Exploring the ripple phase of biomembranes

Monte Carlo simulations are used to study the rippled Pβ′ phase in biomembranes conceived as a lamellar stack of lipid bilayers, each represented by a two-dimensional hexagonal lattice, in aqueous environment. The appearance of this intermediate phase between the ordered Lβ′ phase and the fluid Lα phase in the temperature driven gel–fluid transition is attributed to the coexistence of gel–fluid rich domains in the pre-transition phase. The interfacial energy, that plays a crucial role in domain formation, is governed by a delicate balance between the hydrophobic interaction within the hydrocarbon chains and the headgroup repulsion at the interface. The latter gets disturbed by the increase in the cross-sectional area per molecule at increased temperature and hydration. Periodic surface ripples appear to compensate the packing frustration between the headgroups and the hydrocarbon chains. The Cv plots, produced by the simulation work, confirm the presence of the pre-transition phase. A typical configuration of the Pβ′ phase also shows the corrugated surface profile. The essential presence of certain minimum hydration for the appearance of the Pβ′ phase has also been observed.