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

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.