A cyclic hexapeptide, cyclo(Pro-Gly-ProGly-Pro-Gly), hasbeen synthesized; its solution conformations were examined by 220-MHz nuclear magnetic resonance spectroscopy. The solution structures have been deduced, and shown to vary as a function of solvent polarity. In addition, it has been found that this cyclic peptide binds alkali metal cations. While the predominant conformation of this cyclic peptide is 3-fold symmetric in the apolar solvent methylene chloride, an asymmetric structure is preferred in some polar solvents (water, dimethylsulfoxide). However, addition of alkali metal salts, such as sodium thiocyanate, to dimethylsulfoxide solutions of the peptide shifts the conformational equilibrium in favor of a second type of C3-symmetric structure, presumably the result of the formation of a stable peptide-metal ion complex. Nuclear magnetic resonance data suggest that the peptide in methylene chloride solution takes up a conformation containing three cis' Pro CaC=O bonds and three cis Gly-Pro peptide bonds; that water and dimethylsulfoxide stabilize a conformer in which one (or two) sets of such bonds of a given Pro-Gly unit have undergone interconversion to trans'/trans forms; and that alkali metal cations complex the cyclic peptide in a Cs-symmetric all-trans'/trans structure.The biological activity and structural simplicity of many cyclic peptides have generated considerable interest in the determination of their conformations, with the ultimate goal of ascertaining structure-function relationships. Highresolution nuclear magnetic resonance (NMR) spectroscopy, useful for the study of solution conformations of cyclic peptides, can provide information for structural determinations (1,2).Many naturally-occurring cyclic peptides contain proline, and it is believed that investigations of synthetic prolinecontaining cyclic peptides composed of repeating oligopeptide units (3-5) may clarify the structural role of the prolyl residue in biologically active cyclic peptides. An additional advantage of the investigation of proline peptides is that the number of possible conformers of the peptide backbone is limited by the restrictions to rotation about the N-CCa bond of the prolyl residues. Another consideration is the recent demonstration of the presence of cis-and trane-X-Pro peptide bonds (where X is any amino-acid residue preceeding a proline residue), in both linear (6-8) and cyclic (4, 5) proline-containing peptides. Since the barrier to rotation about the peptide