2-DIMENSIONAL ELECTRON-SPIN-RESONANCE AND SLOW MOTIONS

The slow rotational dynamics of a polyproline peptide with a nitroxide labeled at one end in a glassy medium is probed using two-dimensional (2D) electron spin resonance (ESR). The contributions to the homogene ous relaxation time, T-2, from the overall and/or the internal rotatio ns of the nitroxide is elucidated from the COSY spectra. The use of pu re absorption spectra allows the variation of T-2 across the spectrum to be monitored. It is shown from simulations that the model of anisot ropic Brownian diffusion provides semiquantitative agreement with such a variation. In the 2D ELDOR experiment several mechanisms can lead t o spectral diffusion, which yields a broadening of the hyperfine (hf) auto-peaks with mixing time. We call these spectral diffusion (SD) cro ss-peaks. It is shown that at higher temperatures the principal mechan ism for the formation of SD cross-peaks is the slow reorientation of t he molecule, which modulates the N-14 hf and g tensor interactions. A procedure is shown for extracting a correlation time, tau(c), by monit oring this growth of SD cross-peaks, which is in good agreement with t heory. An anomalous temperature dependence of the experimental tau(c), at very low temperatures, is tentatively attributed to the fast inter nal rotations of the methyl groups on the nitroxide, which leads to sp in-flips of the protons on these methyl groups. The use of pure absorp tion spectra in 2D ELDOR enhances the sensitivity to these cross-peaks .