Nmr studies of the rates of proline cis–trans isomerization in oligopeptides

¹H-Nmr was used to measure the rate of cis–trans interconversion of X-Pro bonds in linear and cyclic oligopeptides. k(cis → trans) = 2.5 × 10^?3 s^?1 at 25°C was found for the zwitterionic form of H-Ala-Pro-OH, in good agreement with earlier measurements. Replacement of Ala by Phe, Tyr, or Trp resulted in a 10-fold slower interconversion rate, whereas after substitution of Ala by His or Glu, the rate decreased only slightly. Independent of the residues X, the interconversion rate was increased by a factor of ca. 20 when the peptide chain was elongated by addition of Ala to the C-terminal Pro. An additional increase by a factor of 6 was observed when going from the protected linear peptide CF₃CO-Gly-Gly-Pro-Ala-OCH₃ to the closely related cyclic compound c[-Gly-Gly-Pro-Gly-Ala-]. These data are evaluated with regard to their possible use in future studies on the role of X-Pro cis–trans isomerization in the kinetics of protein folding.