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

Infrared spectra are reported in the amide I absorption region for som e cyclic peptides of the general formula cyclo[epsilon-Aca-Pro-Xxx] (e psilon-Aca = epsilon-aminocaproyl; Xxx = Ala, Thr(OBz)) and the corres ponding ''dimers'' cyclo[epsilon-Aca-Pro-Xxx-epsilon-Aca-Pro-Xxx] diss olved in D2O and acetonitrile. The acetonitrile solution spectra are v ery different from those measured in D2O. In particular, the D2O solut ion spectra exhibit amide I absorptions at exceptionally low frequenci es (1595-1600 cm-1). The infrared spectra are interpreted as indicatin g 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-tur n. The acetonitrile solution spectra are interpreted as reflecting hyd rogen-bonding characteristics of a beta-turn, with certain compounds a lso showing absorptions that suggest the presence of other hydrogen-bo nded (gamma-turn) conformers.