Tuesday, July 31, 2012

High-Temperature Dynamic Nuclear Polarization Enhanced Magic-Angle-Spinning NMR

Akbey, Ü., A. Linden, and H. Oschkinat, High-Temperature Dynamic Nuclear Polarization Enhanced Magic-Angle-Spinning NMR. Appl. Magn. Reson., 2012. 43(1-2): p. 1-10.


Dynamic nuclear polarization (DNP) transfers electron spin-polarization to nuclear spins in close proximity, increasing sensitivity by two-to-three orders of magnitude. This enables nuclear magnetic resonance (NMR) experiments on samples with low concentrations of analyte. The requirement of using cryogenic temperatures in DNP-enhanced solid-state NMR (ssNMR) experiments may impair the resolution and hence limit its broad application to biological systems. In this work, we introduce a “High-Temperature DNP” approach, which aims at increasing spectral resolution by performing experiments at temperatures of around 180 K instead of ~100 K. By utilizing the extraordinary enhancements obtained on deuterated proteins, still sufficiently large DNP enhancements of 11–18 are obtained for proton and carbon, respectively. We recorded high sensitivity 2D 13 C– 13 C spectra in ~9 min with higher resolution than at 100 K, which has similar resolution to the one obtained at room temperature for some favorable residues.



Thursday, July 26, 2012

Level anti-crossings in ParaHydrogen Induced Polarization experiments with Cs-symmetric molecules

Buljubasich, L., et al., Level anti-crossings in ParaHydrogen Induced Polarization experiments with Cs-symmetric molecules. J. Magn. Reson., 2012. 219(0): p. 33-40.


Hyperpolarization by means of ParaHydrogen Induced Polarization (PHIP) has found increasing applications since its discovery. However, in the last decade only a few experiments have been reported describing the hydrogenation of symmetric molecules. A general AA′BB′ system is studied here. Calculations of the spin dynamics with the density matrix formalism support the experimental findings, providing profound understanding of the experiments in Cs-symmetric molecules. Level anti-crossings between states related to the triplet and the singlet state of one pair of the protons are identified as being responsible for hyperpolarization transfer in a PHIP experiment, when the former p-H2 protons occupy the sites AA′. The hydrogenation of acetylene dicarboxylic acid dimethylester with parahydrogen is used to illustrate the case. The theoretical treatment applied to this particular reaction explains the signal enhancements in both groups of protons in the spectrum when the sample is placed in the proper magnetic field strength, including the phase inversion of the signal of the methyl group. The treatment described here can be extended to every molecule which can be approximated as an AA′BB′ system.

Tuesday, July 24, 2012

Unexpected multiplet patterns induced by the Haupt-effect

This article is not about microwave driven DNP, however, similar effects that can be attributed to the Haupt Effect have been already observed in dissolution-DNP.

Icker, M. and S. Berger, Unexpected multiplet patterns induced by the Haupt-effect. J. Magn. Reson., 2012. 219(0): p. 1-3.


An NMR polarization up to a factor of 100 compared to the room temperature signal of a fully equilibrated sample and up/down multiplets are observed when 4-methyl-pyridine or toluene are taken rapidly from liquid helium temperatures to room temperature by dissolving in acetone-d6. These findings result from the inherent coupling between rotational and nuclear spin states in methyl groups which can act as quantum rotors. The temperature jump causes changes in rotational and spin energy level population due to symmetry rules that is known as the Haupt-effect.

Wednesday, July 18, 2012

Enhanced sensitivity by nonuniform sampling enables multidimensional MAS NMR spectroscopy of protein assemblies

This may look like a bit off-topic. However, in recent years nonuniform sampling and especially the methods to process the experimental data sets have become very stable. In principle uniform sampling can combined with DNP to achieve an even higher sensitivity.


Paramasivam, S., et al., Enhanced sensitivity by nonuniform sampling enables multidimensional MAS NMR spectroscopy of protein assemblies. J Phys Chem B, 2012. 116(25): p. 7416-27.


