Computer simulations of the electron spin resonance spectra of steroid and fatty acid nitroxide probes in bilayer systems

Monte Carlo dynamics (MCD) techniques are used to simulate the orientational behavior and rotational motion of probe molecules in lipid bilayers. The trajectories of molecular orientations generated from the simulations are then used to calculate the order parameters and the orientational time correlation functions. The behavior of the time correlation functions is compared with the predictions of the rotational diffusion (RDM) and the compound motion (CM) models. The MCD trajectories are also used to produce electron-spin resonance (ESR) spectra, employing a recently developed time-domain algorithm. Two questions which have been the subject of debate in the literature are addressed. The first question concerns the discrepancy between the ability of motional models to describe ESR spectra and fluorescence depolarization measurements on rigid molecules in vesicles—while the RDM does an excellent job of fitting the former, the latter require the CM to describe them properly. It is argued that the key to resolving this lies in the fact that the ESR line shapes are sensitive to the tumbling motions of the long molecular axes as well as to rotational motions about them, while fluorescence anisotropy is blind for the latter. The rotation about the long molecular axis introduces a fast decay into the correlation functions in a way independent of the tumbling motion of the axis. The second question concerns the fidelity of reporting by fatty acid spin probes in lipid bilayers. It is shown that the motion of the bulky hydrophillic doxyl group does not, in fact, reflect the motion of the chains about it and consequently these spin probes cannot be considered good reporters for these applications.