Peptide models XXIII.: Conformational model for polar side-chain containing amino acid residues:: A comprehensive analysis of RHF, DFT, and MP2 properties of HCO-L-SER-NH2

Geometric and energetic properties of a diamide of serine, HCO-NH-L-CH(CH2OH)CO-NH2, are investigated by standard methods of computational quantum chemistry. Similarly to other amino acid residues, conformational properties of HCO-L-Ser-NH2 can be derived from the analysis of its E = E(phi, psi; chi(1), chi(2)) hypersurface. Reoptimization of 44 RHF/3-21G conformers at the RHF/6-311++G** level resulted in 36 minima. For all conformers, geometrical properties, including variation of H-bond parameters and structural shifts in the torsional space, are thoroughly investigated. Results from further single-point energy calculations at the RHF, DFT, and MP2 levels, performed on the entire conformational data set, form a database of 224 energy values, perhaps the largest set calculated so far for any single amino acid diamide. A comprehensive analysis of this database reveals significant correlation among energies obtained at six levels of ab initio theory. Regression parameters provide an opportunity for extrapolation in order to predict the energy of a conformer at a high level by doing explicit ab initio computations only for a few selected conformers. The computed conformational and relative energy data are compared with structural and occurrence results derived from a nonhomologous protein database incorporating 1135 proteins. (C) 2000 John Wiley & Sons, Inc.