A Theoretically Improved Perturbation Model for Activity Coefficients of Amino Acids and Peptides in Aqueous Solutions

A theoretically improved model, based on the perturbation theory, to correlate the activity coefficients of amino acids and peptides in aqueous solutions is presented. The radial distribution function for the reference system and for the perturbation term have been obtained from the solution of the Percus−Yevick equation for hard sphere systems. The pair interaction energies required by the model are those due to dispersion forces and dipole−dipole interactions. These interactions are represented by a Lennard−Jones (6−12) and a Keesom expression, respectively. The depth of potential well in the Lennard−Jones model and the size parameter of hard spheres were evaluated by the minimization of the Helmholtz free energy of the systems. The significance of the parameters is justified by comparison of the model with the Gibbs−Bogoliubov inequality. Independently evaluated dipole moments of the amino acids are used. The model can accurately correlate the activity coefficients of amino acids and peptides in aqueous solutions over a wide range of concentration.