SOLVENT-DEPENDENT CONFORMATIONS OF A CYCLIC TETRAPEPTIDE

The conformations of cyclo(-gamma-Abu-L-Pro-)(2) in several solvents h ave been characterized by means of H-1 and C-13 NMR, CD and IR spectra . CD results indicate that the change in polarity of the solvent induc es an inversion of the conformation, revealed by the disappearance of the positive peak near 210 nn and then by the gradual appearance of a negative trough near 210 mm having opposite chirality. The data of NMR spectroscopy in CDCl3, CD2Cl2, CD3OH, (CD3)(2)SO and D2O indicate tha t cyclo(-gamma-Abu-L-Pro-)(2) has a C-2 symmetric conformation consist ing of the cis-trans-cis-trans peptide bond backbone (with two cis gam ma-Abu-L-Pro bonds) in all solvents, and that the change of polarity o f solvent induces the inversion of cis and trans conformations around the Pro C-alpha-C=O single bond. (The trans and cis regions describe t he rotational states of the Pro C-alpha-C=O single bond in which the P ro C-alpha-H is trans and cis to the direction of the Pro carbonyl oxy gen atom, respectively). The cis-trans-cis-trans conformation containi ng the two cis Pro C-alpha-C=O bonds in CDCl3 and CD2Cl2 is stabilized by the presence of seven-membered rings consisting of intramolecular hydrogen bonds between (NH)-N-gamma and C-alpha=O in the gamma-Abu res idue.