MONTE-CARLO SIMULATION OF THE EFFECTS OF CHARGES ON WATER AND IONS IN A TAPERED PORE

Monte-Carlo simulations of the water in a model pore have been carried out in the presence of charges, As the original motivation for the work included modeling a biological pore, which is known to taper and to be critical for ion transport into and out of cells, the model is a cylindrically symmetric pore that tapers at one end, having a funnel-like appearance. The biological pore has protein as its wall, and the dielectric constant of the wall was chosen accordingly as 4. The electric field and potential were calculated for the pore and the surrounding wall, The simulation volume held 50 to 60 water molecules, which are represented explicitly in the simulation. The number of molecules passing through the lower opening of the channel, the energy, the orientation of molecules in the lower section, and the number density of these molecules were determined, The number density varied appreciably with placement and number of fixed charges. With four fixed charges just above the bottom of the lowest section, the energy was lower, the molecules in that section became very strongly oriented, molecules did not pass through the bottom of the channel, and number density rose sharply. Charges below the bottom produced a density minimum and allowed water transport, With K+ ions, generally the water behaved in a similar manner, but the location of the K+ ion in the channel had a strong effect on the surrounding water molecules; this may suggest a mechanism for ion transport. (C) 1995 Academic Press, Inc.