Fluid-gel coexistence in lipid membranes under differential stress

A widely conserved property of many biological lipid bilayers is their asymmetry. In addition to having distinct com-positions on its two sides, a membrane can also exhibit different tensions in its two leaflets, a state known as differential stress. Here, we examine how this stress can influence the phase behavior of the constituent lipid monolayers of a single-component membrane. For temperatures sufficiently close to, but still above, the main transition, molecular dynamics simulations show the emergence of finite gel domains within the compressed leaflet. We describe the thermodynamics of this phenomenon by adding two empirical single-leaflet free energies for the fluid-gel transition, each evaluated at its respective asymmetry-dependent lipid density. Finite size effects arising in simulation are included in the theory through a geometry-dependent interfacial term. Our model reproduces the phase coexistence observed in simulation. It could therefore be used to connect the hidden variableof differential stress to experimentally observable properties of the main phase transition. These ideas could be generalized to any first-order bilayer phase transition in the presence of asymmetry, including liquid-ordered/liquid-disordered phase separation.

Automated summary

A widely conserved property of biological lipid bilayers is their asymmetry, where the two sides of the membrane have distinct compositions and can exhibit different tensions. Molecular dynamics simulations show that at temperatures close to the main transition, finite gel domains emerge within the compressed leaflet. By introducing empirical single-leaflet free energies and considering finite size effects in the simulations, our model reproduces the observed phase coexistence. This model can connect the hidden variable of differential stress to experimentally observable properties of the main phase transition and can be applied to other bilayer phase transitions with asymmetry.