The contryphan family of cyclic peptides, isolated recently from various species of cone shell, has the conserved sequence motif NH3 +-X1COD-WX5PWC-NH2, where X1 is either Gly or absent, O is 4-trans-hydroxyproline, and X5 is Glu, Asp, or Gln. The solution structures described herein of two new naturally occurring contryphan sequences, contryphan-Sm and des[Gly1]-contryphan-R, are similar to those of contryphan-R, the structure of which has been determined recently [Pallaghy et al. (1999) Biochemistry 38, 11553-11559]. The 1H NMR chemical shifts of another naturally occurring peptide, contryphan-P, indicate that it also adopts a similar structure. All of these contryphans exist in solution as a mixture of two conformers due to cis - trans isomerization about the Cys2 - Hyp3 peptide bond. The lower cis - trans ratio for contryphan-Sm enabled elucidation of the 3D structure of both its major and its minor forms, for which the patterns of 3J(HαHN) coupling constants are very different. As with contryphan-R, the structure of the major form of contryphan-Sm (cis Cys2 - Hyp3 peptide bond) contains an N-terminal chain reversal and a C-terminal type I β-turn. The minor conformer (trans peptide bond) forms a hairpin structure with sheetlike hydrogen bonds and a type II β-turn, with the D-Trp4 at the 'Gly position' of the turn. The ratio of conformers arising from cis - trans isomerism around the peptide bond preceding Hyp3 is sensitive to both the amino acid sequence and the solution conditions, varying from 2.7:1 to 17:1 across the five sequences. The sequence and structural determinants of the cis - trans isomerism have been elucidated by comparison of the cis - trans ratios for these peptides with those for contryphan-R and an N-acetylated derivative thereof. The cis - trans ratio is reduced for peptides in which either the charged N-terminal ammonium or the X5 side-chain carboxylate is neutralized, implying that an electrostatic interaction between these groups Stabilizes the cis conformer relative to the trans. These results on the structures and cis - trans equilibrium of different conformers suggest a paradigm of 'locally determined but globally selected' folding for cyclic peptides and constrained protein loops, where the series of stereochemical centers in the loop dictates the favorable conformations and the equilibrium is determined by a small number of side-chain interactions.