SPATIAL ELECTRON-DISTRIBUTION AND POPULATION ANALYSIS OF AMIDES, CARBOXYLIC-ACID, AND PEPTIDES, AND THEIR RELATION TO EMPIRICAL POTENTIAL FUNCTIONS

The properties of the electron distribution in amides, peptides and carboxylic acids, obtained from ab-initio molecular orbital calculations using both minimal and extended basis sets, were studied. These properties are discussed in terms of some of the common assumptions made in empirical conformational calculations of biomolecules. Population analyses of 15 compounds in these families were carried out with both the minimal and extended basis sets and compared with results of CNDO/2 [complete neglect of differential overlap] calculations. Population analysis is a useful tool for recognizing patterns of charge distributions and investigating the transferability of parameters of different functional groups, but its use for providing partial charges for conformational analysis is a questionable procedure. A more detailed analysis of the charge distribution was carried out by calculating the spatial electron distribution in the 4 compounds, N-methylacetamide, acetic acid, diketopiperazine and N-acetyl-N''-methylalanine. Both total electron-density maps and difference-density maps are presented. The properties of the overall shape of the molecule and the atoms in the molecule are discussed in terms of the former along with 3-dimensional shape plots of the total density. The distortion accompanying molecular formation, resulting in such features as the lone pair orbital and bonding densities is discussed in terms of the difference maps. Semiquantitative estimates of the bonding and orbital densities resulting from the integration of the densities are also presented. One of the novel features of the study is the presentation of 3-dimensional surfaces of constant difference densities from which the shapes of the orbitals and bonding densities emerge.