Nuclear magnetic resonance study of fibrinogen-like peptides and their structure in dimethyl sulfoxide and water

The 1H nuclear magnetic resonance (NMR) spectra of four fibrinogen-like oligopeptides (H-Gly-Pro-Ala-NH2, H-Arg-Gly-Pro-Ala-NH2, H-Val-Arg-Gly-Pro-Ala-NH2, and H-Gly-Val-Arg-Gly-Pro-Ala-NH2) in dimethyl-d6 sulfoxide (Me2SO-d6), and of the hexapeptide in water, and the 13C NMR spectrum of H-Gly-Pro-Ala-NH2 in Me2SO-d6, were recorded and interpreted in terms of preferred conformations in solution. Each peptide exists in Me2SO-d6 as in a 30:70 mixture of cis and trans isomers about the Gly-Pro bond, and the hexapeptide in water is solely the trans isomer. For the trans isomers in Me2SO-d6, there is a hydrogen bond between the Gly CO group and one of the C-terminal primary amide hydrogens, and a beta turn involving the Gly-Pro-Ala-NH, section of the molecules. A strong NOE between Pro CalphaH and Ala NH for the trans isomer of the tripeptide in Me2SO-d6 completes the identification of this structural feature as a type II beta turn.