AN ELECTRON-SPIN-RESONANCE STUDY OF INTERACTIONS BETWEEN PHOSPHATIDYLCHOLINE AND PHOSPHATIDYLSERINE IN ORIENTED MEMBRANES

detailed electron spin resonance (ESR) study of mixtures of 1-palmitoy l-2-oleoyl-phosphatidylcholine (POPC) and phosphatidylserine (POPS) in oriented multilayers in the liquid crystalline phase is reported with the purpose of characterizing the effects of headgroup mixing on the structural and dynamical properties of the acyl chains. These studies were performed over a range of blends of POPC and POPS and temperature s, utilizing the spin-labeled lipids 16-phosphatidylcholine and 5-phos phatidylcholine as well as cholestane (CSL). The ESR spectra were anal yzed by nonlinear least-squares fitting using detailed spectral simula tions. Whereas CSL shows almost no variation in ordering and rotationa l dynamics versus mole fraction POPS, (i.e. x(ps)), and 5-PC shows sma ll effects, the weakly ordered end-chain labeled 16-PC shows large rel ative effects, such that the orientational order parameter, S is at a minimum for x(ps) = 0.5 where it is about one-third the value observed for x(ps) = 0 and 1. This is directly reflected in the ESR spectrum a s a substantial variation in the hyperfine splitting with x(ps). The l east-squares analysis also shows a reduction in rotational diffusion c oefficient, R perpendicular to by a factor of 2 for x(ps) = 0.5 and pe rmits the estimation of S-2, the ordering parameter representing devia tions from cylindrically symmetric alignment. These results are contra sted with H-2 NMR studies which were insensitive to effects of mixing headgroups on the acyl chains. The ESR results are consistent with a s omewhat increased disorder in the end-chain region as well as a small amount of chain tilting upon mixing POPC and POPS. They demonstrate th e high sensitivity of ESR to subtle effects in chain ordering and dyna mics.