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Dynamic Nuclear Polarization

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DNP-NMR Literature Blog

Get up-to-date articles about dynamic nuclear polarization enhanced NMR spectroscopy
(DNP-NMR) and related terahertz technology from scientific journals. A free resource courtesy of Bridge12.

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Theoretical Aspects of Dynamic Nuclear Polarization in the Solid State – The Solid Effect

Y. Hovav et al., Theoretical Aspects of Dynamic Nuclear Polarization in the Solid State – The Soild Effect, J. Magn. Reson., 2010, 207(2), 176-189

http://dx.doi.org/10.1016/j.jmr.2010.10.016

Dynamic nuclear polarization has gained high popularity in recent years, due to advances in the experimental aspects of this methodology for increasing the NMR and MRI signals of relevant chemical and biological compounds. The DNP mechanism relies on the microwave (MW) irradiation induced polarization transfer from unpaired electrons to the nuclei in a sample. In this publication we present nuclear polarization enhancements of model systems in the solid state at high magnetic fields.

These results were obtained by numerical calculations based on the spin density operator formalism. Here we restrict ourselves to samples with low electron concentrations, where the dipolar electron–electron interactions can be ignored. Thus the DNP enhancement of the polarizations of the nuclei close to the electrons is described by the Solid Effect mechanism. Our numerical results demonstrate the dependence of the polarization enhancement on the MW irradiation power and frequency, the hyperfine and nuclear dipole–dipole spin interactions, and the relaxation parameters of the system. The largest spin system considered in this study contains one electron and eight nuclei. In particular, we discuss the influence of the nuclear concentration and relaxation on the polarization of the core nuclei, which are coupled to an electron, and are responsible for the transfer of polarization to the bulk nuclei in the sample via spin diffusion.

Theoretical Aspects of Dynamic Nuclear Polarization in the Solid State – The Solid Effect

Y. Hovav et al., Theoretical Aspects of Dynamic Nuclear Polarization in the Solid State – The Soild Effect, J. Magn. Reson., 2010, 207(2), 176-189

http://dx.doi.org/10.1016/j.jmr.2010.10.016

Dynamic nuclear polarization has gained high popularity in recent years, due to advances in the experimental aspects of this methodology for increasing the NMR and MRI signals of relevant chemical and biological compounds. The DNP mechanism relies on the microwave (MW) irradiation induced polarization transfer from unpaired electrons to the nuclei in a sample. In this publication we present nuclear polarization enhancements of model systems in the solid state at high magnetic fields.

These results were obtained by numerical calculations based on the spin density operator formalism. Here we restrict ourselves to samples with low electron concentrations, where the dipolar electron–electron interactions can be ignored. Thus the DNP enhancement of the polarizations of the nuclei close to the electrons is described by the Solid Effect mechanism. Our numerical results demonstrate the dependence of the polarization enhancement on the MW irradiation power and frequency, the hyperfine and nuclear dipole–dipole spin interactions, and the relaxation parameters of the system. The largest spin system considered in this study contains one electron and eight nuclei. In particular, we discuss the influence of the nuclear concentration and relaxation on the polarization of the core nuclei, which are coupled to an electron, and are responsible for the transfer of polarization to the bulk nuclei in the sample via spin diffusion.

Linearly Polarized Modes of a Corrugated Waveguide

E.J. Kowalski et al., Linearly Polarized Modes of a Corrugated Waveguide, IEEE Trans. on Mic. Theo. and Tech., 2010, 58(11), 2772-2780

http://dx.doi.org/10.1109/TMTT.2010.2078972

A linearly polarized $({rm LP}_{mn})$ mode basis set for oversized, corrugated, metallic waveguides is derived for the special case of quarter-wavelength-depth circumferential corrugations. The relationship between the ${rm LP}_{mn}$ modes and the conventional modes $({rm HE}_{mn},{rm EH}_{mn},{rm TE}_{0n},{rm TM}_{0n})$ of the corrugated guide is shown.

The loss in a gap or equivalent miter bend in the waveguide is calculated for single-mode and multimode propagation on the line. In the latter case, it is shown that modes of the same symmetry interfere with one another, causing enhanced or reduced loss, depending on the relative phase of the modes. If two modes with azimuthal $(m)$ indexes that differ by one propagate in the waveguide, the resultant centroid and the tilt angle of radiation at the guide end are shown to be related through a constant of the motion. These results describe the propagation of high-power linearly polarized radiation in overmoded corrugated waveguides.

