Quantitative evaluation of gamma-turn conformation in proline-containing peptides using 13C N.M.R.

The dependence of the 13C shift difference of proline carbons C beta and C gamma on the dihedral angle psi has been studied using the model peptide acetyl-D-proline N-methylamide. The shift difference delta beta gamma is shown to be correlated with the percent cis isomer about the acetylproline bond, both factors depending strongly on the degree of intermolecular hydrogen bonding. Both the fraction of trans peptide bond and the fractional gamma-turn conformation increase as the sample concentration is decreased in CDCl3. delta beta gamma values have been used to evaluate the fractional gamma-turn probabilities in a number of cyclic and linear peptides including thyrotropin releasing factor and bradykinin. Using this parameter, it is concluded that in bradykinin the gamma-turn probability is low in D2O and not strongly temperature dependent. In contrast, studies of a model peptide for the portion of bradykinin believed to adopt a gamma-turn conformation are consistent with an increased gamma-turn probability inless polar solvents. Data for X-Pro-Y peptides (Y = imino acid) indicate significantly reduced values of delta beta gamma, and this appears to be a useful basis for assigning the Pro C beta resonances corresponding to this sequence.