Protein dynamics studied by rotating frame 15N spin relaxation times.

Conformational rate processes in aqueous solutions of uniformly 15N-labeled pancreatic trypsin inhibitor (BPTI) at 36 degrees C were investigated by measuring the rotating frame relaxation times of the backbone 15N spins as a function of the spin-lock power. Two different intramolecular exchange processes were identified. A first local rate process involved the residues Cys38 and Arg39, had a correlation time of about 1.3 ms, and was related to isomerization of the chirality of the disulfide bond Cys14-Cys38. A second, faster motional mode was superimposed on the disulfide bond isomerization and was tentatively attributed to local segmental motions in the polypeptide sequence -Cys14-Ala15-Lys16-. The correlation time for the overall rotational tumbling of the protein was found to be 2 ns, using the assumption that relaxation is dominated by dipolar coupling and chemical shift anisotropy modulated by isotropic molecular reorientation.