Linearly Polarized Modes of a Corrugated Waveguide

E.J. Kowalski et al., Linearly Polarized Modes of a Corrugated Waveguide, IEEE Trans. on Mic. Theo. and Tech., 2010, 58(11), 2772-2780

http://dx.doi.org/10.1109/TMTT.2010.2078972

A linearly polarized $({rm LP}_{mn})$ mode basis set for oversized, corrugated, metallic waveguides is derived for the special case of quarter-wavelength-depth circumferential corrugations. The relationship between the ${rm LP}_{mn}$ modes and the conventional modes $({rm HE}_{mn},{rm EH}_{mn},{rm TE}_{0n},{rm TM}_{0n})$ of the corrugated guide is shown.

The loss in a gap or equivalent miter bend in the waveguide is calculated for single-mode and multimode propagation on the line. In the latter case, it is shown that modes of the same symmetry interfere with one another, causing enhanced or reduced loss, depending on the relative phase of the modes. If two modes with azimuthal $(m)$ indexes that differ by one propagate in the waveguide, the resultant centroid and the tilt angle of radiation at the guide end are shown to be related through a constant of the motion. These results describe the propagation of high-power linearly polarized radiation in overmoded corrugated waveguides.

Solid-State NMR Spectroscopy on Complex Biomolecules

M. Renault et al., Solid-State NMR Spectroscopy on Complex Biomolecules, Ang. Chem. Int. Ed., 2010, 49(45), 8346-8357

http://dx.doi.org/10.1002/anie.201002823

Biomolecular applications of NMR spectroscopy are often merely associated with soluble molecules or magnetic resonance  imaging. However, since the late 1970s, solid-state NMR (ssNMR) spectroscopy has demonstrated its ability to provide atomic-level insight into complex biomolecular systems ranging from lipid bilayers to complex biomaterials. In the last decade, progress in the areas of NMR spectroscopy, biophysics, and molecular biology have significantly expanded the repertoire of ssNMR spectroscopy for biomolecular studies. This Review discusses current approaches and methodological challenges, and highlights recent progress in using ssNMR spectroscopy at the interface of structural and cellular biology.

Solid-State NMR Spectroscopy on Complex Biomolecules

M. Renault et al., Solid-State NMR Spectroscopy on Complex Biomolecules, Ang. Chem. Int. Ed., 2010, 49(45), 8346-8357

http://dx.doi.org/10.1002/anie.201002823

Biomolecular applications of NMR spectroscopy are often merely associated with soluble molecules or magnetic resonance  imaging. However, since the late 1970s, solid-state NMR (ssNMR) spectroscopy has demonstrated its ability to provide atomic-level insight into complex biomolecular systems ranging from lipid bilayers to complex biomaterials. In the last decade, progress in the areas of NMR spectroscopy, biophysics, and molecular biology have significantly expanded the repertoire of ssNMR spectroscopy for biomolecular studies. This Review discusses current approaches and methodological challenges, and highlights recent progress in using ssNMR spectroscopy at the interface of structural and cellular biology.

Dynamic Nuclear Polarization of Deuterated Proteins

U. Akbey, Dynamic Nuclear Polarization of Deuterated Proteins, Angew. Chem. Int. Ed., 2010, 49(42), 7803-7806

http://dx.doi.org/10.1002/anie.201002044

Magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy has evolved as a robust and widely applicable technique for investigating the structure and dynamics of biological systems. It is in fact rapidly becoming an indispensable tool in structural biology studies of amyloid, nanocrystalline, and membrane proteins. However, it is clear that the low sensitivity of MAS experiments to directly detected 13C and 15N signals limits the utility of the approach, particularly when working with systems which are difficult to obtain in large quantities.

This limit provides the impetus to develop methods to enhance the sensitivity of MAS experiments, the availability of which will undoubtedly broaden the applicability of the technique. Remarkable progress towards this goal has been achieved by incorporating high-frequency dynamic nuclear polarization (DNP) into the MAS NMR technique. The DNP method exploits the microwave-driven transfer of polarization from a paramagnetic center, such as nitroxide free radical, to the nuclear spins, and has been demonstrated to produce uniformly polarized macromolecular samples. In principle signal enhancements, e = 660 can be obtained for 1H and recently signal enhancements of e = 100–200 were observed in model compounds. However, in applications of DNP to MAS spectra of biological systems, including studies of lysozyme, and bacteriorhodopsin, the enhancements have been smaller, e=40–50. An exception is the amyloidogenic peptide GNNQQNY7–13 which forms nanocrystals for which the proton T1 time is long and e ~ 100.

