SPECTROSCOPIC STUDY OF THE CONFORMATIONS OF PROLINE-CONTAINING OLIGOPEPTIDES IN THE CRYSTALLINE STATE AND IN SOLUTION

A conformational study has been carried out on a series of linear proline-containing oligopeptides (ZGP, ZGPL, ZGPLG and ZGPLGP) in both the crystalline state and in DMSO-d₆, solution, using Raman and n.m.r. spectroscopy. The amide I and HI bands in the Raman spectra of the crystalline forms indicate the presence of a type I β-bend conformation in ZGPLG and ZGPLGP, but not in ZGP and ZGPL. This result is in agreement with X-ray data on these mole cules. The Raman spectra of these peptides in solution indicate that more than one conformation is present, i.e. that the β-bend structure of the solid form of ZGPLG and ZGPLGP is destabilized by DMSO-d₆. ¹³C and ¹H n.m.r. data also demonstrate the presence of more than one conformation in ZGP, ZGPL, ZGPLG and ZGPLGP in DMSO-d₆ solution. These isomers differ in their con formation (cis and trans) about their Gly-Pro peptide bonds and possibly about the Cα-C' bond of the C-terminal proline in ZGPLGP. For ZGP, ZGPL, ZGPLG and ZGPLGP, the ensemble of conformations in DMSO-d₆ includes C₅ and C₇ hydrogen-bonded rings; in addition, ZGPLGP may contain a small percentage of a β-bend conformation (at Pro₂-Leuj₃) with trans peptides in both Gly-Pro moieties.