Theoretical study of beta(2,3)-peptide models

Conformational features of alpha,beta-disubstituted beta(2,3)-dipeptide mod els have been studied with quantum mechanics method. Geometries were optimi zed with the HF/6-31G** method, and energies were evaluated with the B3LYP/ 6-31G** method. Solvent effect was evaluated with the SCIPCM method. For (2 S,3S)-beta(2,3)-dipeptide model 1, a six-membered-ring hydrogen bonded stru cture is most stable. However, the conformation corresponding to the format ion of the 14-helix is only about 1.7 kcal/mol less stable in methanol solu tion, indicating that the 14-helix is favored if a (2S,3S)-beta(2,3)-polype ptide contains more than 5 residues. On the other hand, the conformation co rresponding to the formation of beta-sheet is most stable for (2R,3S)-beta( 2,3)-dipeptide model 2, suggesting that this type of beta-peptides is intri nsically favored for the formation of P-sheet secondary structure.