Nuclear Magnetic Resonance--new Methods And Molecular St

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We have continued our study of biological macromolecules that are weakly aligned relative to the magnetic field. Study of 31P chemical shift anisotropy and 31P-H3? dipolar couplings independently validate that the structure derived of a model DNA oligonucleotide derived from dipolar couplings is considerably more accurate than the conventional NMR structure or structures observed in the crystalline state. Tracer diffusion anisotropy measurements indicate that the commonly used bicellar liquid crystalline medium does not consist of disks but of strongly perforated bilayers. A modification to the recently proposed gel method for aligning macromolecules has been developed that permits stretching of the gel instead of compression. It has been demonstrated for the first time that detergent-solubilized polypeptides can be aligned in such a medium, providing new opportunities for the study of such systems. A dipolar coupling study of Ca2+-calmodulin reveals that the interhelical angles in the N-terminal domain differ substantially from those observed in the crystalline state, indicative of considerable plasticity in this regulatory protein. In contrast, the sidechain chi-1 angles of the majority of target-interacting residues are locked into a single rotameric state, and local plasticity in the target-interacting surface is dominated by flexibility in the chi-2 and chi-3 angles of 8 methionine residues.