NMR study of T1 magnetic relaxation dispersion. I. Theoretical considerations of relaxation of scalar coupled spins at arbitrary magnetic field. J. Chem. Phys., 2008. 129(23): p. 234513-12.

A theoretical approach to the description of longitudinal T1 relaxation in scalar coupled systems of spin 1/2 nuclei at arbitrary magnetic field is developed, which is based on the Redfield theory. The consideration is addressed to field-cycling relaxometry experiments with high-resolution NMR detection, in which the field dependence of T1-relaxation times, the nuclear magnetic relaxation dispersion NMRD, can be studied for individual spins of the molecule. Our study reveals well-pronounced effects of spin-spin couplings on the NMRD curves. First, coupled spins having completely different high-field T1 times tend to relax at low field with a common relaxation time. Second, the NMRD curves exhibit sharp features at the fields corresponding to the positions of nuclear spin level anticrossings. Such effects of spin-spin couplings show up not only for individual spins but also for the T1-relaxation of the total spin magnetization of the molecule. The influence of spin-spin coupling is of importance as long as the coupling strength J is larger than the inverse T1-relaxation times of the spins. Around J·T1=1 there is also a coherent contribution to the relaxation kinetics resulting in an oscillatory component of the kinetic curves. Application of the theory to experimental examples will be described in subsequent publications.

Feb 25, 2012

Practical exercises for learning to construct NMR/MRI probe circuits

An article about magnetic resonance instrumentation in general, but still of interest for researchers that build there own NMR/DNP equipment.

Wheeler, D.D. and M.S. Conradi, Practical exercises for learning to construct NMR/MRI probe circuits. Concepts in Magnetic Resonance Part A, 2012. 40A(1): p. 1-13.

Nine sets of laboratory exercises are designed to acquaint the student with many aspects of NMR/MRI probe resonant circuits. These include demonstrations of stray inductance and capacitance, matching a resonant circuit to 50 Ω, inductive coupling, doubly resonant circuits, loading effects of tissue, and radiation from the probe circuit. Many of the exercises are for the bench, but some are performed in an NMR system. The exercises should lead to an understanding of, as well as some intuitive knowledge about, basic NMR/MRI probe circuits.

Feb 23, 2012

BCN Biosolids NMR and DNP symposium

The NMR laboratory of the University of Barcelona is part of the Spanish network of Scientific and Technological Infrastructures. It is located in theBarcelona Science Parc and is part of two excellence campus (Barcelona Knowledge Campus –BKC-and Health Universitat de Barcelona-HUBc-). Together with other large infrastructures in the same area (the synchrotron light source Alba, and the Barcelona Supercomputing Center) constitute one of the pillars of the present and future competitiveness of biotechnology and structural biology in the Barcelona area, Catalonia and Spain. To serve its mission to provide access to the latest enabling technologies for the biosciences, the University of Barcelona NMR laboratory plans to focus on two rapidly emerging fields: the study of biological solids by NMR and Dynamic Nuclear Polarization (DNP). While the former holds promise to open extremely important areas of research for biomedicine such as membrane proteins and amyloids, the later breaks the largest individual limitation of NMR which is its low sensitivity. The initial successes achieved by leading researchers in the field in bringing together solid state NMR and DNP are imposing a change in paradigm.

The University of Barcelona, in collaboration with Bruker, is organizing for March 14th 2012 a one-day symposium in which members of the International Advisory Board of the Barcelona NMR laboratory will present the state of the art in Biosolids NMR and DNP as well as in the coordination of NMR in other structural biology hubs in Europe.

The BCN Biosolids NMR and DNP symposium will take place just after another NMR related event, the BioNMR meeting with computational experts that will be held in the same venue March 12 and 13 (http://mmb.irbbarcelona.org/BCN_BSandDNP/). Together, the two meetings will provide a forum to analyze challenges and opportunities of state of the art NMR in biomedicine and its linkage to other platforms.

The following speakers have already accepted to participate in the BCN Biosolids and DNP symposium:

Mikael Akke (Lund)
Marc Baldus (Utrecht)
Geoffrey Bodenhausen (Paris and Lausanne)
Eike Brunner (Dresden)
Ulrich Günther (Birmingham)
Jochem Stuppe (Billerica)

Attendance is open but registration is requested for organization purposes. Scientific sessions will run from 10 to 17 with coffee and lunch breaks. You can register by clicking here.

