Ion binding of cyclolinopeptide A: An nmr and CD conformational study

CD and nmr techniques have been used to study, in acetonitrile solution, the ion‐complexing capability of Cyclolinopeptide A (CLA), a cyclic nonapeptide of sequence\documentclass{article}\pagestyle{empty}\begin{document}$$ cyclo{\rm - }\left({{\rm Pro - Pro - Phe - Phe - Leu - Ile - Ile - Leu - Val}} \right) $$\end{document}endowed with remarkable cytoprotective ability in vitro, and the conformation of the Ba 2+ / CLA complex. At room temperature, CLA in acetonitrile shows a proton nmr spectrum characteristic of the coexistence of many different conformers in intermediate exchange. The backbone contains a cis Pro‐Pro bond, with all other peptide bonds in the trans conformation. CLA binds Ba 2+ more tightly than the other cations studied, namely K + , Na + , Mg 2+ , and Ca 2+ ; CD data are indicative of the presence of both 1 : 2 (sandwich) and 1 : 1 (equimolar) type complexes, depending on the Ba 2+ ion concentration, whereas nmr data are consistent with an equimolar form. The relevant conformational features of the equimolar Ba 2+ /CLA complex are that the backbone contains all trans peptide bonds, a type I 6 → 3 β‐turn and a 3 → 1 γ‐turn (or a distorted 3 → 9 β‐turn). The global shape of the complexed peptide can be described as a bowl, with the concave (polar) side hosting Ba 2+ and the convex side predominantly apolar.