Molecular Signatures of Pressure-Induced Phase Transitions in a Lipid Bilayer.

Understanding how lipid bilayers respond to pressure is essential for interpreting the coupling between membrane proteins and their native environments. Here, we use all-atom molecular dynamics to examine the pressure-temperature behavior of model membranes composed of dimyristoylphosphatidylcholine (DMPC) or its cis-unsaturated analogue Δ9-cis-PC. Within the studied range (288-308 K, 1-2000 bar), DMPC undergoes a liquid-gel transition, while Δ9-cis-PC remains fluid due to unsaturation. The CHARMM36 force field reproduces experimental boundaries with high fidelity: simulated DMPC transitions fall within 5-10 K and 100-300 bar of experimental values, and Δ9-cis-PC exhibits no transition. Hysteresis is modest but most pronounced when starting from low-temperature gels; we propose a split-phase simulation protocol that alleviates the hysteresis problem. We identify the area per lipid, bilayer thickness, and acyl-chain gauche fractions as sensitive phase markers; among these, the gauche fraction provides the most robust signature. Simulations indicate that an interdigitated gel is the equilibrium structure under finite-size conditions, and we propose a novel metric to quantify the extent of this phenomenon. However, at low temperature and high pressure, interdigitation decreases, consistent with the experimental lamellar gel phase. This long-lived interdigitation critically impacts standard order parameters, specifically, area per lipid and membrane thickness. Finally, we discuss in detail how finite-size effects influence phase transition and interdigitation. Overall, these results underscore the accuracy of modern force fields and highlight how simulations are essential to mechanistically complement experimental studies of pressure-sensitive membranes.