Hydrogen bonds in the tripeptide Pro-Leu-Gly-NH2O and H studies

¹⁷O---NMR measurements of labeled Pro-Leu-Gly-NH₂ were carried out at different pH levels and in mixed solvents of water/acetonitrile. Complementary studies of the amide protons were carried out in acetonitrile-d₃. Only the prolyl C = ¹⁷O group was sensitive to the pH level. Protonation of the amine group resulted in an upfield chemical shift of 18 ppm. The chemical shifts of each of the three oxygen sites was sensitive to the ratio water: acetonitrile. Solvent composition dependence of the chemical shift and linewidth suggests that the prolyl C = ¹⁷O is involved in intramolecular hydrogen bond formation when Pro-Leu-Gly-NH₂ is dissolved in acetonitrile, while in water there is no intramolecular H bond.