Intramolecular Hydrogen Bonding and Conformation of Small Peptides: Variable-Temperature FTIR Study on N-Acetyl- l -Pro- l -Leu-Gly-NH2 and Related Compounds

Intramolecular hydrogen bonding and conformation of tripeptide amides in CDCl 3 solutions have been examined by variable-temperature FTIR spectroscopy. Absorption in the NH stretching region of N -acetyl- l -Pro- l -Leu-Gly-NH 2 was decomposed into some component bands by least-squares fitting. Two broad bands at around 3350 and 3290 cm -1 were assigned to intramolecularly hydrogen-bonded NH groups in different conformers. The other relatively sharp bands were assigned to hydrogen-bond-free NHs of the three different amide groups by referring to spectra of analogous amide compounds. From temperature dependence of the band intensities, it is concluded that the C-terminal amide group takes part in the hydrogen bonding in both hydrogen-bonded conformers. From comparison with spectra of N -trifluoroacetyl- l -Pro- l -Leu-Gly-NH 2 and l -Pro- l -Leu-Gly-NH 2, the 3290-cm -1 band has been assigned to a 13-membered ring with the hydrogen bond between NH of the C-terminal amide group and CO of the N-terminal acetyl group. The 3350-cm -1 band, on the other hand, has been assigned to a 10-membered hydrogen-bonded ring between the C-terminal amide NH and a prolyl carbonyl group. A van't Hoff analysis shows that the 13-membered hydrogen-bonded ring is enthalpically more favorable but entropically less favorable than the 10-membered hydrogen-bonded ring.