CONFORMATIONAL STUDIES OF CYCLO-(-PRO-GLY-)(3) AND ITS COMPLEXES WITH CATIONS BY VIBRATIONAL CIRCULAR-DICHROISM

We report vibrational circular dichroism (VCD) and infrared absorption spectra of a cyclic hexapeptide, cyclo-(-Pro-Gly-)(3) in aqueous and nonaqueous media, and in the presence of a number of mono- and divalent metal cations. Cyclo-(-Pro-Gly-)(3) was studied Previously by CD, Raman, and NMR spectroscopies, and its ionophoric properties were recognized. VCD in the amide I spectral region monitors the relative orientation of amide linkages of the peptide with respect to each other. We find that VCD is an enormously sensitive technique to monitor the conformation of small peptides, and can distinguish more than a dozen different molecular shapes, depending on the ionic strength of the solvent media.