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°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 anistropy modulated by isotropic molecular reorientation.