Simulation of water in a small pore: Effect of electric field and density

The behavior of water in a pore of nanometer dimensions is studied by Monte Carlo simulation over a series of densities, and with five different electric charge configurations, providing external fields from zero to extremely high values, exceeding 3 x 10(9) V m(-1). The pore contained a tapered section that had an opening of radius 0.25 nm into a secondary tapered section below. The pore wall was a medium of dielectric constant 4 (comparable to the value in a protein), and the electric charges were placed in the wall. A reservoir, with which the remainder of the volume could exchange water molecules, was kept at constant density. Quantities that were obtained included the energy of the system, the orientation of the water molecules in the tapered section of the pore (and the remainder of the volume, but that proved to show little orientation), the density in response to the density of the reservoir section, molecular distributions, and electric potential and field. We observed that a high field, as expected, lowered the energy of the water molecules, the density of the pore responded to the density of the reservoir, and the orientation of the molecules in the tapered section responded to the field of the fixed charges in the wall. The larger fields pulled molecules close to the wall, on average. The differences in the behavior of the tapered section and the cylindrical section are particularly interesting: small changes in geometry produce significant changes in water structure and apparent rigidity as shown by the high average orientation. To the extent that the pore can be thought of as a model for a protein channel, it suggests that small changes in an amino acid side chain, whether by mutation, proton transfer, or simply reorientation, could have major consequences for the function of the protein; this includes geometric effects, as well as effects upon the electric field, and through the field, on the water in the pore.