Progress in FY2010 was made in the following areas: (1) LOW-TEMPERATURE DYNAMIC NUCLEAR POLARIZATION (DNP). DNP is a phenomenon in which irradiation of electron spin transitions with microwaves leads to enhancements of nuclear spin polarizations, and hence enhancements of NMR signals. We have constructed an apparatus for DNP at 9.4 Tesla magnetic fields (corresponding to 264 GHz microwave frequency) that allows us to assess the magnitudes of NMR signal enhancements as a function of temperature, down to approximately 10 K. We find that signal enhancements at 10 K can be as large as a factor of 80, with only 30 mW of microwave power, and enhancements at 30 K can be approximately 30. These enhancements are relative to thermal equilibrium signals at each temperature, so a factor of 80 at 10 K corresponds to a factor of 30 X 80 = 2400 relative to room-temperature NMR signals. We have also found that (of paramagnetic compounds we have tested), a triradical compound containing three nitroxide groups produces the largest signal enhancements in frozen aqueous solutions. These results are very encouraging, and will be extended to solid state NMR measurements on biological systems under magic-angle spinning conditions in the next fiscal year. We have begun to investigate whether these large signal enhancements may enable magnetic resonance imaging (MRI) of subcellular structures with spatial resolution below 1 micron. Our experimental measurements of absolute signal-to-noise ratios and of NMR linewidths indicate that sub-micron MRI should indeed be possible, with spatial resolution possibly as good as 300 nm X 300 nm X 300 nm. (2) FREQUENCY-SELECTIVE DIPOLAR RECOUPLING. Dipolar recoupling techniques are techniques for measuring nuclear magnetic dipole-dipole couplings and hence internuclear (or interatomic) distances in solid state NMR. We have developed a new approach to dipolar recoupling that allows measurements of distances between specific pairs of carbon-13 nuclei in proteins that are uniformly labeled with carbon-13. This technique, for the first time, allows us to measure quantitatively the distances between carbon atoms that define backbone and sidechain torsion angles in uniformly-labeled proteins. We have successfully tested this technique on microcrystalline model proteins with known structure, and we have begun to apply this technique in structural studies of HET-s prion fibrils and beta-amyloid fibrils. (3) SEMI-AUTOMATED RESONANCE ASSIGNMENTS IN SOLID STATE NMR. We have developed a new computational protocol for sequential assignment of chemical shifts in multidimensional solid state NMR spectra of uniformly 15N,13C-labeled proteins. This method uses a novel Monte Carlo/simulated annealing (MC/SA) approach to assign crosspeak signals to individual residues in a manner that maximizes a scoring function, which favors sequential matches of relevant chemical shifts, disfavors mismatches, and favors contiguous stretches of residues with assigned signals. This computational approach replaces manual assignment procedures that are highly subjective, error-prone, and incomplete in their analysis of the information content of the NMR data.
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