CORRELATION-ENERGY, THERMAL-ENERGY, AND ENTROPY EFFECTS IN STABILIZING DIFFERENT SECONDARY STRUCTURES OF PEPTIDES

Quantum chemical calculations on some typical elements of secondary st ructure in peptides and proteins (beta sheets, beta and gamma turns) a t the Hartree-Fock and MP2 correlation energy levels show considerable differences in the stability orders of alternative structures. The co rrelation energy data indicate an overestimation of hydrogen-bonded st ructures. Thus, correlation energy data may be misleading when compari ng peptide structures of different type, as for instance, conformation s with and without hydrogen bonds or with a different number of hydrog en bonds. This effect is corrected at the Gibbs free energy level when including thermal energy and entropy contributions. Considerable comp ensation of correlation energy and entropy contributions is mainly res ponsible for the relatively good correspondence of Hartree-Fock energy differences obtained with more extended basis sets and the free entha lpy data at the correlation energy level.