Simulation study of a gramicidin/lipid bilayer system in excess water and lipid. II. Rates and mechanisms of water transport

A gramicidin channel in a fluid phase DMPC bilayer with excess lipid and wa ter has been simulated. By use of the formal correspondence between diffusi on and random walk, a permeability for water through the channel was calcul ated, and was found to agree closely with the experimental results of Rosen berg and Finkelstein (Rosenberg, P. A., and A. Finkelsiein. 1978. J. Gen. P hysioi. 72:327-340; 341-350) for permeation of water through gramicidin in a phospholipid membrane. By using fluctuation analysis, components of resis tance to permeation were computed for movement through the channel interior , for the transition step at the channel mouth where the water molecule sol vation environment changes, and for the process of diffusion up to the chan nel mouth. The majority of the resistance to permeation appears to occur in the transition step at the channel mouth. A significant amount is also due to structurally based free energy barriers within the channel. Only small amounts are due to local friction within the channel or to diffusive resist ance for approaching the channel mouth.