Local control of peptide conformation: stabilization of cis proline peptide bonds by aromatic proline interactions

In the native state of proteins there is a marked tendency for an aromatic amino acid to precede a cis proline. There are also significant differences between the three aromatic amino acids with Tyr exhibiting a noticeably higher propensity than Phe or Trp to precede a cis proline residue. In order to study the role that local interactions play in these conformation preferences, a set of tetrapeptides of the general sequence acetyl-Gly-X-Pro-Gly-carboxamide (GXPG), where X = Tyr, Phe, Trp, Ala, or cyclohexyl alanine, were synthesized and studied by nmr. Analysis of the nmr data shows that none of the peptides adopt a specific backbone structure. Ring current shifts, the equilibrium constant, the Van't Hoff enthalpy, and the measured rate of cis-trans isomerization all indicate that the cis proline conformer is stabilized by favorable interactions between the aromatic ring and the proline residue. Analysis of the side chain conformation of the aromatic residue and analysis of the chemical shifts of the pyrrolidine ring protons shows that the aromatic side chain adopts a preferred conformation in the cis form. The distribution of rotamers and the effect of an aromatic residue on the cis-trans equilibrium indicate that the preferred conformation is populated to approximately 62% for the Phe containing peptide, 67% for the Tyr containing peptide, and between 75 and 80% for the Trp containing peptide. The interaction is unaffected by the addition of 8M urea. These local interactions favor an aromatic residue immediately preceding a cis proline, but they cannot explain the relative propensities for Phe-Pro, Tyr-Pro, and Trp-Pro cis peptide bonds observed in the native state of proteins. In the model peptides the percentage of the cis proline conformer is 21% GYPG while it is 17% for GFPG. This difference is considerably smaller than the almost three to one preponderance observed for cis Tyr-Pro peptide bonds vs cis Phe-Pro peptide bonds in the protein database.