MONTE-CARLO CALCULATION OF THE CONFORMATIONAL FREE-ENERGY AND ENTROPY OF A POLYPEPTIDE-CHAIN NEAR A CHARGED SURFACE

The conformational behavior of a molecular model of enkephalin near a charged plane is studied using a Monte Carlo procedure. The variation of the dielectric permittivity near the charged surface is taken into account and the electric image method is used in order to obtain the interaction energy of the molecule with the plane as well as the electrostatic interactions between partial atomic charges. Evaluation of the free energy and entropy of conformation are made as functions of the molecule to plane distance. The molecular chain adopts folded structures near the surface and extended ones far from it, but the folding of the molecule and its attraction or repulsion by the plane depend largely on the charge density of that surface and on the charged ends of the molecule.