Molecular dynamics simulation study of a two-dimensional fluid mixture system: a model for biological membranes.

Abstract

The computer simulation technique of molecular dynamics was applied to a model two-dimensional fluid mixture system to examine the short-range ordering of lipid and protein molecules in biological membranes. The model system consists of small disks (lipids) and large disks (proteins) with a radius ratio of 6, constrained to move in a plane. The particles interact with pairwise additive repulsive short range potentials, so as to simulate hard disks. Periodic boundary conditions are assumed in order to minimize boundary effects. For values of the number density of the small disks and of the temperature appropriate for a lipid membrane, the fraction, f, of small disks 'next to' at least one large disk was computed by molecular dynamics. This was done as a function of concentration and for several definitions of 'next to'. The molecular dynamics results show that, at moderately low mole fractions of the large disks, the calculated values of f deviate noticeably from the linear relation which would be expected in the absence of protein-protein proximity effects. The results are discussed in terms of current models of lipid-protein ordering in biological membranes.