Feb 21, 2012

Feb 17, 2012

An ultrasensitive tool exploiting hydration dynamics to decipher weak lipid membrane-polymer interactions

Pike, K.J., et al., A spectrometer designed for 6.7 and 14.1 T DNP-enhanced solid-state MAS NMR using quasi-optical microwave transmission. J. Magn. Reson., 2012. 215(0): p. 1-9.

A Dynamic Nuclear Polarisation (DNP) enhanced solid-state Magic Angle Spinning (MAS) NMR spectrometer operating at 6.7 T is described and demonstrated. The 187 GHz TE13 fundamental mode of the FU CW VII gyrotron is used as the microwave source for this magnetic field strength and 284 MHz 1H DNP-NMR. The spectrometer is designed for use with microwave frequencies up to 395 GHz (the TE16 second-harmonic mode of the gyrotron) for DNP at 14.1 T (600 MHz 1H NMR). The pulsed microwave output from the gyrotron is converted to a quasi-optical Gaussian beam using a Vlasov antenna and transmitted to the NMR probe via an optical bench, with beam splitters for monitoring and adjusting the microwave power, a ferrite rotator to isolate the gyrotron from the reflected power and a Martin-Puplett interferometer for adjusting the polarisation. The Gaussian beam is reflected by curved mirrors inside the DNP-MAS-NMR probe to be incident at the sample along the MAS rotation axis. The beam is focussed to a ~1mm waist at the top of the rotor and then gradually diverges to give much more efficient coupling throughout the sample than designs using direct waveguide irradiation. The probe can be used in triple channel HXY mode for 600 MHz 1H and double channel HX mode for 284 MHz 1H, with MAS sample temperatures >85 K. Initial data at 6.7 T and ~1 W pulsed microwave power are presented with 13C enhancements of 60 for a frozen urea solution (1H-13C CP), 16 for bacteriorhodopsin in purple membrane (1H-13C CP) and 22 for 15N in a frozen glycine solution (1H-15N CP) being obtained. In comparison with designs which irradiate perpendicular to the rotation axis the approach used here provides a highly efficient use of the incident microwave beam and an NMR-optimised coil design.

Feb 15, 2012

An ultrasensitive tool exploiting hydration dynamics to decipher weak lipid membrane-polymer interactions

Cheng, C.-Y., et al., An ultrasensitive tool exploiting hydration dynamics to decipher weak lipid membrane-polymer interactions. J. Magn. Reson., 2012. 215(0): p. 115-119.

We introduce a newly developed tool, 1H Overhauser Dynamic Nuclear Polarization (ODNP), to sensitively explore weak macromolecular interactions by site-specifically probing the modulation of the translational dynamics of hydration water at the interaction interface, in the full presence of bulk water. Here, ODNP is employed on an illustrative example of a membrane-active triblock copolymer, poloxamer 188 (P188), which is known to restore the integrity of structurally compromised cell membranes. We observe a distinct change in the translational dynamics of the hydration layer interacting with the lipid membrane surface and the bilayer-interior as P188 is added to a solution of lipid vesicles, but no measurable changes in the dynamics or structure of the lipid membranes. This study shows that hydration water is an integral constituent of a lipid membrane system, and demonstrates for the first time that the modulation of its translational diffusivity can sensitively report on weak polymer–membrane interactions, as well as mediate essential lipid membrane functions. ODNP holds much promise as a unique tool to unravel molecular interactions at interfaces even in the presence of bulk water under ambient conditions.

DNP in MRI: An in-bore approach at 1.5 T

Krummenacker, J.G., et al., DNP in MRI: An in-bore approach at 1.5 T. J. Magn. Reson., 2012. 215(0): p. 94-99.

We have used liquid state ("Overhauser") Dynamic Nuclear Polarization (DNP) to significantly enhance the signal to noise ratio (SNR) of Magnetic Resonance Imaging (MRI). For the first time this was achieved by hyperpolarizing directly in the MRI-scanner field of 1.5 T in continuous flow mode and immediately delivering the hyperpolarized substance to the imaging site to ensure maximum contrast between hyperpolarized sample and sample at thermal polarization. We achieve a maximum absolute signal enhancement factor of 98; while the hyperpolarized sample is transported at a flow rate of up to 30 ml/h yielding an average flow speed up to 470 mm/s over a distance of approximately 80 mm. A spatial imaging resolution of 100 micro meter with a signal to noise ratio of 25 was achieved on the flowing sample. Application to MRI contrast enhancement or microfluidic imaging can be envisaged immediately.