We report dramatic sensitivity enhancements in multidimensional MAS NMR spectra by the use of nonuniform sampling (NUS) and introduce maximum entropy interpolation (MINT) processing that assures the linearity between the time and frequency domains of the NUS acquired data sets. A systematic analysis of sensitivity and resolution in 2D and 3D NUS spectra reveals that with NUS, at least 1.5- to 2-fold sensitivity enhancement can be attained in each indirect dimension without compromising the spectral resolution. These enhancements are similar to or higher than those attained by the newest-generation commercial cryogenic probes. We explore the benefits of this NUS/MaxEnt approach in proteins and protein assemblies using 1-73-(U-(13)C,(15)N)/74-108-(U-(15)N) Escherichia coli thioredoxin reassembly. We demonstrate that in thioredoxin reassembly, NUS permits acquisition of high-quality 3D-NCACX spectra, which are inaccessible with conventional sampling due to prohibitively long experiment times. Of critical importance, issues that hinder NUS-based SNR enhancement in 3D-NMR of liquids are mitigated in the study of solid samples in which theoretical enhancements on the order of 3-4 fold are accessible by compounding the NUS-based SNR enhancement of each indirect dimension. NUS/MINT is anticipated to be widely applicable and advantageous for multidimensional heteronuclear MAS NMR spectroscopy of proteins, protein assemblies, and other biological systems.



Monday, July 16, 2012

Theoretical Aspects of Dynamic Nuclear Polarization in the Solid State: The Influence of High Radical Concentrations on the Solid Effect and Cross Effect Mechanisms

Hovav, Y., et al., Theoretical Aspects of Dynamic Nuclear Polarization in the Solid State: The Influence of High Radical Concentrations on the Solid Effect and Cross Effect Mechanisms. Appl. Magn. Reson., 2012: p. 1-21.


Dynamic nuclear polarization (DNP) is used to enhance signals in NMR and MRI experiments. During these experiments microwave (MW) irradiation mediates transfer of spin polarization from unpaired electrons to their neighboring nuclei. Solid state DNP is typically applied to samples containing high concentrations (i.e. 10–40 mM) of stable radicals that are dissolved in glass forming solvents together with molecules of interest. Three DNP mechanisms can be responsible for enhancing the NMR signals: the solid effect (SE), the cross effect (CE), and thermal mixing (TM). Recently, numerical simulations were performed to describe the SE and CE mechanisms in model systems composed of several nuclei and one or two electrons. It was shown that the presence of core nuclei, close to DNP active electrons, can result in a decrease of the nuclear polarization, due to broadening of the double quantum (DQ) and zero quantum (ZQ) spectra. In this publication we consider samples with high radical concentrations, exhibiting broad inhomogeneous EPR line-shapes and slow electron cross-relaxation rates, where the TM mechanism is not the main source for the signal enhancements. In this case most of the electrons in the sample are not affected by the MW field applied at a discrete frequency. Numerical simulations are performed on spin systems composed of several electrons and nuclei in an effort to examine the role of the DNP inactive electrons. Here we show that these electrons also broaden the DQ and ZQ spectra, but that they hardly cause any loss to the DNP enhanced nuclear polarization due to their spin-lattice relaxation mechanism. Their presence can also prevent some of the polarization losses due to the core nuclei.



Saturday, July 14, 2012

Cross Polarization for Dissolution Dynamic Nuclear Polarization Experiments at Readily Accessible Temperatures 1.2 < T < 4.2 K

Bornet, A., et al., Cross Polarization for Dissolution Dynamic Nuclear Polarization Experiments at Readily Accessible Temperatures 1.2 < T < 4.2 K. Appl. Magn. Reson., 2012: p. 1-11.


Cross polarization can provide significant enhancements with respect to direct polarization of low-γ nuclei such as 13 C. Substantial gains in sample throughput (shorter polarization times) can be achieved by exploiting shorter build-up times τ DNP ( 1 H) < τ DNP ( 13 C). To polarize protons rather than low-γ nuclei, nitroxide radicals with broad ESR resonances such as TEMPO are more appropriate than Trityl and similar carbon-based radicals that have narrow lines. With TEMPO as polarizing agent, the main Dynamic Nuclear Polarization (DNP) mechanism is thermal mixing (TM). Cross polarization makes it possible to attain higher polarization levels at 2.2 K than one can obtain with direct DNP of low-γ nuclei with TEMPO at 1.2 K, thus avoiding complex cryogenic technology.



Thursday, July 12, 2012

The Different Magnetic Resonance Communities Join Forces for Progress in DNP: an Editorial for the Special Issue on DNP

This is the foreword for the special issue on DNP published by Applied Magnetic Resonance (Springer).

Atsarkin, A.V. and W. Köckenberger, The Different Magnetic Resonance Communities Join Forces for Progress in DNP: an Editorial for the Special Issue on DNP. Appl. Magn. Reson., 2012: p. 1-2.