Theoretical Studies of β-Peptide Models

The key conformations of β-dipeptide models 4−9 have been studied with quantum mechanics calculations including a self-consistent isodensity solvation model to evaluate the tendency of β-sheet, 14-helix, and 12-helix formation of β-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 β-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...