Feb 13, 2012

A large sample volume magic angle spinning nuclear magnetic resonance probe for in situ investigations with constant flow of reactants

This post is not particular about DNP. However, it is a very interesting piece of instrumentation that demonstrates the state-of-the-art of instrumentation for MAS-NMR.

Hu, J.Z., et al., A large sample volume magic angle spinning nuclear magnetic resonance probe for in situ investigations with constant flow of reactants. Phys. Chem. Chem. Phys., 2012. 14(7): p. 2137-2143.

A large-sample-volume constant-flow magic angle sample spinning (CF-MAS) NMR probe is reported for in situ studies of the reaction dynamics, stable intermediates/transition states, and mechanisms of catalytic reactions. In our approach, the reactants are introduced into the catalyst bed using a fixed tube at one end of the MAS rotor while a second fixed tube, linked to a vacuum pump, is attached at the other end of the rotor. The pressure difference between both ends of the catalyst bed inside the sample cell space forces the reactants flowing through the catalyst bed, which improves the diffusion of the reactants and products. This design allows the use of a large sample volume for enhanced sensitivity and thus permitting in situ13C CF-MAS studies at natural abundance. As an example of application, we show that reactants, products and reaction transition states associated with the 2-butanol dehydration reaction over a mesoporous silicalite supported heteropoly acid catalyst (HPA/meso-silicalite-1) can all be detected in a single 13C CF-MAS NMR spectrum at natural abundance. Coke products can also be detected at natural 13C abundance and under the stopped flow condition. Furthermore, 1H CF-MAS NMR is used to identify the surface functional groups of HPA/meso-silicalite-1 under the condition of in situ drying. We also show that the reaction dynamics of 2-butanol dehydration using HPA/meso-silicalite-1 as a catalyst can be explored using 1H CF-MAS NMR.

Dynamic Tunneling Polarization as a Quantum Rotor Analogue of Dynamic Nuclear Polarization and the NMR Solid Effect

Horsewill, A.J. and S.M.M. Abu-Khumra, Dynamic Tunneling Polarization as a Quantum Rotor Analogue of Dynamic Nuclear Polarization and the NMR Solid Effect. Phys. Rev. Lett., 2011. 107(12): p. 127602.

The populations of the tunneling states of CH3 are manipulated by rf irradiation of weakly allowed sideband transitions within the manifold of tunneling-magnetic levels. Substantial positive and negative CH3 tunneling polarizations are observed, providing a quantum rotor analogue of dynamic nuclear polarization and the solid effect in NMR. The field-cycling NMR technique used in the experiments employs level crossings between tunneling and Zeeman systems to report on the tunneling polarization. The tunneling lifetimes are measured and the field dependence investigated.

Feb 12, 2012

Scalable Spin Amplification with a Gain Over a Hundred

Negoro, M., et al., Scalable Spin Amplification with a Gain Over a Hundred. Phys. Rev. Lett., 2011. 107(5): p. 050503.

We propose a scalable and practical implementation of spin amplification which does not require individual addressing nor a specially tailored spin network. We have demonstrated a gain of 140 in a solid-state nuclear spin system of which the spin polarization has been increased to 0.12 using dynamic nuclear polarization with photoexcited triplet electron spins. Spin amplification scalable to a higher gain opens the door to the single spin measurement for a readout of quantum computers as well as practical applications of nuclear magnetic resonance spectroscopy to infinitesimal samples which have been concealed by thermal noise.

Optically Erasing Disorder in Semiconductor Microcavities with Dynamic Nuclear Polarization

Liew, T.C.H. and V. Savona, Optically Erasing Disorder in Semiconductor Microcavities with Dynamic Nuclear Polarization. Phys. Rev. Lett., 2011. 106(14): p. 146404.

The mean squared value of the photonic disorder is found to be reduced by a factor of 100 in a typical GaAs based microcavity when exposed to a circularly polarized continuous wave optical pump without any special spatial patterning. Resonant excitation of the cavity mode excites a spatially nonuniform distribution of spin-polarized electrons, which depends on the photonic disorder profile. Electrons transfer spin to nuclei via the hyperfine contact interaction, inducing a long-living Overhauser magnetic field able to modify the potential of exciton polaritons.

