Determination of proton transfer energies and lattice energies of several amino acid zwitterions

The gas-phase proton transfer energies of several amino acids, Gly, Ala, Val, Ser, Thr, Cys, δ-His, ∈-His, and 4-OH-Pro were calculated with ab initio 6-31G** optimized geometries. For accurate descriptions of electrostatic interaction energies and the transferability of point charges from one molecule to another, a method for calculating empirical point charges, the Mulliken-Population-Constrained-Potential-Derived (MPCPD) method is proposed. The MPCPD point charges accurately reproduce ab initio electrostatic potentials and have good transferability. Lattice energies of molecular crystals of amino acids in the zwitterionic form were calculated from the proton transfer energies and heats of sublimation data. The interaction energies other than electrostatic energy which contribute to the lattice energies were also estimated using the electrostatic energies calculated from several point-charge sets.