Multiple Conformations and Conformational Change in the Acetonitrile of Cation-binding Cyclic Tetrapeptide, Cyclo[Gly–l-Cys(Bzl(OMe))–Sar–l-Pro

Abstract

Abstract The conformations of the cyclic tetrapeptide, cyclo[Gly–l-Cys(Bzl(OMe))–Sar–l-Pro] (CGCSP), in solution, were studied by means of 1H-, 13C-nuclear magnetic resonance (NMR), and circular dichroism (CD). The CGCSP equilibrates among three conformers, M, m, and n, in acetonitrile, providing three sets of resonances for each proton in the 1H-NMR spectrum. An inspection of a CPK model indicated that the three structures were sterically allowed to occur by the cis-trans isomerism around the Cys-Sar and Sar-Pro peptide bond. The temperature dependence of the amide proton resonances in 1H-NMR suggested that one γ-turn (1←3 hydrogen bond from the Cys to the Pro) is present in the two conformers, M and m. Thus, the three conformations in acetonitrile were deduced from all of the spectral data. This cyclic tetrapeptide is capable of binding a monovalent cation, Li+, and divalent cations, Mg2+, Ca2+, and Ba2+. Its CD titration curves demonstrated that at least three types of complexes with Ca2+ and Ba2+ were formed. In contrast, Li+ and Mg2+ were shown to form the 1 : 1 complex with this pep tide. The NMR data suggest that the conformational change of the cyclic backbone takes place by changing the geometry of the peptide bonds around the Cys–Sar–Pro sequence on the addition of a metal cation.