Tuesday, May 31, 2011

The Effect of Sample Age on Dynamic Nuclear Polarization Parameters in Asphalt Suspensions

Kirimli, H.E., et al., The Effect of Sample Age on Dynamic Nuclear Polarization Parameters in Asphalt Suspensions. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2011. 33(7): p. 604 - 611

The Overhauser effect type dynamic nuclear polarization was used to study some colloidal suspensions with benzene, toluene, ethylbenzene, mesitylene, tetralin, and chlorobenzene solvent media of asphaltene obtained from MC-30 liquid asphalt in a weak magnetic field of 1.437 mT at room temperature. For each sample of solvent medium, the samples were prepared in three different asphaltene concentrations. The samples were held for ten years and then dynamic nuclear polarization parameters were found via dynamic nuclear polarization experiments that were performed again. Thus, the effect of sample age to dynamic nuclear polarization parameters was investigated. The results were discussed.

Friday, May 20, 2011

High-field Overhauser dynamic nuclear polarization in silicon below the metal--insulator transition

Dementyev, A.E., D.G. Cory, and C. Ramanathan, High-field Overhauser dynamic nuclear polarization in silicon below the metal--insulator transition. J. Chem. Phys., 2011. 134(15): p. 154511-9

Single crystal silicon is an excellent system to explore dynamic nuclear polarization (DNP), as it exhibits a continuum of properties from metallic to insulating as a function of doping concentration and temperature. At low doping concentrations DNP has been observed to occur via the solid effect, while at very high-doping concentrations an Overhauser mechanism is responsible. Here we report the hyperpolarization of 29Si in n-doped silicon crystals, with doping concentrations in the range of (1–3)×1017 cm−3. In this regime exchange interactions between donors become extremely important. The sign of the enhancement in our experiments and its frequency dependence suggest that the 29Si spins are directly polarized by donor electrons via an Overhauser mechanism within exchange-coupled donor clusters. The exchange interaction between donors only needs to be larger than the silicon hyperfine interaction (typically much smaller than the donor hyperfine coupling) to enable this Overhauser mechanism. Nuclear polarization enhancement is observed for a range of donor clusters in which the exchange energy is comparable to the donor hyperfine interaction. The DNP dynamics are characterized by a single exponential time constant that depends on the microwave power, indicating that the Overhauser mechanism is a rate-limiting step. Since only about 2% of the silicon nuclei are located within 1 Bohr radius of the donor electron, nuclear spin diffusion is important in transferring the polarization to all the spins. However, the spin-diffusion time is much shorter than the Overhauser time due to the relatively weak silicon hyperfine coupling strength. In a 2.35 T magnetic field at 1.1 K, we observed a DNP enhancement of 244 ± 84 resulting in a silicon polarization of 10.4 ± 3.4% following 2 h of microwave irradiation.

Quantifying the transfer and settling in NMR experiments with sample shuttling

Granwehr, J., et al., Quantifying the transfer and settling in NMR experiments with sample shuttling. J. Chem. Phys., 2010. 132(24): p. 244507-13

Nuclear magnetic resonance NMR in combination with pulsed magnetic field gradients has proven very successful for measuring molecular diffusion, where the correlation time of the motion is much shorter than the timescale of the experiment. In this article, it is demonstrated that a single-scan NMR technique to measure molecular diffusion can be employed to also study incoherent random motions over macroscopic length scales that show correlation times similar to the timescale of the experiment. Such motions are observed, for example, after the mixing of two components or after transferring a sample from one container into another. To measure the fluid settling, a series of magnetization helices were encoded onto a sample. Stimulated gradient echo trains were then generated after different mixing times, which enabled the determination of an effective dispersion coefficient for the fluid. This technique was used to optimize the timing of NMR experiments combined with dissolution dynamic nuclear polarization, where a sample was shuttled between two magnets. In addition to the decay of fluid turbulences, the presence of microbubbles in the sample tube at the end of the shuttling step was identified as another contribution to the NMR linewidth. Microbubbles could be indirectly observed through the line broadening effect on the NMR signal due to their different susceptibility compared to the solvent, which induced field gradients near the interfaces. Using these data, the signal attenuation caused by sample motion in single-scan two-dimensional correlation spectroscopy NMR experiments could be predicted with reasonable accuracy.

