Structural characterization of alpha-helices of implicitly solvated poly-alanine

The structural characteristics of alpha-helices in poly-alanine-based peptides have been investigated via molecular dynamics simulation with the goal of understanding the basic features of peptide simulations within the context of a model system, classical molecular dynamics with generalized Born (GB) solvation, and to shed insight into the formation and stabilization of alpha-helices in short peptides. The effects of peptide length, terminal charges, proline substitution, and temperature on the alpha-helical secondary structure have been studied. The simulations have shown that distinct secondary structure begins to develop in peptides with lengths approaching 10 residues while ambiguous structures occur in shorter peptides. The helical content of peptides with lengths > or =10 amino acids is observed to be nearly constant up to (Ala)(40). Interestingly, terminal charges and proline in the second position from the N-terminus alter the secondary structure locally with little effect on the overall alpha-helical content of the peptide. The free energy profile of helix formation was also investigated. A large increase in free energy accompanying the formation of helices with more than two consecutive hydrogen bonds in the (i, i + 4) pattern was observed while the free energy increases linearly with additional hydrogen bonds. Values for the change in enthalpy and entropy of helix nucleation and propagation are reported. Additionally the results obtained from the GB model are compared to explicit solvent simulations of two synthetic alanine-based peptides.