Asymmetry and Rippling in Mixed Surfactant Bilayers from All-Atom and Coarse-Grained Simulations: Interdigitation and Per Chain Entropy

Abstract

Understanding lateral organizations of self-assembled bilayers is crucial to gain a control on functionally relevant topologies. We study self-assembled bilayers composed of a surfactant, behenyl trimethyl ammonium chloride (BTMAC), a cosurfactant, stearyl alcohol (SA), at a ratio of 2:1 in the presence of water at 283 K employing subsequent all-atom (AA) and coarse-grained (CG) molecular dynamics simulations. Differences in initial configurations lead to the formation of bilayers at ripple or square phases or interdigitated gel phases of varying trans-leaflet asymmetry. The AA ripple and gel phases are reproduced well at the CG level using bonded potentials from Boltzmann inversion of AA canonical sampling and nonbonded potentials from MARTINI. Inhomogeneous populations of disordered chains with higher per chain configurational entropy and tilt result in rippling stabilized by periodic hydrophobic energy barrier and strong interdigitation. Order parameters of the asymmetric bilayers are sufficiently coupled to the per chain entropies at both levels of resolutions to serve as a reflector of the per chain configurational entropy inaccessible by experiments. Thus, trans-bilayer asymmetry may be a controlling parameter to induce rippling in a bilayer of industrial importance. This work will be useful for future investigation on domain-associated transport and signaling in biomembranes at a low temperature.