CONFORMATIONS OF CYCLIC PEPTIDES .4. NUCLEAR MAGNETIC RESONANCE STUDIES OF CYCLO-PENTAGLYCYL-L-LEUCYL AND CYCLO-DIGLYCYL-L-HISTIDYLDIGLYCYL-L-TYROSYL

Proton magnetic resonance spectra of cyclo- Gly-Gly-L-His-Gly-Gly-L-Tyr (I) and a partially C-deuterated derivative, in dimethyl sulfoxide and in water, are interpreted to indicate a peptide backbone containing two short transannularly hydrogen-bonded antiparallel peptide segments ioined at their ends by peptide bonds perpendicular to the average ring plane. In this peptide and in cyclo-Gly5-L-Tyr (II) and cyclo-Gly5-L-Leu (III), which have the same backbone, the side chains are shown to be attached at “corner” α-carbons, rather than at the α-carbons in the centers of the extended segments. The structures proposed are based primarily on studies of the peptide proton resonances, which indicate four protons exposed to solvent and two shielded from it, and on spin-decoupling studies used to determine which α-protons and peptide protons correspond to individual residues. Study of the peptide proton region of the spectrum of a C-deuterated derivative of I, in mixtures of water and dimethyl sulfoxide, demonstrate that the backbone conformation of this peptide does not change in going from one pure solvent to the other. Syntheses of I, of III, and of Gly-L-His-Gly-Gly-Gly-Gly are described. © 1969, American Chemical Society. All rights reserved.