Monday, December 24, 2012

Solid state nuclear magnetic resonance with magic-angle spinning and dynamic nuclear polarization below 25K

Thurber, K.R., et al., Solid state nuclear magnetic resonance with magic-angle spinning and dynamic nuclear polarization below 25K. J. Magn. Reson., 2013. 226(0): p. 100-106.

We describe an apparatus for solid state nuclear magnetic resonance (NMR) with dynamic nuclear polarization (DNP) and magic-angle spinning (MAS) at 20-25K and 9.4Tesla. The MAS NMR probe uses helium to cool the sample space and nitrogen gas for MAS drive and bearings, as described earlier [1], but also includes a corrugated waveguide for transmission of microwaves from below the probe to the sample. With a 30mW circularly polarized microwave source at 264GHz, MAS at 6.8kHz, and 21K sample temperature, greater than 25-fold enhancements of cross-polarized (13)C NMR signals are observed in spectra of frozen glycerol/water solutions containing the triradical dopant DOTOPA-TEMPO when microwaves are applied. As demonstrations, we present DNP-enhanced one-dimensional and two-dimensional (13)C MAS NMR spectra of frozen solutions of uniformly (13)C-labeled l-alanine and melittin, a 26-residue helical peptide that we have synthesized with four uniformly (13)C-labeled amino acids.

Happy Holidays and a Happy New Year

Happy Holidays and a Happy New Year to all the DNP-NMR blog readers. I will be taking a short break but will be back in the second week of January with new updates and DNP literature.

Happy Holidays or Fröhliche Weihnachten as we say in Germany.

Friday, December 21, 2012

Frequency dependence of electron spin relaxation times in aqueous solution for a nitronyl nitroxide radical and perdeuterated-tempone between 250MHz and 34GHz

Biller, J.R., et al., Frequency dependence of electron spin relaxation times in aqueous solution for a nitronyl nitroxide radical and perdeuterated-tempone between 250MHz and 34GHz. J. Magn. Reson., 2012. 225(0): p. 52-57.


Electron spin relaxation times of perdeuterated tempone (PDT) 1 and of a nitronyl nitroxide (2-(4-carboxy-phenyl)-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl) 2 in aqueous solution at room temperature were measured by 2-pulse electron spin echo (T(2)) or 3-pulse inversion recovery (T(1)) in the frequency range of 250MHz to 34GHz. At 9GHz values of T(1) measured by long-pulse saturation recovery were in good agreement with values determined by inversion recovery. Below 9GHz for 1 and below 1.5GHz for 2,T(1) approximately T(2), as expected in the fast tumbling regime. At higher frequencies T(2) was shorter than T(1) due to incomplete motional averaging of g and A anisotropy. The frequency dependence of 1/T(1) is modeled as the sum of spin rotation, modulation of g and A-anisotropy, and a thermally-activated process that has maximum contribution at about 1.5GHz. The spin lattice relaxation times for the nitronyl nitroxide were longer than for PDT by a factor of about 2 at 34GHz, decreasing to about a factor of 1.5 at 250MHz. The rotational correlation times, tau(R) are calculated to be 9ps for 1 and about 25ps for 2. The longer spin lattice relaxation times for 2 than for 1 at 9 and 34GHz are due predominantly to smaller contributions from spin rotation that arise from slower tumbling. The smaller nitrogen hyperfine couplings for the nitronyl 2 than for 1 decrease the contribution to relaxation due to modulation of A anisotropy. However, at lower frequencies the slower tumbling of 2 results in a larger value of omegatau(R) (omega is the resonance frequency) and larger values of the spectral density function, which enhances the contribution from modulation of anisotropic interactions for 2 to a greater extent than for 1.



Wednesday, December 19, 2012

On the spin order transfer from parahydrogen to another nucleus

Bär, S., et al., On the spin order transfer from parahydrogen to another nucleus. J. Magn. Reson., 2012. 225(0): p. 25-35.


