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Proton magnetic resonance spectra of the cyclic hexapeptides cyclo-(Gly-L-Leu-Gly)2, cyclo-(Gly-cf2-L-Tyr-Gly)2, cyclo-(Gly-L-His-Gly-L-Ala-L-Tyr-Gly), and cyclo(Gly-L-His-Gly-L-Tyr-L-Ala-Gly) were measured, largely at 220 MHz, and compared with the spectra of three previously studied cyclic hexapeptides, cyclo-(Gly-Gly-**4 5oxytocin,6 ferrichrome,6 and evolidine.7 (In each of these there is at least one turn of the peptide chain similar to half the cyclic hexapeptide backbone shown in Figure 1, with a solvent-shielded peptide proton inside that turn.) In the present work we use in addition a recently developed correlation between H-N-C"-H dihedral angle and coupling constant to establish details of conformation. The dihedral angle data are then used in constructing molecular models from which values of the backbone conformational angles and are estimated.

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