DYNAMICS AND ORDERING IN MIXED-MODEL MEMBRANES OF DIMYRISTOYLPHOSPHATIDYLCHOLINE AND DIMYRISTOYLPHOSPHATIDYLSERINE - A 250-GHZ ELECTRON-SPIN-RESONANCE STUDY USING CHOLESTANE

We report here on a 250-GHz electron spin resonance (ESR) study of mac roscopically aligned model membranes composed of mixtures of dimyristo ylphosphatidylcholine (DMPC) and dimyristoylphosphatidylserine (DMPS), utilizing the nixtroxide-labeled cholesterol analog cholestane (CSL). Two clearly resolved spectral components, distinct in both their orde ring and dynamics, are resolved. The major component in membranes comp osed mostly of DMPC shows typical characteristics, with the long axis of CSL parallel to the bilayer normal with slow (10(6) less than or eq ual to R less than or equal to 10(7) s(-1)) rotational diffusion rates , as expected for cholesterol. The second component grows in as the mo le fraction of DMPS increases. A detailed analysis shows that CSL sens es a local, strongly biaxial environment. Our results imply that the i nefficient packing between cholesterol and DMPS occurs probably becaus e of the strong interactions between the PS headgroups, which provide the local biaxiality. Such a packing of the headgroups has been predic ted by molecular dynamics simulations but had not been observed experi mentally. The analysis of these spectral components was greatly aided by the excellent orientational resolution provided by the 250-GHz spec tra. This enabled the key qualitative features of this interpretation to be ''read'' off the spectra before the detailed analysis.