Applicability of a continuum solvation model to the octanol water transfer: CFF91 based model for amino acids

A continuum hydration model based upon the atomic charges provided with the CFF91 force field [A. B. Schmidt and R. M. Fine (1994) Molecular Simulation, 13, 347-365] has been extended to the octanol-water transfer. The electrostatic component of the transfer free energy is calculated using the finite-difference solution to the Poisson-Boltzmann equation while the nonpolar contributions are assumed to be proportional to the solute-excluded volume in water. All atomic charges and radii besides the aromatic carbon radius are equal in both solvents. The octanol dielectric constant and the probe radius are the main fitting parameters defining the octanol phase. The model has been tested for 38 organic molecules related to the amino acid residues and generally provides a high accuracy. In particular, the mean unsigned error for N-acetyl amino acid amides is 0.5 kcal/mol.