Water Structure around Enkephalin near a PE Surface: A Molecular Dynamics Study

Abstract

A molecular model was constructed simulating a pentapeptide protein, leucine enkephalin, near a crystalline polyethylene (PE) surface surrounded by explicit water molecules. A molecular dynamic simulation of the model was performed to examine the forces exerted on the protein by the surface and the water molecules. The orientational distribution functions of the water molecules were examined to determine if and how structured or ordered water in the model contributed to the attractive and repulsive forces between the protein and the surface. The results of the simulations showed that there is relatively strong spatial and orientational structuring of the water adjacent to the PE surface, and a lesser amount of structuring near the protein. The distribution is a function of the orientation of the protein with respect to the surface. The proximity of the protein to the surface has a strong influence on the density and structure of water between the protein and surface. A linear correlation between the force of water on the protein and the angular distribution of water molecules was observed in the most dense region of water between the protein and surface. When the protein presents its polar side toward the nonpolar PE surface, the water structure near the protein disrupts the normal structure of water near the surface. Additionally, the orientational structure of the water between them causes an asymmetric force on the protein, pushing it away from the surface. When the nonpolar side of the protein is presented toward the PE, there is less disruption of water structure, and the interaction forces are attractive.