Mammalian oxytocin and its derivatives are a series of polypeptides containing S—S bonds. By utilizing ABEEMσπ model (Atom Bond Electronegativity Equalization Method at σπ level), their properties involving the intra-molecular hydrogen bond (HB) and the global softness are investigated. Firstly, the energies of intra-molecular HB are calculated under the point charge model, which are basically consistent with the experimental and theoretical results. Then, the relationship of molecular bioactivities of mammalian oxytocin and its derivatives is successfully interpreted, according to the fact that the larger global softness corresponds to the stronger bioactivities. In the present work, ABEEMσπ model is tentatively applied to study the related properties of polypeptides with S—S bonds. The results are in basic accordance with the previous experimental and theoretical investigations. Therefore, these studies provide a foundation for extending the ABEEMσπ model to investigate the similar properties of other polypeptides.

Key words: ABEEMσπ model, oxytocin, derivatives, hydrogen bond, global softness, bioactivity