Cyclic peptides. 23. Conformations of an ion-binding cyclic peptide analog of valinomycin, cyclo(L-Val-Gly-Gly-L-Pro)3

Abstract

A 270-MHz 1H NMR investigation of an ion-binding cyclic peptide analogue of valinomycin, cyclo(L-Val-Gly-Gly-L-Pro)3, and its cation complexes is reported. In CD2Cl2 and CDCl3, the peptide is proposed to occur in a C3-symmetric conformer with the N-H''s of all 6 glycine residues intramolecularly H bonded. This conformation is different from the familiar valinomycin bracelet structure and lacks any cavity. Cations do not bind, or bind only weakly, to the peptide in these solvents. Uncomplexed cyclo(L-Val-Gly-Gly-L-Pro)3 in acetonitrile appears to be averaging among several conformations with no evidence found for any preferred intramolecular H bonds. The strong 1:1 complexes of cyclo(L-Val-Gly-Gly-L-Pro)3 with K+ and Ba2+ in acetonitrile are structurally analogous to the bracelet conformation of valinomycin and involve the N-H''s of the Val residues and of the Gly''s preceding Pro in intramolecular H bonding. Tl+ also formed strong 1:1 complexes with the dodecapeptide.