Progress in FY2008 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. (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 are currently applying the new frequency-selective recoupling techniques in our structural studies of various amyloid and prion fibrils.
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