Spin-label HF-EPR of lipid ordering in cholesterol-containing membranes

Conventional oxazolidine spin-labelled lipids have the axial N-14-hyperfine tensor z-axis directed along the long axis of the lipid chain. Investigation of lateral ordering of the lipids in membranes requires measurement of the x-y Zeeman anisotropy of the nonaxial g-tensor at high fields. Both the lateral and transverse ordering of the lipid chains in membranes of dimyristoyl phosphatidylcholine containing 40 mol% cholesterol in the liquid-ordered phase have been studied with 94 GHz electron paramagnetic resonance spectroscopy. This has been done by using probe amounts of phosphatidylcholine systematically spin-labelled at positions n along the length of the sn-2 chain [n-PCSL, l-acyl-2-(n-(4,4-dimethyloxazolidine-N-oxyl) stearoyl)-sn-glycero-3-phosphocholine]. Nonaxial (g(xx)-g(yy)) anisotropy of the spin-labelled lipid chains is detected over a wide range of lemperature throughout the liquid-ordered phase. The transverse profile of lateral ordering with position, n, of chain labelling follows the profile of the rigid steroid nucleus of cholesterol. It becomes progressively averaged towards the terminal methyl group of the sn-2 chain, in the region of the flexible hydrocarbon chain of cholesterol. The nonaxial lipid ordering may be related to lipid domain formation in membranes containing cholesterol and saturated-chain lipids.