QUANTITATIVE EVALUATION OF γ‐TURN CONFORMATION IN PROLINE‐CONTAINING PEPTIDES USING13 C N.M.R.

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Abstract
The dependence of the 13C shift difference of proline carbons Cβ and Cγ on the dihedral angle ø has been studied using the model peptide acetyl‐d‐proline N‐methylamide. The shift difference Δβγ 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 γ‐turn conformation increase as the sample concentration is decreased in CDCl3. Δβγ values have been used to evaluate the fractional γ‐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 γ‐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 γ‐turn conformation are consistent with an increased γ‐turn probability in less polar solvents. Data for X‐Pro‐Y peptides (Y = imino acid) indicate significantly reduced values of Δβγ, and this appears to be a useful basis for assigning the Pro Cβ resonances corresponding to this sequence.