Feb 11, 2012

Quantitative analysis of high field liquid state dynamic nuclear polarization

van Bentum, P.J.M., et al., Quantitative analysis of high field liquid state dynamic nuclear polarization. Phys. Chem. Chem. Phys., 2011. 13(39): p. 17831-17840.

Dynamic Nuclear Polarization (DNP) in the liquid state has become the focus of attention to improve the NMR sensitivity of mass limited samples. The Overhauser model predicts a fast reduction in DNP enhancement at high magnetic fields where the Electron Larmor frequency exceeds the typical inverse correlation time of the magnetic interaction between a radical spin and proton spins of the water molecules. Recent experiments have shown that an appreciable DNP enhancement in the liquid state is possible also at magnetic fields of 3 to 9 Tesla. At present it is not clear whether the Overhauser model needs to be adapted to explain these results. In the present paper we aim to resolve this question by a combination of in situ temperature dependent NMR relaxation measurements, EPR and DNP experiments. Enhancement factors of up to -165 are obtained with microwave powers below 500 mW. We conclude that at 3.4 Tesla (95 GHz) the various measurements are consistent with each other and in quantitative agreement with Overhauser theory. Microwave heating of the sample does play an important role to reduce the correlation times and allow a substantial Overhauser DNP. The typical enhancement factors may allow new applications in microfluidic NMR.

Dynamic nuclear polarization of quadrupolar nuclei using cross polarization from protons: surface-enhanced aluminium-27 NMR

Vitzthum, V., et al., Dynamic nuclear polarization of quadrupolar nuclei using cross polarization from protons: surface-enhanced aluminium-27 NMR. Chemical Communications, 2012. 48(14): p. 1988-1990.

The surface of [gamma]-alumina nanoparticles can be characterized by dynamic nuclear polarization (DNP) surface-enhanced NMR of 27Al. DNP is combined with cross-polarization and MQ-MAS to determine local symmetries of 27Al sites at the surface.

Feb 10, 2012

Overhauser DNP with 15N labelled Fremy's salt at 0.35 Tesla

Turke, M.-T., et al., Overhauser DNP with 15N labelled Fremy's salt at 0.35 Tesla. Phys. Chem. Chem. Phys., 2012. 14(2): p. 502-510.

The effectiveness of dynamic nuclear polarization (DNP) as a tool to enhance the sensitivity of liquid state NMR critically depends on the choice of the optimal polarizer molecule. In this study the performance of 15N labelled Fremy's salt as a polarizing agent in Overhauser DNP is investigated in detail at X-band (0.35 T, 9.7 GHz EPR, 15 MHz 1H NMR) and compared to that of TEMPONE-D,15N employed in previous studies. Both radicals provide similar maximum enhancements of the solvent water protons under similar conditions but a different saturation behaviour. The factors determining the enhancement and effective saturation were measured independently by EPR, ELDOR and NMRD and are shown to fulfil the Overhauser equation. In particular, following the theory of EPR saturation we provide analytical solutions for the dependence of the enhancement on the microwave field strength in terms of saturation transfer between two coupled hyperfine lines undergoing spin exchange. The negative charge of the radical in Fremy's salt solutions can explain the peculiar properties of this polarizing agent and indicates different suitable application areas for the two types of nitroxide radicals.

Feb 8, 2012

Mesoscale spatial distribution of electron spins studied by time-resolved small-angle and ultrasmall-angle neutron scattering with dynamic nuclear polarization: A case of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) doped in high-density polyethylene

Kumada, T., Y. Noda, S. Koizumi, and T. Hashimoto, J. Chem. Phys., 133, (2010)

We carried out time-resolved small-angle neutron scattering (SANS) and ultrasmall-angle neutron scattering (USANS) studies of dynamically polarized high-density polyethylene (HDPE) doped with 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) persistent free radicals. We observed a remarkable enhancement of the scattering intensity shortly after a switching of microwave frequency from positive (negative) to negative (positive) dynamic nuclear polarization (DNP). The enhancement was found to be due to spatially heterogeneous proton-spin polarization generated as a result of heterogeneously distributed TEMPO in the HDPE sample. The spatial fluctuation of the polarization ranged up to the length-scale of ≥ 100 nm. This result strongly suggests that the TEMPO free radicals are localized more in nonfibrils but less in fibrils of HDPE. In this way, we propose that the time-resolved DNP-SANS and DNP-USANS be general techniques to determine mesoscale spatial distribution of electron spins in dielectric materials.