Dynamic Nuclear Polarization of Deuterated Proteins

U. Akbey, Dynamic Nuclear Polarization of Deuterated Proteins, Angew. Chem. Int. Ed., 2010, 49(42), 7803-7806

http://dx.doi.org/10.1002/anie.201002044

Magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy has evolved as a robust and widely applicable technique for investigating the structure and dynamics of biological systems. It is in fact rapidly becoming an indispensable tool in structural biology studies of amyloid, nanocrystalline, and membrane proteins. However, it is clear that the low sensitivity of MAS experiments to directly detected 13C and 15N signals limits the utility of the approach, particularly when working with systems which are difficult to obtain in large quantities.

This limit provides the impetus to develop methods to enhance the sensitivity of MAS experiments, the availability of which will undoubtedly broaden the applicability of the technique. Remarkable progress towards this goal has been achieved by incorporating high-frequency dynamic nuclear polarization (DNP) into the MAS NMR technique. The DNP method exploits the microwave-driven transfer of polarization from a paramagnetic center, such as nitroxide free radical, to the nuclear spins, and has been demonstrated to produce uniformly polarized macromolecular samples. In principle signal enhancements, e = 660 can be obtained for 1H and recently signal enhancements of e = 100–200 were observed in model compounds. However, in applications of DNP to MAS spectra of biological systems, including studies of lysozyme, and bacteriorhodopsin, the enhancements have been smaller, e=40–50. An exception is the amyloidogenic peptide GNNQQNY7–13 which forms nanocrystals for which the proton T1 time is long and e ~ 100.

EPR detected polarization transfer between GD3+ and protons at low temperature and 3.3T: The first step of dynamic nuclear polarization

Nagarajan V. et al., EPR detected polarization transfer between Gd3+ and protons at low temperature and 3.3 T: The first step of dynamic nuclear polarization, J. Chem. Phys., 2010, 132, 214504

http://dx.doi.org/10.1063/1.3428665

Electron-electron double resonance pulsed electron paramagnetic resonance (EPR) at 95 GHz (3.3 T) is used to follow the dynamics of the electron spin polarization during the first stages of dynamic nuclear polarization in solids. The experiments were performed on a frozen solution of Gd+3 (S=7/2) in water/glycerol. Focusing on the central |−1/2>→|+1/2> transition we measured the polarization transfer from the Gd3+ electron spin to the adjacent 1H protons.

The dependence of the echo detected EPR signal on the length of the microwave irradiation at the EPR “forbidden” transition corresponding to an electron and a proton spin flip is measured for different powers, showing dynamics on the microsecond to millisecond time scales. A theoretical model based on the spin density matrix formalism is suggested to account for this dynamics. The central transition of the Gd3+ ion is considered as an effective S=1/2 system and is coupled to 1H (I=1/2) nuclei. Simulations based on a single electron-single nucleus four level system are shown to deviate from the experimental results and an alternative approach taking into account the more realistic multinuclei picture is shown to agree qualitatively with the experiments.

EPR detected polarization transfer between GD3+ and protons at low temperature and 3.3T: The first step of dynamic nuclear polarization

Nagarajan V. et al., EPR detected polarization transfer between Gd3+ and protons at low temperature and 3.3 T: The first step of dynamic nuclear polarization, J. Chem. Phys., 2010, 132, 214504

http://dx.doi.org/10.1063/1.3428665

Electron-electron double resonance pulsed electron paramagnetic resonance (EPR) at 95 GHz (3.3 T) is used to follow the dynamics of the electron spin polarization during the first stages of dynamic nuclear polarization in solids. The experiments were performed on a frozen solution of Gd+3 (S=7/2) in water/glycerol. Focusing on the central |−1/2>→|+1/2> transition we measured the polarization transfer from the Gd3+ electron spin to the adjacent 1H protons.

The dependence of the echo detected EPR signal on the length of the microwave irradiation at the EPR “forbidden” transition corresponding to an electron and a proton spin flip is measured for different powers, showing dynamics on the microsecond to millisecond time scales. A theoretical model based on the spin density matrix formalism is suggested to account for this dynamics. The central transition of the Gd3+ ion is considered as an effective S=1/2 system and is coupled to 1H (I=1/2) nuclei. Simulations based on a single electron-single nucleus four level system are shown to deviate from the experimental results and an alternative approach taking into account the more realistic multinuclei picture is shown to agree qualitatively with the experiments.

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