Two-Dimensional Electron Spin Resonance and Slow Motions

Abstract

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 homogeneous relaxation time, T_2, from the overall and/or the internal rotations of the nitroxide is elucidated from the COSY spectra. The use of pure absorption spectra allows the variation of T_2 across the spectrum to be monitored. It is shown from simulations that the model of anisotropic Brownian diffusion provides semiquantitative agreement with such a variation. In the 2D ELDOR experiment several mechanisms can lead to spectral diffusion, which yields a broadening of the hyperfine (hf) auto-peaks with mixing time. We call these spectral diffusion (SD) cross-peaks. It is shown that at higher temperatures the principal mechanism for the formation of SD cross-peaks is the slow reorientation of the molecule, which modulates the ^(14)N hf and g tensor interactions. A procedure is shown for extracting a correlation time, τ_c, by monitoring this growth of SD cross-peaks, which is in good agreement with theory. An anomalous temperature dependence of the experimental τ_c, at very low temperatures, is tentatively attributed to the fast internal rotations of the methyl groups on the nitroxide, which leads to spin-flips of the protons on these methyl groups. The use of pure absorption spectra in 2D ELDOR enhances the sensitivity to these cross-peaks.