Enthalpy Drives Phase Separation in Model Membranes: Evidence from Molecular Dynamics Simulations of Non-Lipid Amphiphiles Vitamin-E, triton-X, and Benzyl Alcohol

Abstract

Phase separation of lipid bilayers into liquid-ordered and liquid-disordered regions facilitates compartmentalization of membrane proteins, an important prerequisite for cellular biochemical signaling. In this study, we investigated the phase behavior of binary and ternary lipid mixtures under the influence of three non-lipid amphiphiles, Vitamin-E, Triton-X 100, and benzyl alcohol. Molecular dynamics (MD) simulations of these additives in fluid-phase lipid bilayers were performed to investigate their effect on membrane thickness and fluidity as readouts of enthalpic and entropic changes, respectively. Simulation results indicate that Vitamin-E increases bilayer thickness and lowers the fluidity, while Triton-X and benzyl alcohol each decrease bilayer thickness and increase fluidity. Experimentally, in both binary and ternary lipid mixtures, Vitamin-E induced ideal mixing of the lipids whereas Triton-X and benzyl alcohol either resulted in sustained phase separation or induced large-scale phase separation. Because changes in bilayer thickness determined by MD correlate better than fluidity with phase separation observed in model membranes we conclude that hydrophobic mismatch of different lipid types (enthalpic effect) dominate the effect of fluidity (entropic effect) as the key driving force for the phase separation behavior observed in lipid bilayers. Moreover, these non-lipid amphiphiles provide new tools to tune raft formation/disruption and to study raft-related cell membrane functions through modulation of non-raft membrane thickness.