CHAIN REVERSALS IN MODEL PEPTIDES - STUDIES OF CYSTINE-CONTAINING CYCLIC-PEPTIDES .3. CONFORMATIONAL FREE-ENERGIES OF CYCLIZATION OF TETRAPEPTIDES OF SEQUENCE AC-CYS-PRO-X-CYS-NHME

Six tetrapeptides of amino acid sequence Ac-Cys-Pro-X-Cys-NHMe, where X = Asn, Gly, Ser, Phe, Val, and Aib, respectively, were synthesized, and the molecular structures of the cyclic forms of two of them (with X = Ser and Val, respectively) were determined by X-ray diffraction. These two cyclic peptides were found to adopt a type I beta-turn conformation centered at Pro-X. The molecular structures of two of the cyclic peptides (with X = Gly and Ser, respectively) were examined by two-dimensional NMR spectroscopy in aqueous solution. The Gly peptide exhibited a type II beta-tum at Pro-Gly, and Ser peptide exhibited a structure similar to the X-ray structure, with evidence of about 10% of a different conformation. The chemical equilibrium between the oxidized (cyclic) and reduced (acyclic) forms of this series of peptides was examined in aqueous solution at 25-degrees-C and pH 8 to obtain quantitative information about the beta-turn-forming potential for each Pro-X pair. The method of disulfide exchange was employed together with automated HPLC for the analysis of the thiol/disulfide equilibrium mixture. The peptide with X = Aib was used as the formal oxidant. These tetrapeptides provide experimental information indicating that residue X leads to a decreasing tendency in the order Asn, Aib, Gly, Ser, Phe, and Val for the Pro-X fragment to form a beta-turn.