Characterization of the stereochemical selectivity of β-hairpin formation by molecular dynamics simulations

The stability of secondary structure motifs found in proteins is influenced by the choice of the configuration of the chiral centers present in the amino acid residues (i.e., D vs L). Experimental studies showed that the structural properties of the tetrapeptide ^LV ^LP ^LA ^LL (all-L) are drastically altered upon mutating the L-proline and the L-alanine by their D-enantiomers [J. Am. Chem. Soc. 1996, 118, 6975]. The all-L diastereomer is unstructured, experiencing little or no β-hairpin formation, while the ^LV ^DP ^DA ^LL peptide exhibits a substantial population of β-hairpin conformation. In this study, we perform molecular dynamics simulations to investigate the folding propensity of these two model peptides. The results confirm the experimental findings, namely, that the presence of D-amino acids in the loop region strongly induces β-hairpin formation (a population increase from about 1.5% to 50% is observed). The major factor determining the different behavior is found to be the large difference in energy between the two diastereomers, approximately 22 kJ/mol, when they adopt a β-hairpin structure. The higher energy observed for the all-L peptide is a consequece of none-ideal hydrogen bond formation and of steric repulsions. The results suggest that selective incorporation of D-amino acids in proteins can be used to enhance certain secondary structure elements. The kinetic behavior of the folding process observed in the simulations is also investigated. We find that the decay rate of the folded structure fits to a biexponential function, suggesting that the folding/unfolding process of a β-hairpin is governed by two different mechansims.