Abstract Proton NMR spectra of a cyclic peptide, cyclo-(S-acetarnidoinethyl-l-cysteinyl)-d-leucyl-l-histidyl-(S-acetamidomethyl-l-cysteinyl)-d-leucyl-l-histidyl, were measured and analyzed in order to investigate its conformation and its interaction with Cu(II). A linear peptide, [N-(t-butoxycarbonyl)-S-acetamidomethyl-l-cysteinyl]-d-leucyl-l-histidyl-(S-acetamidomethyl-l-cysteinyl)-d-leucyl-l-histidine methyl ester, was also investigated for comparison. It was strongly suggested that the cyclic hexapeptide forms transannular hydrogen bonds between the cysteine residue peptide bonds and thus two β-turns exist in the d-leucyl-l-histidyl sequences. This finding is particularly noteworthy because the present cyclic peptide has no prolines which are frequently seen in β-turn structures of proteins, naturally occurring peptides or synthetic peptides and because the cyclic peptide we studied is composed of amino acids all bearing the large side chains which seemed likely to prevent β-turn formation due to their steric hindrance. The interactions of the cyclic peptide and the linear peptide with Cu(II) were also studied by observing 1H NMR spectral changes on addition of CuCl2. It was suggested that the imidazolyl groups in the peptides are directly coordinated to Cu(II).