Simulation and generalized Langevin equation study of lipid subdiffusion in biomembrane phases

In biomembranes, lipid mobility exhibits deviations from the classical diffusive behavior of Brownian particles, i.e., "anomalous" diffusion. The question arises as to how this anomalous diffusive behavior varies in gel, ripple, and fluid biomembrane phases. In this study, we perform all-atom molecular dynamics simulations of dimyristoylphosphatidylcholine bilayers in the three different phases and analyze the results using the framework of the generalized Langevin equation. This analysis emphasizes subdiffusive behavior on the relatively short, picosecond to nanosecond timescales, capturing local molecular constraints and transient caging effects during the crossover of atomic dynamics from vibrational to incipient anharmonic motion. The ripple and gel phases are found to exhibit strong transient caging and prolonged memory effects resulting in distinct subdiffusive behavior. The role of hydrogen bonding in lipid confinement is also examined, demonstrating its influence on phase-dependent molecular ordering and on short-time diffusional constraints. These findings demonstrate the generalized Langevin equation framework's utility in characterizing molecular transport and lipid dynamics, with implications for longer timescale membrane dynamics.