Soliton Based Dynamic Nuclear Polarization: An Overhauser Effect in Cyclic Polyacetylene at High Field and Room Temperature.

Polyacetylene, a versatile material with an electrical conductivity that can span 7 orders of magnitude, is the prototypical conductive polymer. In this letter, we report the observation of a significant Overhauser effect at the high magnetic field of 14.1 T that operates at 100 K and room temperature in both linear and cyclic polyacetylene. Significant NMR signal enhancements ranging from 24 to 45 are obtained. The increased sensitivity enabled the characterization of the polymer chain defects at natural abundance. The absence of end methyl group carbon-13 signals provides proof of the closed-loop molecular structure of cyclic polyacetylene. The remarkable efficiency of the soliton based Overhauser effect DNP mechanism at high temperature and high field holds promise for applications and extension to other conductive polymer systems.

High-resolution, >1 GHz NMR in unstable magnetic fields.

Resistive or hybrid magnets can achieve substantially higher fields than those available in superconducting magnets, but their spatial homogeneity and temporal stability are unacceptable for high-resolution NMR. We show that modern stabilization and shimming technology, combined with detection of intermolecular zero-quantum coherences (iZQCs), can remove almost all of the effects of inhomogeneity and drifts, while retaining chemical shift differences and J couplings. In a 25-T electromagnet (1 kHz/s drift, 3 kHz linewidth over 1 cm(3)), iZQC detection removes >99% of the remaining inhomogeneity, to generate the first high-resolution liquid-state NMR spectra acquired at >1 GHz.

Exploring surfaces and cavities in lipoxygenase and other proteins by hyperpolarized xenon-129 NMR.

This paper presents an exploratory study of the binding interactions of xenon with the surface of several different proteins in the solution and solid states using both conventional and hyperpolarized (129)Xe NMR. The generation of hyperpolarized (129)Xe by spin exchange optical pumping affords an enhancement by 3-4 orders of magnitude of its NMR signal. As a result, it is possible to observe Xe directly bound to the surface of micromolar quantities of lyophilized protein. The highly sensitive nature of the (129)Xe line shape and chemical shift are used as indicators for the conditions most likely to yield maximal dipolar contact between (129)Xe nuclei and nuclear spins situated on the protein. This is an intermediate step toward achieving the ultimate goal of NMR enhancement of the binding-site nuclei by polarization transfer from hyperpolarized (129)Xe. The hyperpolarized (129)Xe spectra resulting from exposure of four different proteins in the lyophilized, powdered form have been examined for evidence of binding. Each of the proteins, namely, metmyoglobin, methemoglobin, hen egg white lysozyme, and soybean lipoxygenase, yielded a distinctly different NMR line shape. With the exception of lysozyme, the proteins all possess a paramagnetic iron center which can be expected to rapidly relax the (129)Xe and produce a net shift in its resonance position if the noble gas atom occupies specific binding sites near the iron. At temperatures from 223 to 183 K, NMR signals were observed in the 0-40 ppm chemical shift range, relative to Xe in the gas phase. The signals broadened and shifted downfield as the temperature was reduced, indicating that Xe is exchanging between the gas phase and internal or external binding sites of the proteins. Additionally, conventional (129)Xe NMR studies of metmyoglobin and lipoxygenase in the solution state are presented. The temperature dependence of the chemical shift and line shape indicate exchange of Xe between adsorption sites on lipoxygenase and Xe in the solvent on the slow to intermediate exchange time scale. The NMR results are compared with N(2), Xe, and CH(4) gas adsorption isotherms. It is found that lipoxygenase is unique among the proteins studied in possessing a relatively high affinity for gas molecules, and in addition, demonstrating the most clearly resolved adsorbed (129)Xe NMR peak in the lyophilized state.

Microscopic interpretation of optically pumped NMR signals in GaAs.

By numerical modelling and least squares fitting to the field, time and polarization dependence of optically pumped 69Ga NMR data in semi-insulating GaAs at low temperature and high field, a microscopic picture of this phenomenon emerges. Numerical values can be obtained for the key optical pumping parameters: the correlation time for the electron-nuclear contact interaction, and the electronic g-factor. The values of these parameters, which are in close agreement with previously reported values, can be combined with literature estimates for the average hyperfine interaction and spin diffusion coefficient D to generate numerical solutions for the Zeeman nuclear spin order, (r,t). The modelling permits the influence of variations with any other relevant parameter to be predicted. Experimentally, the optical polarization and time dependence data are presented. Least squares fits to the optical polarization dependence yield a value for the optically induced electron spin polarization at the given field, temperature, and excitation intensity. With the numerical solutions (r,t) that best fit the available data, the hyperfine frequency shift can be used to simulate the time evolution of the 69Ga NMR line shape. These simulations provide guidance on how to optimize the experimental conditions to most effectively observe hyperfine effects near shallow donors in this particular material.

Nuclear magnetic resonance by measuring reaction yield of spin symmetry species.

It is proposed that the nuclear magnetic resonance of sites which release dihydrogen can be obtained by measuring the branching fraction to the ortho and para forms. The motivation is to transform the sensitivity problem from that of detecting magnetization into the more tractable one of establishing the para and ortho content of free H2. It is shown with a density operator formalism that the para mole fraction reports directly on the zero-quantum coherence of the precursor and that other spin operators may be observed indirectly. Spectra are simulated for the case of a surface site from which H2 is released.