Quantitative dynamic nuclear polarization-NMR on blood plasma for assays of drug metabolism

Lerche, M.H., et al., Quantitative dynamic nuclear polarization-NMR on blood plasma for assays of drug metabolism. NMR in Biomedicine, 2011. 24(1): p. 96-103

Analytical platforms for the fast detection, identification and quantification of circulating drugs with a narrow therapeutic range are vital in clinical pharmacology. As a result of low drug concentrations, analytical tools need to provide high sensitivity and specificity. Dynamic nuclear polarization-NMR (DNP-NMR) in the form of the hyperpolarization–dissolution method should afford the sensitivity and spectral resolution for the direct detection and quantification of numerous isotopically labeled circulating drugs and their metabolites in single liquid-state NMR transients. This study explores the capability of quantitative in vitro DNP-NMR to assay drug metabolites in blood plasma. The lower limit of detection for the anti-epileptic drug 13C-carbamazepine and its pharmacologically active metabolite 13C-carbamazepine-10,11-epoxide is 0.08 µg/mL in rabbit blood plasma analyzed by single-scan 13C DNP-NMR. An internal standard is used for the accurate quantification of drug and metabolite. Comparison of quantitative DNP-NMR data with an established analytical method (liquid chromatography-mass spectrometry) yields a Pearson correlation coefficient r of 0.99. Notably, all DNP-NMR determinations were performed without analyte derivatization or sample purification other than plasma protein precipitation. Quantitative DNP-NMR is an emerging methodology which requires little sample preparation and yields quantitative data with high sensitivity for therapeutic drug monitoring

Spin-Labeled Heparins as Polarizing Agents for Dynamic Nuclear Polarization

Dollmann, B.C., et al., Spin-Labeled Heparins as Polarizing Agents for Dynamic Nuclear Polarization. ChemPhysChem, 2010. 11(17): p. 3656-3663

A potentially biocompatible class of spin-labeled macromolecules, spin-labeled (SL) heparins, and their use as nuclear magnetic resonance (NMR) signal enhancers are introduced. The signal enhancement is achieved through Overhauser-type dynamic nuclear polarization (DNP). All presented SL-heparins show high 1H DNP enhancement factors up to E=−110, which validates that effectively more than one hyperfine line can be saturated even for spin-labeled polarizing agents. The parameters for the Overhauser-type DNP are determined and discussed. A striking result is that for spin-labeled heparins, the off-resonant electron paramagnetic resonance (EPR) hyperfine lines contribute a non-negligible part to the total saturation, even in the absence of Heisenberg spin exchange (HSE) and electron spin-nuclear spin relaxation (T1ne). As a result, we conclude that one can optimize the use of, for example, biomacromolecules for DNP, for which only small sample amounts are available, by using heterogeneously distributed radicals attached to the molecule.

Kinetics from Indirectly Detected Hyperpolarized NMR Spectroscopy by Using Spatially Selective Coherence Transfers

T. Harris et al., Kinetics from Indirectly Detected Hyperpolarized NMR Spectroscopy by Using Spatially Selective Coherence Transfers, Chemistry – A European Journal, 2011, 17, 697-703.

An important recent development in NMR spectroscopy is the advent of ex situ dynamic nuclear polarization (DNP) approaches, which are capable of yielding liquid-state sensitivities that exceed considerably those afforded by the highest-field spectrometers. This increase in sensitivity has triggered new research avenues, particularly concerning the in vivo monitoring of metabolism and disease by NMR spectroscopy. So far such gains have mainly materialized for experiments that focus on nonprotonated, low-γ nuclei; targets favored by relatively long relaxation times T1, which enable them to withstand the transfer from the cryogenic hyperpolarizer to the reacting centers of interest. Recent studies have also shown that transferring this hyperpolarization to protons by indirectly detected methods could successfully give rise to 1H NMR spectra of hyperpolarized compounds with a high sensitivity. The present study demonstrates that, when merged with spatially encoded methods, indirectly detected 1H NMR spectroscopy can also be exploited as time-resolved hyperpolarized spectroscopy. A methodology is thus introduced that can successfully deliver a series of hyperpolarized 1H NMR spectra over a minutes-long timescale. The principles and opportunities presented by this approach are exemplified by following the in vitro phosphorylation of choline by choline kinase, a potential metabolic marker of cancer; and by tracking acetylcholine’s hydrolysis by acetylcholine esterase, an important enzyme partaking in synaptic transmission and neuronal degradation.