Interpretation of 13C spin-lattice relaxation resulting from ring puckering in proline

A calculation of 13C relaxation parameters for a system undergoing overall isotropic motion as well as internal conformational fluctuation between two stable states is presented. Dependence of T1 on the lifetimes of the two conformations, the angle defined by the particular C-H vector and the effective axis about which it rotates, and the range through which it rotates are considered. Relaxation data for proline and proline-containing peptides have been analyzed on the basis of several models for proline puckering: interconversion between two envelope forms with C/sup γ/ puckered, interconversion between two C/sup β/--C/sup γ/ half-chair forms, and an intermediate case. Consideration of the 13C T1 values for each proline carbon indicates that for most cases the two stable conformational states correspond to approximate C/sup β/--C/sup γ/ half-chairs with C/sup γ/ exhibiting a somewhat greater angular displacement than C/sup β/. Reasonable values for the NT1/sup γ//NT1/sup α/ ratio can be obtained by requiring roughly equal lifetimes approximately 10-11--10-12 s for the two states and a typical range of motion for the C/sup γ/--H vectors approximately 50 to 700. These results appear to be consistent with available theoretical calculations, x-ray data, and conclusions based on 1H--1H scalar coupling studies