Nmr studies of the rates of proline <i>cis</i>–<i>trans</i> isomerization in oligopeptides

Abstract

Abstract 1 H‐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 CF 3 CO‐Gly‐Gly‐Pro‐Ala‐OCH 3 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.