Quantitative Evaluation of Γ‐turn Conformation in Proline‐containing Peptides Using13 C N.M.R.

Abstract

The dependence o f the 13C shift difference o f proline carbons Cp and C, on the dihedral angle $ has been studied using the model peptide acetyl-D-proline Nmethylamide. The shift difference Ap, 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 y-turn conformation increase as the sample concentration is decreased in CDC13. A h values have been used to evaluate the fractional 7-turn probabilities in a number o f cyclic and linear peptides including thyrotropin releasing factor and bradykinin. Using this parameter, it is concluded that in bradykinin the y-turn probability is low in D 2 0 and not strongly ternperature dependent. In contrast, studies o f a model peptide f o r the portion of bradykinin believed to adopt a y-turn conformation are consistent with an increased y-turn probability in less polar solvents. Data for X-Pro-Y peptides ( Y = imino acid) indicate significantly reduced values of A h , and this appears to be a useful basis for assigning the Pro Cp resonances corresponding to this sequence.