The hyperpolarization of nuclear spins holds great potential e.g. for biomedical research. Strong signal enhancements have been demonstrated e.g. by transforming the spin order of parahydrogen (pH(2)) to net polarization of a third nucleus (e.g. (13)C) by means of a spin-order-transfer (SOT) sequence. The polarization achieved is vitally dependent on the sequence intervals, which are a function of the J-coupling constants of the molecule to be polarized. How to derive the SOT sequence intervals, the actual values for molecules as well as the (theoretical) polarization yield and robustness, however, are not fully described. In this paper, (a) we provide the methods to obtain the SOT intervals for a given set of J-coupling constants (i.e. of a new hyperpolarization agent); (b) exemplify these methods on molecules from literature, providing the hitherto missing intervals and simulated polarization yield; and (c) assess the robustness of the sequences towards B(1) and J-coupling errors. Close to unity polarization is obtained for all molecules and sequences. Furthermore, the loss of polarization caused by erroneous B(1) and J-coupling constants is reduced by choosing the channel and phase of some pulses in the SOT sequences appropriately.



Monday, December 17, 2012

Indirect and Direct 29Si Dynamic Nuclear Polarization of Dispersed Nanoparticles

Lafon, O., et al., Indirect and Direct 29Si Dynamic Nuclear Polarization of Dispersed Nanoparticles. Chemical Communications, 2012.


We show how the 29Si NMR signals of dispersed inorganic nanoparticles of laponite[registered sign] can be enhanced by Dynamic Nuclear Polarization (DNP). The direct DNP enhances the signals of 29Si nuclei near to unpaired electrons, whereas the indirect DNP via 1H enhances the signals of more remote sites.



Friday, December 14, 2012

Theoretical aspects of dynamic nuclear polarization in the solid state - spin temperature and thermal mixing

Hovav, Y., A. Feintuch, and S. Vega, Theoretical aspects of dynamic nuclear polarization in the solid state - spin temperature and thermal mixing. Phys. Chem. Chem. Phys., 2013. 15(1): p. 188-203.

Dynamic nuclear polarization is a method which allows for a dramatic increase of the NMR signals due to polarization transfer between electrons and their neighboring nuclei, via microwave irradiation. These experiments have become popular in recent years due to the ability to create hyper-polarized chemically and biologically relevant molecules, in frozen glass forming mixtures containing free radicals. Three mechanisms have been proposed for the polarization transfer between electrons and their surrounding nuclei in such non-conducting samples: the solid effect and cross effect mechanisms, which are based on quantum mechanics and relaxation on small spin systems, and thermal mixing, which originates from the thermodynamic macroscopic notion of spin temperature. We have recently introduced a spin model, which is based on the density matrix formalism and includes relaxation, and applied it to study the solid effect and cross effect mechanisms on small spin systems. In this publication we use the same model to describe the thermal mixing mechanism, and the creation of spin temperature. This is obtained without relying on the spin temperature formalism. Simulations of small model systems are used on systems with homogeneously and inhomogeneously broadened EPR lines. For the case of a homogeneously broadened line we show that the nuclear enhancement results from the thermal mixing and solid effect mechanisms, and that spin temperatures are created in the system. In the inhomogeneous case the enhancements are attributed to the solid effect and cross effect mechanisms, but not thermal mixing.



Wednesday, December 12, 2012

Combustion resistance of the 129Xe hyperpolarized nuclear spin state

Stupic, K.F., et al., Combustion resistance of the 129Xe hyperpolarized nuclear spin state. Phys. Chem. Chem. Phys., 2013. 15(1): p. 94-97.

Using a methane-xenon mixture for spin exchange optical pumping, MRI of combustion was enabled. The 129Xe hyperpolarized nuclear spin state was found to sufficiently survive the complete passage through the harsh environment of the reaction zone. A velocity profile (Vz(z)) of a flame was recorded to demonstrate the feasibility of MRI velocimetry of transport processes in combustors.



Monday, December 10, 2012

Sensitivity enhancement and low-field spin relaxation in singlet NMR

This article does not describe any DNP experiments. However, singlet states can be useful for dissolution DNP experiments.



Bocan, J., G. Pileio, and M.H. Levitt, Sensitivity enhancement and low-field spin relaxation in singlet NMR. Phys. Chem. Chem. Phys., 2012. 14(46): p. 16032-16040.


