Determination of Potential Parameters for Amino Acid Zwitterions

Abstract

An empirical potential energy function for description of the interactions between amino acid zwitterions has been developed. Potential energy surfaces of molecular ion pairs, methylammonium ion−acetate ion, glycine zwitterion−methyl ammonium ion, and glycine zwitterion−acetate ion, were obtained with 6-31G** ab initio molecular orbital calculations. The point charges of the ions were obtained with the modified partial equalization of orbital electronegativity (M-PEOE) method, the coefficients of the attractive part of the nonbonded potential were calculated from charge-dependent effective atomic polarizabilities, and the coefficients of the repulsive part of the nonbonded potential were obtained from the equilibrium conditions of molecules in molecular crystals and by reproducing the lattice energies of amino acid molecular crystals. The hydrogen-bond model proposed by No et al. was introduced, and the potential parameters of the model were determined in order that the potential energy function could reproduce both the ab initio potential energy surfaces and the experimental structures of some amino acid molecular crystals. The minimum-energy crystal structures obtained through crystal packing computations with the empirical potential energy function agreed well with X-ray crystal structures.