VIBRATIONAL CIRCULAR-DICHROISM STUDIES OF MOLECULAR-CONFORMATION AND ASSOCIATION OF DIPEPTIDES

Vibrational circular dichroism (VCD) and infrared (IR) absorption spectra in the NH stretching region have been measured for the dipeptides, R'-CO-AA-NH-R'' (R'=Me and tertBu; AA=Ala, Leu, Val and Phe; R''=Me, isoBu and neoPe). Analyses of the VCD and absorption spectra indicated that the VCD bands for the NH stretching are quite sensitive to the state of hydrogen bonding as well as the local conformation of oligopeptides. VCD spectra exhibit a negative VCD band at 3420-3405 cm-1 due to the C5 conformer with an intramolecularly hydrogen-bonded five-membered ring. The intermolecularly hydrogen-bonded N-H stretching vibration exhibits a characteristic negative-positive couplet from the high wavenumber side due to the antiparallel C5-C5 dimer formation. Hydrogen-bonded oligomers beyond the dimer formed in highly concentrated solutions give rise to an additional negative VCD band on the lower wavenumber side of the hydrogen-bonded absorption band.