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 cyclo-(Pro-Pro-Phe-Phe-Leu-Ile-Ile-Leu-Val) 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 Ba2+ more tightly than the other cations studied, namely K+, Na+, Mg2+, and Ca2+; CD data are indicative of the presence of both 1:2 (sandwich) and 1:1 (equimolar) type complexes, depending on the Ba2+ ion concentration, whereas nmr data are consistent with an equimolar form. The relevant conformational features of the equimolar Ba2+/CLA complex are that the backbone contains all trans peptide bonds, a type I 6----3 beta-turn and a 3----1 gamma-turn (or a distorted 3----9 beta-turn). The global shape of the complexed peptide can be described as a bowl, with the concave (polar) side hosting Ba2+ and the convex side predominantly apolar.