Microwave frequency dependence of ESR spectra from spin labels undergoing two-site exchange in myelin proteolipid membranes.

Measurement at two microwave frequencies allows the unambiguous assignment of two-component spin-label ESR spectra such as are observed frequently from biological membranes and reconstituted protein-lipid complexes. Consistent spectral subtractions were obtained with 9 and 34 GHz ESR spectra of spin-labeled lipids from lipid-protein complexes for two related myelin proteins, and the 34 GHz difference spectra further showed restriction of axial lipid rotation at the hydrophobic protein surface. Extension of lineshape simulations with the exchange-coupled Bloch equations to 34 GHz, by allowing for nonaxial g tensors and including linear dispersion distortions, yielded consistent rates of lipid exchange at the protein interface and reflected the different lipid selectivities for the two proteins. The present data at two microwave frequencies leave little doubt that the spin-label ESR spectra from these myelin protein-lipid complexes consist of two components in slow exchange.