Molecular Dynamics Simulations of In-Plane Density Fluctuations in Phospholipid Bilayers

Abstract

At room temperature many lipids form the bilayer structure of biological membranes, which resembles liquid crystals. The structural features of the fluid lipid bilayer are well-known, but many details of the dynamic behavior of biological membranes still remain to be understood.Molecular motions in liquids can be monitored by the intermediate scattering function, F(q,t). In membranes it probes the propagation and decay of in-plane density flutuations at wave vector q. An attractive property of the intermediate scattering function is that it can equally well be determined from scattering experiments as from molecular dynamics simulations, opening the possibility of direct comparison between experimental and simulation data. Density fluctuations in lipid bilayers can stay correlated for hundreds of nanoseconds which implies that in contrast to simple liquids, an exponential decay of F(q,t) as suggested by a purely single diffusive model, does not describe how fluid membranes behave. Microsecond molecular dynamics simulations on thousands of lipids, both atomistic and coarse-grained, has been used to explore F(q,t). The atomistic simulations span membrane patches of tens of nanometers while the coarse-grained simulations, including almost 100 000 lipids, reach the low micrometer domain. The main purpose was to establish a dispersion relation for the density fluctuations, i.e. a relation between the wave vector and the decay rate. Different model functions are compared to find the dispersion relation that best fits the simulation data. The important question of how material properties (bending modulus and area compressibility) are related to the molecular structure of the complex liquid is adressed in the context of the different models.