Simulation studies of high-field EPR spectra of spin-labeled lipids in membranes

The high-field (i.e., 94 GHz) membrane EPR spectra of lipids spin labeled i n their fatty acid chains have been simulated by using two limiting motiona l models. The aim was to identify the dynamic origin of the residual (g(xx) - g(yy)) anisotropy observed in the nonaxial EPR spectra of cholesterol-co ntaining membranes. It is concluded that the residual spectral anisotropy a rises from inplane ordering of the lipid chains by cholesterol. The partial averaging of the (g(xx) - g(yy)) anisotropy was best described by restrict ed axial rotation with a frequency in the region of tau (-1)(R parallel to) similar to 0.5-1 x 10(9) s(-1). Simulations for slower axial rotation of u nrestricted amplitude produced less satisfactory fits. In phospholipid memb ranes not containing cholesterol, the nonaxial anisotropy is completely ave raged in the fluid phase and substantially reduced even in the gel phase. T he unrestricted axial rotation in the gel phase is of comparable frequency to that of the limited axial rotation in the liquid-ordered phase of membra nes containing cholesterol. These results on in-plane ordering by cholester ol in the liquid-ordered phase could be significant for current proposals r egarding domain formation in cellular membranes. (C) 2000 academic Press.