Li. Horvath et al., MICROWAVE FREQUENCY-DEPENDENCE OF ESR-SPECTRA FROM SPIN LABELS UNDERGOING 2-SITE EXCHANGE IN MYELIN PROTEOLIPID MEMBRANES, Journal of magnetic resonance. Series B, 105(2), 1994, pp. 120-128
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.