Development of Solid State NMR Methods and Technology

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Progress in FY2011 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 and tested a new cryostat that permits "double-resonance" DNP measurements (1H and 13C) at temperatures down to 8 K. Preliminary studies indicate DNP signal enhancements up to 20X. 2D 13C-13C exchange measurements on amyloid fibrils have been carried out to determine local peptide backbone conformations. We have also constructed a new magic-angle spinning probe with DNP capabilities that operates down to 20 K. Testing of this probe is in progress. (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 applied frequency-selective 13CO-13CO recoupling methods developed in our group to model microcrystaline proteins (SH3 and GB1) and to beta-amyloid fibrils. Along with 15N-15N recoupling measurements, these data place new constraints on backbone conformations in uniformly labeled systems. (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. In FY2011, we have developed a very general version of the MC/SA program that is applicable to virtually any combination of multidimensional chemical shift correlation data in solid state NMR.