Theoretical study of peptides formed by aminoxy acids

Quantum mechanics methods have been applied to study the conformational features of peptides formed by aminoxy acids. Geometry and vibration frequencies were calculated by the HF/6-31G** method. Energy was further evaluated using the MP2/6-31G** or B3LYP/6-31G* calculation. Solvent effect was modeled by the self-consistent isosurface polarization continuum model with the HF/6-31G** method. There is a significant preference for the formation of an eight-membered-ring hydrogen bond between adjacent amino acid residues, which resembles a γ-turn. The rotation direction of the C8 structure is determined by the chirality of the Cα center and is independent upon the size of the alkyl side chain. There is a cooperative effect for the formation of adjacent C8 structures, which promotes the formation of helix. Thus, a homo (S)-oxa-polypeptide forms a right-handed 1.8₈-helix, with each turn of the helix containing about 1.8 units of aminoxy acids.