CONFORMATIONS OF CYCLIC PEPTIDES .3. CYCLOPENTAGLYCYLTYROSYL AND RELATED COMPOUNDS

Proton magnetic resonance studies of cyclopentaglycyl-L-tyrosyl and a partially C-deuterated derivative, using water, dimethyl sulfoxide, and trifluoroacetic acid as solvents, show that the six amide protons of this peptide are divided into two groups. Two protons are shielded from the solvent and four are exposed to it. This observation strongly supports an internally hydrogen-bonded structure, shown in Figure 1, as the stable form of the cyclic hexapeptide backbone. To this stable backbone conformation the single side chain is attached at one of the “corner” positions (1, 3, 4, and 6 in Figure 1), at least in dimethyl sulfoxide or a dimethyl sulfoxide-water mixture. Additional evidence is adduced from the spectra to define more narrowly, although with less certainty, details of the location and conformation of the side chain. The syntheses of the cyclic peptides are described. © 1969, American Chemical Society. All rights reserved.