The singlet states of nuclear spin-1/2 pairs often display extended lifetimes that can be an order of magnitude longer than conventional relaxation times. We show that, in favourable circumstances, acquisition of the NMR signal during an extended multiple spin-echo train, followed by suitable data processing, enhances the signal-to-noise ratio of singlet NMR by up to an order of magnitude. The achievable enhancement depends on the transverse relaxation time constant, the magnetic field inhomogeneity, and the acceptable degradation in digital spectral resolution. We use the combination of singlet NMR and multiple spin-echo data acquisition to study the low-field nuclear relaxation processes of 15N-labelled nitrous oxide (15N2O) in solution. A general relaxation theory for coupled 2-spin-1/2 systems in low magnetic field is developed. Experimental trajectories of the nuclear spin observables are compared with theoretical expressions, including dipole-dipole and spin-rotation relaxation mechanisms. The estimated values of the spin-rotation tensors are compared with previous estimations from NMR and molecular beam electric resonance.



Friday, December 7, 2012

Development of DNP-Enhanced High-Resolution Solid-State NMR System for the Characterization of the Surface Structure of Polymer Materials

Horii, F., et al., Development of DNP-Enhanced High-Resolution Solid-State NMR System for the Characterization of the Surface Structure of Polymer Materials. J. Infrared Millim. Te., 2012. 33(7): p. 756-765.


A dynamic nuclear polarization (DNP)-enhanced cross-polarization/magic-angle spinning (DNP/CP/MAS) NMR system has been developed by combining a 200 MHz Chemagnetics CMX-200 spectrometer operating at 4.7 T with a high-power 131.5 GHz Gyrotron FU CW IV. The 30 W sub-THz wave generated in a long pulse TE $ _{{41}}^{{(1)}} $ mode with a frequency of 5 Hz was successfully transmitted to the modified Doty Scientific low-temperature CP/MAS probe through copper smooth-wall circular waveguides. Since serious RF noises on NMR signals by arcing in the electric circuit of the probe and undesired sample heating were induced by the continuous sub-THz wave pulse irradiation with higher powers, the on-off sub-THz wave pulse irradiation synchronized with the NMR detection was developed and the appropriate setting of the irradiation time and the cooling time corresponding to the non-irradiation time was found to be very effective for the suppression of the arcing and the sample heating. The attainable maximum DNP enhancement was more than 30 folds for C1 13 C-enriched D -glucose dissolved in the frozen medium containing mono-radical 4-amino-TEMPO. The first DNP/CP/MAS 13 C NMR spectra of poly(methyl methacrylate) (PMMA) sub-micron particles were obtained at the dispersed state in the same frozen medium, indicating that DNP-enhanced 1 H spins effectively diffuse from the medium to the PMMA particles through their surface and are detected as high-resolution 13 C spectra in the surficial region to which the 1 H spins reach. On the basis of these results, the possibility of the DNP/CP/MAS NMR characterization of the surface structure of nanomaterials including polymer materials was discussed.



Thursday, December 6, 2012

Postdoctoral researcher in Gyrotron DNP NMR

I think I posted that earlier, but this position is still available:


Postdoctoral researcher in Gyrotron DNP NMR

€Competitive | Switzerland | 03 Dec 2012
Dynamic Nuclear Polarization for NMR is a very strong and exciting area of research at EPFL, thanks to a campus-wide collaboration among scientists and with two companies.
Proficiency in experimental physics is required, preferably with knowledge in magnetic resonance or millimeter wave technology.
Funding of this position comes from EPFL, leaving full freedom in the choice of projects. Contract renewable up to 4 years.
Location: Lausanne, Switzerland
Salary min: CHF 75,000
Contact: jean-philippe.ansermet@epfl.ch
See: http://lpmn.epfl.ch

DNP-NMR is becoming more popular

So I don't know if you are paying attention to the pageview count that is displayed on the front page to the right. Well, I do, and not too boost my ego, but because I'm generally interested how the blog is doing and what topics are interesting for the readers.

If you follow that page counter you probably noticed that the page view count almost doubled from October to November. Now this number is a cumulative sum over the last 30 days but you can see the same behavior in the monthly counts (shown in the figure below).



I'm super excited about that trend. It shows that the interest in DNP-NMR increases. Why the sudden increase? Well, over the last few months I was able to get more inbound links to the blog that makes the blog more visible and that increased the page view count significantly.