Simulation of the Electron Spin Resonance Spectra of Spin-Labeled Fatty Acids in Protein-Containing Model Membranes

In this review we show that Electron Spin Resonance (ESR) can be used to investigate the time scale and specificity of lipid-lipid and lipid—protein contacts in membranes. Details are given of the simulation of the spectra obtained with labeled lipids either covalently attached to the boundary of rhodopsin or freely diffusing in rhodopsin containing vesicles. It is shown that the time scale of conventional ESR is idealy suited to the determination of the collision frequencies between lipids and proteins and/or lipids-lipids in membranes. In addition we show that the use of a mixture of 14N and 15N nitroxides allows one to detect unambiguously the collision frequency between unlike molecules such as two different phospholipids or a phospholipid and a labeled protein.