Molecular dynamics of a bilayer membrane

A molecular dynamics simulation was carried out for a bilayer of 2 × 64 decane molecules as a model for a biological membrane. Two dimensional periodic conditions were used and head groups were subjected to a simple mean force interaction. Four runs at two different temperatures and head group densities were executed, each over 80 ps. Molecular order parameters were analysed and compared with experiment. A fluctuating tilt of the molecules occurs with respect to the bilayer normal with a spatial correlation of about 1 nm and a temporal correlation of 8 ps. The molecules do not rotate isotropically around their long axes, and a different behaviour of odd and even numbered bonds is apparent. The density profile shows a dip near the middle of the bilayer and the lateral pressure profile shows a negative region near the head groups compensated by a positive region near the tails. Translational diffusion is inhibited by a long negative tail of the velocity autocorrelation function, but still exceeds experimental values. The results are discussed in terms of implications for membrane structure. Results confirm earlier simulations on a smaller model (J. chem. Phys., 1982, 76, 3271), except for the cooperativity of molecular tilt.