A new theoretical approach to simulation of EPR spectra of spin-labels in protein and membrane systems

Abstract

A certain model of nitroxyl radial movement is used to interpret EPR spectra of spin-labeled products. In the model proposed a fast reorientation of a spin label occurred due to the order parameter, and a slow motion of the carrier of a spin label, already with partially averaged magnetic parameters, is determined by the rotational correlation time of the macromolecule. By the example of different proteins we demonstrate how to get a fairly good coincidence of simulated EPR spectra with experimental ones within that simple model in solution and using an adsorbent. It is minimalistic in sense of involved motion parameters, and thus attempts to minimize the ambiguity of spectra interpretation. We show that TEMPO radical in lipid phase obeys two-motion model, undergoing fast anisotropic reorientation due to being incorporated into axial phase, and, is additionally affected by slow stochastic process characterized by effective nanosecond-scale correlation time, which can be attributed to domain structure of the membrane.