Molecular dynamics study of a lipid bilayer and a polymer liquid

Abstract

We present molecular dynamics simulation of a lipid bilayer of 2 × 10 freely jointed chain molecules of 16 beads each. All beads interact with each other (via a Lennard-Jones potential) and with an aqueous medium. Terminal beads are bound near the bilayer surfaces by a harmonic potential. An algorithm for integrating the equations of motion of a molecular system in the presence of geometrical constraints, such as fixed bond lengths, is described. All results are compared with those obtained for an isotropic polymer liquid with the same Lennard-Jones interaction and density as for the bilayer. Equilibrium properties of the chains such as end-to-end distance, radius of gyration, molecular order parameters and pair correlation functions are computed. Dynamic properties considered include the time autocorrelation functions of various variables, the translational motion of chain beads and the space-time van Hove correlation functions. It is found that compared with the polymer liquid, the molecular motions in the anisotrpic bilayer and its structure are quite different. Such differences arise mainly from the strong correlations existing in the bilayer.