MICROWAVE FREQUENCY-DEPENDENCE OF ESR-SPECTRA FROM SPIN LABELS UNDERGOING 2-SITE EXCHANGE IN MYELIN PROTEOLIPID MEMBRANES

Measurement at two microwave frequencies allows the unambiguous assign ment of two-component spin-label ESR spectra such as are observed freq uently from biological membranes and reconstituted protein-lipid compl exes. Consistent spectral subtractions were obtained with 9 and 34 GHz ESR spectra of spin-labeled lipids from lipid-protein complexes for t wo 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 incl uding linear dispersion distortions, yielded consistent rates of lipid exchange at the protein interface and reflected the different lipid s electivities for the two proteins. The present data at two microwave f requencies leave little doubt that the spin-label ESR spectra from the se myelin protein-lipid complexes consist of two components in slow ex change. (C) 1994 Academic Press, Inc.