MOLECULAR-DYNAMICS OF A WATER-LIPID BILAYER INTERFACE

We present results of molecular dynamics simulations of a glycerol 1-m onooleate bilayer in water. The total length of analyzed trajectories is 5 ns. The calculated width of the bilayer agrees wi:ll with the exp erimentally measured value. The interior of the membrane is in a highl y disordered fluid state. Atomic density profiles, orientational and c onformational distribution functions, and order parameters indicate th at disorder increases toward the center of the bilayer. Analysis of ou t-of-plane thermal fluctuations of the bilayer surfaces occurring at t he time scale of the present calculations reveals that the distributio n of modes agrees with predictions of the capillary wave model. Fluctu ations of both bilayer surfaces are uncorrelated, yielding Gaussian di stribution of instantaneous widths of the membrane. Fluctuations of th e width produce transient thinning defects in the bilayer which occasi onally span almost half of the membrane. The leading mechanism of thes e fluctuations is the orientational and conformational motion of head groups rather than vertical motion of the whole molecules. Water consi derably penetrates the head group region of the bilayer but not its hy drocarbon core. The total net excess dipole moment of the interfacial water points toward the aqueous phase, but the water polarization prof ile is non-monotonic. Both water and head groups significantly contrib ute to the surface potential across the interface. The calculated sign of the surface potential is in agreement with that from experimental measurements, but the value is markedly overestimated. The structural and electrical properties of the water-bilayer system are discussed in relation to membrane functions, in particular transport of ions and n onelectrolytes across membranes.