HIGH-FREQUENCY, SPIN-LABEL EPR OF NONAXIAL LIPID ORDERING AND MOTION IN CHOLESTEROL-CONTAINING MEMBRANES

The EPR spectra of spin-labeled lipid chains in fully hydrated bilayer membranes of dimyristoyl phosphatidylcholine containing 40 mol % of c holesterol have been studied in the liquid-ordered phase at a microwav e radiation frequency of 94 GHz, At such high field strengths, the spe ctra should be optimally sensitive to lateral chain ordering that is e xpected in the formation of in-plane domains, The high-field EPR spect ra from random dispersions of the cholesterol-containing membranes dis play very little axial averaging of the nitroxide g-tensor anisotropy for lipids spin labeled toward the carboxyl end of the sn-2 chain (dow n to the 8-C atom). For these positions of labeling, anisotropic N-14- hyperfine splittings are resolved in the g(zz) and g(yy) regions of th e nonaxial EPR spectra. For positions of labeling further down the lip id chain, toward the terminal methyl group, the axial averaging of the spectral features systematically increases and is complete at the 14- C atom position. Concomitantly, the time-averaged [A(zz)] element of t he N-14-hyperfine tensor decreases, indicating that the axial rotation at the terminal methyl end of the chains arises from correlated torsi onal motions about the bonds of the chain backbone, the dynamics of wh ich also give rise to a differential line broadening of the N-14-hyper fine manifolds in the g(zz) region of the spectrum. These results prov ide an indication of the way in which lateral ordering of lipid chains in membranes is induced by cholesterol.