QUANTITATION OF LATERAL STRESS IN LIPID LAYER CONTAINING NONBILAYER PHASE PREFERRING LIPIDS BY FREQUENCY-DOMAIN FLUORESCENCE SPECTROSCOPY

Frequency-resolved fluorescence measurements have been performed to quantitate the lateral stress of the lipid layer containing nonbilayer phase preferring dioleoylphosphatidylethanolamine (DOPE). On the basis of a new rotational diffusion model, the wobbling diffusion constant (D(W)), the curvature-related hopping diffusion constant (D(H)), and the two local orientational order parameters ([P2] and [P4]) of 1-palmitoyl-2-[[2-[4-(6-phenyl-trans-1,3,5-hexatrienyl)phenyl]ethyl]carbonyl]-3-sn-phosphatidylcholine (DPH-PC) in fully hydrated DOPE and DOPE/dioleoylphosphatidylcholine (DOPC) mixtures were calculated from the frequency-domain anisotropy data. The values of [P2], [P4], and D(H) for DOPE were found to increase significantly at approximately 12-degrees-C, the known lamellar liquid crystalline (L(alpha)) to inverted hexagonal (H(II)) phase transition temperature of DOPE. Similar features as well as a decline of D(W) were detected in the DOPE/DOPC mixtures as the DOPE content was increased from 85% to 90% at 23-degrees-C, corresponding to the known lyotropic phase transition of the DOPE/DOPC. In contrast, for DOPC (0-40-degrees-C) and DOPE/DOPC (0-100% DOPE at 3-degrees-C), which remained in the L(alpha) phase, these changes were not detected, The most probable local orientation of DPH-PC in the DOPE/DOPC mixtures shifted progressively toward the normal of the lipid/water interface as the content of DOPE increased. We concluded that the curvature-related lateral stress in the lipid layer increases with the content of the nonbilayer phase preferring lipids.