Inductive Effects on the Energetics of Prolyl Peptide Bond Isomerization: Implications for Collagen Folding and Stability

Publisher Website
Google Scholar
Abstract
The hydroxylation of proline residues in collagen increases the stability of the collagen triple helix. Previous X-ray diffraction analyses had demonstrated that the presence of an electron-withdrawing substituent on the pyrrolidine ring of proline residues has significant structural consequences [Panasik, N., Jr.; Eberhardt, E. S.; Edison, A. S.; Powell, D. R.; Raines, R. T. Int. J. Pept. Protein Res.1994 , 44, 262−269]. Here, NMR and FTIR spectroscopy were used to ascertain kinetic and thermodynamic properties of N-acetyl-[β,γ-13C]d,l-proline methyl ester (1); N-acetyl-4(R)-hydroxy-l-proline [13C]methyl ester (2); and N-acetyl-4(R)-fluoro-l-proline methyl ester (3). The pKa's of the nitrogen atom in the parent amino acids decrease in the following order: proline (10.8) > 4(R)-hydroxy-l-proline (9.68) > 4(R)-fluoro-l-proline (9.23). In water or dioxane, amide I vibrational modes decrease in the following order: 1 > 2 > 3. At 37 °C in dioxane, the rate constants for amide bond isomerization are greater for 3 than 1. Each of these results is consistent with the traditional picture of amide resonance coupled with an inductive effect that results in a higher bond order in the amide CO bond and a lower bond order in the amide C−N bond. Further, at 37 °C in water or dioxane equilibrium concentrations of the trans isomer increase in the order: 1 < 2 < 3. Inductive effects may therefore accelerate the folding and enhance the stability of collagen, which has a preponderance of hydroxyproline residues, all with peptide bonds in the trans conformation.