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

A Monte Carlo simulation of water in a small pore has been modified to trace individual molecules. The locations, orientations, and local density of those that move less than 1 Angstrom during the simulation are shown to depend heavily on the locations of other molecules that are fixed on the sides of the pores. The electric field produces secondary effects, while density, which produces major effects on the average water properties (Green, M. E.; Lu, J. J. Phys. Chem. B 1999, 101, 6512.) is secondary in determining the properties of individual water molecules; in particular, the location of those molecules so immobile as to form an effective block of the pore is little affected by field or density. The electric field does have an effect on the orientation of the immobilized molecules large enough to observe clearly, but it is not a large effect. In this set of simulations, the number of moves per molecule was extended to ensure that the effect was real, and not an artifact of certain molecules starting slowly. Principal results included approximate 4-fold symmetry of the immobilization, corresponding to the location of water molecules fixed to the wall, and with fixed orientation; these molecules were attached to ensure that the pore was not entirely hydrophobic. If the pore were in a protein, one should expect instead of hydrogen bonding water molecules, hydrogen bonding amino acid side chains. Nevertheless, the effect would be expected to be similar, albeit with different symmetry. The role of density was also limited, as the symmetry of the location of the fixed molecules, and their orientation, turned out not to be heavily dependent on density. Instead, the density effects seem to be strongest on the mobile molecules, as can be seen by comparing these results with those from Green and Lu.(1) It appears that the water molecules could well block a pore of appropriate dimensions; a rearrangement of hydrogen bonds, possibly under influence of an electric field, could in turn unblock it.