Workshop on Treatment of Electrostatic Interactions in Computer Simulations of Condensed Media

Santa Fe, June 23-25, 1999

Electrostatic Properties of Interfaces between Water and Hexane, Octanol and Membranes Probed by Small Solutes

Andrew Pohorille, Karl Schweighofer and Michael A. Wilson

Biomolecular and Cellular Modeling Program, NASA-Ames Research Center and Department of Pharmaceutical Chemistry, University of California, San Francisco

Interfaces between water, and nonpolar or amphiphilic media play an important role in chemistry and biology. They mediate such diverse phenomena as heterogeneous catalysis, uptake of pollutants by water droplets, folding of membrane-bound proteins and peptides, and, possibly, the phenomenon of anesthesia. The interfaces are electrostatically complex environments due to rapid density changes and anisotropic orientational distributions of interfacial molecules which, in turn, generate large, local electric fields. These complications considerably limit applications of continuum dielectric approaches to interfacial phenomena since, in these approaches, an interface is somewhat arbitrarily represented by either a sharp dielectric discontinuity or a continuously and monotonically changing dielectric constant. To determine how interfaces can be best approximated in continuum theories, it is first necessary to characterize them accurately using molecularly detailed computer simulation methods. To this end, we present results of molecular dynamics simulations of interactions between four different interfaces and several small, spherical or nearly spherical solutes with differing polarities. Interfaces between water and two bulk solvents, hexane and octanol, and two bilayer-forming lipids, glycerol 1-monooleate and 1-palmitoyl 2-oleoyl sn-glycero 3-phosphocholine (POPC), have been considered. The solutes we investigated include the noble gases and methane and its four fluorinated derivatives. For each of these molecules, the excess chemical potential was calculated as a function of the coordinate perpendicular to the interface using the Widom particle insertion method. This method yields highly accurate results for a given choice of intermolecular potentials.

The results reveal both similarities and differences in the behaviors of the solutes at different interfaces. For nonpolar solutes, the chemical potential is dominated by the reversible work required to create cavities that are large enough to accommodate the solute molecules. This work is greater in water than in nonpolar phases (e.g. hexane or the interior of a bilayer). At the interface between weakly interacting liquids (e.g. water and hexane), the chemical potential exhibits a weak, local minimum. In contrast, it has a maximum at the densely packed interfaces between water and phospholipid head groups. In both cases, however, the solutes tend to accumulate in the nonpolar phase. For polar solutes, such as the partially fluorinated methanes, the chemical potential is a balance between nonelectrostatic (cavity formation) and electrostatic terms. In all cases, the electrostatic contributions change oppositely to nonelectrostatic terms. They are more favorable in water than in nonpolar environments. Furthermore, at the water-POPC interface, the electrostatic term has a minimum in the highly polar POPC head group region. As a result of the balance between electrostatic and nonelectrostatic contributions, the chemical potential of small, polar solutes exhibits a minimum at the water-hexane interface whereas, in the water-POPC system, it has a maximum at the interface between water and phospholipid head groups and a minimum on the hydrocarbon side of head groups.The situation is even more complicated at the water-octanol interface due to ordering of octanol molecules extending beyond the first interfacial layer.