Peptide models XXIII. Conformational model for polar side-chain containingamino 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(CH2O H)CO-NH2, are investigated by standard methods of computational quantum che mistry. Similarly to other amino acid residues, conformational properties o f 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, geometrica l properties, including variation of H-bond parameters and structural shift s in the torsional space, are thoroughly investigated. Results from further single-point energy calculations at the RHF, DFT, and MP2 levels, performe d on the entire conformational data set, form a database of 224 energy valu es, perhaps the largest set calculated so far for any single amino acid dia mide. A comprehensive analysis of this database reveals significant correla tion 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 computat ions only for a few selected conformers. The computed conformational and re lative energy data are compared with structural and occurrence results deri ved from a nonhomologous protein database incorporating 1135 proteins. (C) 2000 John Wiley & Sons, Inc.