Molecular signatures of pressure-induced phase transitions in a lipid bilayer

Abstract

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 DMPC or $Δ$9-cis-PC. Within the studied range (288-308 K, 1-2000 bar), DMPC undergoes a liquid--gel transition, while $Δ$9-cis-PC remains fluid due to unsaturation. The CHARMM36 force field reproduces experimental boundaries with high fidelity: simulated DMPC transitions deviate by only 5-10 K and 100-300 bar, and $Δ$9-cis-PC exhibits no transition. Hysteresis is modest but most pronounced when starting from low-temperature gels. We identify area per lipid, bilayer thickness, and acyl-chain gauche fraction as sensitive phase markers; among these, the gauche fraction provides the most robust signature. Simulations indicate an interdigitated gel is the equilibrium structure under finite-size conditions. 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. These results underscore the accuracy of modern force fields and highlight how simulations mechanistically complement experimental studies of pressure-regulated membranes.

Figures (16)

• Figure 1: Thermodynamics of DMPC bilayer in water. Volumes have been normalized to the condition with the highest volume among all conditions: 1 bar and 308 K. Values presented here correspond to the running averages computed over 5 ns for all pressures and temperatures conditions simulated.
• Figure 2: Molecular phase transition descriptors. (a) The gauche fraction of the dihedral angles of the acyl chains of the DMPC lipids and the (b) DMPC area per lipid are shown as a function of time. Each panel corresponds to a different simulated temperature (288 K, 293 K, 298 K and 308 K). The simulated pressures at each temperature are shown on the corresponding panels. The values for each replicas are represented by a different line style with replica 1 in solid, replica 2 in dashed, and replica 3 in dotted style. Values presented here correspond to the running averages computed over 20 ns.
• Figure 3: Morphology of DMPC bilayers Molecular rendering of the various DMPC phases and bilayer organizations observed. (a) The crystal-liquid phase, taken here after 1 µs of simulation at 1 bar and 308 K. (b) The gel phase with no major interdigitations, observed after 1 µs of simulation at 750 bar and 288 K. (c) Finally, the interdigitated gel phase, observed in most conditions where the fluid-to-gel transition occurred, here taken after 1 µs of simulation at 2000 bar and 308K. Images rendered with VMD.VMD
• Figure 4: DMPC phase diagram. The experimental phase diagram of DMPC bilayer (a) adapted from Ichimori et al.ICHIMORI_1998_DMPC_phasediagram shows the experimental melting line between gel and fluid (upper most dashed line) and ripple and lamellar gel (lower dashed line). The DMPC phase diagram built from the MD simulations (b) shows the temperature and pressure conditions under which we conducted our molecular dynamics simulations, starting from a liquid phase. After 1 µs of simulation, we obtained either the crystal liquid phase (indicated by circles), or the gel phase (indicated by triangles), or metastable states where some replicas stayed in the crystal liquid phase and some replicas experienced fluid-to-gel phase transition (indicated by hexagons). Points are colored according to the % of interdigitated lipids in the gel phase. The values were calculated from the last 300 ns of simulation. The interdigitation values shown here were calculated as the mean over the replicas. The thick pink line corresponds to the fluid-to-gel transition we can build from the simulations. To facilitate comparison with experimental literature, temperature values are presented in both Celsius and Kelvin.
• Figure 5: Fraction of interdigitated lipids as a function of (P, T). The population of interdigitated lipids in DMPC bilayer at each temperature and pressure condition studied. Percentages presented here were calculated from the distributions of Figure \ref{['fig:DMPC_interdigitations_dist']} as explained in Methods. Data points are the average among three replicas, with error bars corresponding to the SD among these replicas.