Optimal control design of NMR and dynamic nuclear polarization experiments using monotonically convergent algorithms

Maximov, I.I., Z. Tosner, and N.C. Nielsen, J. Chem. Phys., 128, (2008)

Optimal control theory has recently been introduced to nuclear magnetic resonance (NMR) spectroscopy as a means to systematically design and optimize pulse sequences for liquid- and solid-state applications. This has so far primarily involved numerical optimization using gradient-based methods, which allow for the optimization of a large number of pulse sequence parameters in a concerted way to maximize the efficiency of transfer between given spin states or shape the nuclear spin Hamiltonian to a particular form, both within a given period of time. Using such tools, a variety of new pulse sequences with improved performance have been developed, and the NMR spin engineers have been challenged to consider alternative routes for analytical experiment design to meet similar performance. In addition, it has lead to increasing demands to the numerical procedures used in the optimization process in terms of computational speed and fast convergence. With the latter aspect in mind, here we introduce an alternative approach to numerical experiment design based on the Krotov formulation of optimal control theory. For practical reasons, the overall radio frequency power delivered to the sample should be minimized to facilitate experimental implementation and avoid excessive sample heating. The presented algorithm makes explicit use of this requirement and iteratively solves the stationary conditions making sure that the maximum of the objective is reached. It is shown that this method is faster per iteration and takes different paths within a control space than gradient-based methods. In the present work, the Krotov approach is demonstrated by the optimization of NMR and dynamic nuclear polarization experiments for various spin systems and using different constraints with respect to radio frequency and microwave power consumption.

PhD Position: Liquid state NMR experiments using a dedicated DNP NMR spectrometer based on a unique dual centre magnet

PhD Position: Liquid state NMR experiments using a dedicated DNP NMR spectrometer based on a unique dual centre magnet

The Dynamic Nuclear Polarisation (DNP) research group in Nottingham has designed and built a unique prototype DNP NMR facility based on a magnet with two iso-centres (3.4T and 9.4T)1. Hyperpolarised spin systems can be prepared by low temperature DNP (1.4K) at the 3.4T isocentre.2 Subsequently the sample is shuttled in solid state to the high field centre and rapidly dissolved by injection of small amounts of hot water. After this temperature jump the sample containing the highly polarised molecules is in liquid state at ambient temperature. The resulting signal enhancement compared to a thermally polarised sample which is used in conventional NMR can reach more than 10e4. 

Feb 1, 2012

A Slowly Relaxing Rigid Biradical for Efficient Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy: Expeditious Characterization of Functional Group Manipulation in Hybrid Materials

Zagdoun, A., et al., A Slowly Relaxing Rigid Biradical for Efficient Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy: Expeditious Characterization of Functional Group Manipulation in Hybrid Materials. J. Am. Chem. Soc., 2011. 134(4): p. 2284-2291.

A new nitroxide-based biradical having a long electron spin-lattice relaxation time (T1e) has been developed as an exogenous polarization source for DNP solid-state NMR experiments. The performance of this new biradical is demonstrated on hybrid silica-based mesostructured materials impregnated with 1,1,2,2-tetrachloroethane radical containing solutions, as well as in frozen bulk solutions, yielding DNP enhancement factors of over 100 at a magnetic field of 9.4 T and sample temperatures of ~100 K. The effects of radical concentration on the DNP enhancement factors and on the overall sensitivity enhancements are reported. The relatively high DNP efficiency of the biradical is attributed to an increased T1e, which enables more effective saturation of the electron resonance. This new biradical is shown to outperform the polarizing agents used so far in DNP surface-enhanced NMR spectroscopy of materials, yielding a 113-fold increase in overall sensitivity for silicon-29 CPMAS spectra as compared to conventional NMR experiments at room temperature. This results in a reduction in experimental times by a factor >12700, making the acquisition of 13C and 15N one- and two-dimensional NMR spectra at natural isotopic abundance rapid (hours). It has been used here to monitor a series of chemical reactions carried out on the surface functionalities of a hybrid organic-silica material.