Stability order of basic peptide conformations reflected by density functional theory

Ab initio density functional methods at the B3LYP/6-311+G(d, p) and 6-31G(d) levels were performed on several basic peptide conformations representing typical elements of secondary structure (beta-sheets, beta- and gamma-turns). The results are compared with those from Hartree-Fock and MP2 correlation energy calculations. Whereas the geometries of the structures are well described at all approximation levels, there are considerable discrepancies of the stability orders. Contrary to the Hartree-Fock calculations, the correlation energy methods provide the more compact structures with intramolecular hydrogen bonds distinctly favoured over extended conformations when comparing the energy differences. However, due to considerable compensation of correlation energy and entropy contributions, the stability order at the Gibbs free energy level closely corresponds to that at the Hartree-Fock level.