Nitroxide Spin−Relaxation over the Entire Motional Range

Homogeneous line widths, characterized by the phase memory time, T m , of a nitroxide spin-labeled peptide, were measured over the entire motional range (∼10 - 1 0 -10 - 4 s at the spectrometer resonance frequency of 9.7 GHz) using spin-echo correlated spectroscopy (SECSY) ESR. Phase memory times as short as 20 ns could be determined by using schemes designed to minimize the effects of pulse ring-down. Experimental T m versus temperature curve (from 192 to 310 K) clearly resolves the T m minimum, which provides great sensitivity to the details of molecular dynamics.