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The synthesis of cyclo-(~-Val-Gly-Gly-~-Pro)3 is reported. This homodetic cyclic dodecapeptide contains only naturally occurring amino acids and is a model of an ion carrier related to valinomycin. Titration curves obtained from circular dichroism data in acetonitrile solutions reveal three kinds of cyclic dodecapeptide (P)-cation (C) complexes: PC (1:l complex), P2C (peptide-sandwich complex), and PC2 (ion-sandwich complex). Binding constants of several cations and molar ellipticities of the bound species were determined using a new computer program. The stabilities of the 1: 1 complexes are found to be correlated with the diameter of the divalent cation in the series Mg2+ << Ca2+ << Ba2+. The charge of the cation is also an important factor: Ca2+ is more strongly bound than K+, which has a diameter close to that of Ba2+. Nevertheless, the stability of the peptide-K+ complex is comparable to that of valinomycin-K+. Another important finding is that P2C complexes are stable; indeed, these are the major species for small cations, e.g., Li+, Mg2+, and Na+.Current interest in the functions of biological membranes, particularly ion transport, has led to several studies of ion binding and transport with antibiotics4 and other natural peptides5 and with synthetic ionophore^.^,' Extrapolation of