Tigerinins isolated from the skin secretions of the frog Hoplobatrachus tigerinus are cyclic peptides composed of 11-12 amino acids, which could be characterized by a broad spectrum of biological activities. For the tigerinin peptides and their stereoisomeric forms, a comprehensive conformational analysis was performed applying theoretical methods. The results pointed out that for the stereoisomers of tigerinins, the types I and III β-turns appeared in certain tetrapeptide units, while inverse γ-turns were determined in certain tripeptide units. The typical intramolecular H-bonds, such as the i←i+3 and i←i+2 H-bonds, were identified, which stabilized the β-turn and inverse γ-turn structures, respectively. The results led to the conclusion that the occurrence of different turn structures correlated well with the appearance of various intramolecular H-bonds. Besides studying the backbone conformations, for the side-chains of amino acids, the preferred rotamer states were also determined. The comparison of conformational features indicated that the different stereoisomeric forms were characterized by typical structural properties; however, both differences and similarities could be observed for the three-dimensional structure of stereoisomers. On the whole, in the course of our theoretical study, the types and sequential locations of turn structures were accurately determined, and our results indicated that the β-turn structure could be important for the biological activity of tigerinin peptides, described in the literature.
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