Self-organization and phase structure of ternary lipid membranes: A spin label EPR study

Abstract

The phase structure of membranes formed by ternary lipid mixtures, palmitoyl sphingomyelin (PSM)/palmitoyl oleoyl phosphatidylcholine (POPC)/Cholesterol (Chol), the composition of which simulates that of cell membranes, was studied by EPR using spin-labeled phospholipids. It is shown that, in the two-component POPC/Chol membranes and ternary membranes with a low (less than 24%) fraction of PSM, no phase boundaries are observed and the transition from the liquid disordered (ld) phase to the liquid ordered (lo) phase occurs continuously. In contrast, two-component EPR spectra and tie lines are detected in the other two-component systems, PSM/POPC and PSM/Chol, as well as in the ternary systems with a low content of Chol or POPC, respectively. These features are indicative of the coexistence of the gel phase (Lβ) and two liquid phases, ld or lo, respectively. The tie lines are approximately parallel to the Gibbs triangle sides Chol = 0 and POPC = 0, respectively. In the central part of the Gibbs triangle, where a sufficiently fast rotational diffusion of the spin labels indicates the membrane liquid state, we also observed two-component EPR spectra with quite different magnetic anisotropy and tie lines, indicating the coexistence of ld and lo phases. The boundaries are found for all the two-phase coexistence regions. The localization of the three-phase (Lβ, ld, and lo) coexistence region between the two-phase regions (Lβ-ld, Lβ-lo, and ld-lo) was proposed on the basis of an EPR-line shape analysis.