Electronic structure and bonding of the amino acids containing first row atoms.

The electronic structures of the amino acids containing first row atoms have been determined for the zwitterionic form using an approximate self-consistent field method, partial retention of diatomic differential overlap. Various energetic quantities including certain proton affinities are present as are eigenvalues for the highest occupied and lowest unoccupied molecular orbitals. It is found that our method, in common with all methods employing minimum basis sets, yields eigenvalues for the highest occupied molecular orbital that are too high. The method does predict the location of this orbital correctly when compared to calculations employing larger basis sets. It is predicted that electron loss due to ionizing radiation should occur from the carboxylate group for the nonaromatic amino acids, while for tyrosine and tryptophan, electron loss should occur from the ring system. No choice between these two sites can be made for phenylalanine. Charge distributions have been obtained which show that only partial zwitterionic character is found in the backbone and that little delocalization of charge from the backbone to the side chain occurs. Localized molecular orbitals have been obtained using the Boys criteria and the bonding in the amino acids is disscussed in terms of these orbitals. Hybridization of various bonds and bond polarities are discussed as is the phenomenon of fractional bonding to carbon.