OSMOTIC PERMEABILITY IN A MOLECULAR-DYNAMICS SIMULATION OF WATER TRANSPORT THROUGH A SINGLE-OCCUPANCY PORE

The aim of this work is to determine plausible values for the rate con stants of kinetic models representing water transport through narrow p ores. We present here the results of molecular dynamics simulations of the movement of water molecules through a single-site hydrophilic por e. The system consists of a rectangular box of water molecules, some o f which are positionally restrained so as to act as a membrane. This m embrane separates two compartments where water molecules move freely; one of the positions in the membrane is initially vacant( the 'single- site pore'), but can be occupied by mobile molecules. To analyze the r esults, we represented the pore by a two-state kinetic diagram in whic h the vacant and occupied states are linked by transitions correspondi ng to the binding and release of water molecules. The mean occupancy a nd vacancy times directly yield the rate constants of binding and rele ase, which in turn yield the osmotic water permeability coefficient pe r pore p(f). We also compute the apparent activation energies Delta E for the rate constants and for p(f). The p(f) value was (1.56 +/- 0.0 4). 10(-11) cm(3)/s (at 307 K), which is much larger than those determ ined for CHIP28 and for gramicidin A (of about 10(-13) and 10(-14) res pectively). These values were compared with those arising from a model of a symmetric single-file pore through which one-vacancy-mediated wa ter transport takes place. The model yields an expression for p, as a function of the rate constants and of the number of molecular position s (n) in the file. When n = 1, this expression becomes the one corresp onding to the single-site pore studied in our current simulation. Usin g the rate constants of binding and release derived from our simulatio n, the p, values are consistent with an occupancy value of 5-6 found f or gramicidin A, and with occupancies of 4-7 that can be estimated for the single-file pore of a recently proposed model for CHIP28. Delta E for p(f) is 3.0 kcal/mol, a value similar to that determined for CHI P28. Hence, the system simulated here appears plausible and can be use d to mimic some physical properties of water transport through biologi cal pores.