TURNS IN SMALL CYCLIC-PEPTIDES - CAN INFRARED-SPECTROSCOPY DETECT AND DISCRIMINATE AMONGST THEM

Infrared spectra are reported in the amide I absorption region for some cyclic peptides of the general formula cyclo[epsilon-Aca-Pro-Xxx] (epsilon-Aca = epsilon-aminocaproyl; Xxx = Ala, Thr(OBz)) and the corresponding ''dimers'' cyclo[epsilon-Aca-Pro-Xxx-epsilon-Aca-Pro-Xxx] dissolved in D2O and acetonitrile. The acetonitrile solution spectra are very different from those measured in D2O. In particular, the D2O solution spectra exhibit amide I absorptions at exceptionally low frequencies (1595-1600 cm-1). The infrared spectra are interpreted as indicating an unusual D2O solution structure involving three-centered hydrogen bonding to the Aca carbonyl group, two amide protons interacting with the C=O group to simultaneously close both a gamma-turn and a beta-turn. The acetonitrile solution spectra are interpreted as reflecting hydrogen-bonding characteristics of a beta-turn, with certain compounds also showing absorptions that suggest the presence of other hydrogen-bonded (gamma-turn) conformers.