Theoretical studies of β-peptide models

The key conformations of beta-dipeptide models 4-9 have been studied with quantum mechanics calculations including a self-consistent isodensity solvation model to evaluate the tendency of beta-sheet, 14-helix, and 12-helix formation of beta-peptide models. The most stable conformation of dipeptide models 5-7 is a formal six-membered-ring (C6) hydrogen-bonded structure, although the hydrogen bond is very weak because of a bad N-H- - -O angle. Many local conformational minima with folded structures are found. This is attributed to internal non-hydrogen-bonded electrostatic (or dipole) interactions. Most interestingly, for dipeptide model 7, the most stable conformation in polar solvent is predicted to correspond to the 14-helix. The conformations for beta-sheet, 14-helix, and 12-helix are much destabilized by electrostatic interactions in the gas phase but significantly benefit from the polar solvent effect. The 12-helix is intrinsically less favorable than the 14-helix. The key difference between 14- and 12-helices is the dihedral angle (mu) about the C-alpha-C-beta bond--the former is about 60 degrees while the latter is about 90 degrees. Comparatively, beta(3)-peptides have greater 14-helical propensity than beta(2)-peptides. The five-membered and six-membered rings in dipeptide models 8 and 9 promote the 12-helix and 14-helix conformations, respectively. Calculations for beta-hexapeptide models 10 and 11 indicate somewhat stronger hydrogen bonding in the 12-helix than in the 14-helix structure.