Cyclic peptide disulfides: consecutive β-turn conformation of a synthetic model peptide corresponding to the active site of thioredoxin

Abstract

A synthetic model peptide, which mimics the active-site disulfide loop of thioredoxin has been prepared. 270 MHz ¹H-NMR studies establish that Cys-4 and methylamide NH groups are solvent-shielded, using hydrogen-deuterium exchange, solvent and temperature dependence of chemical shifts and nitroxide radical-induced broadening as diagnostic criteria. Infrared measurements provide supporting evidence for intramolecularly hydrogen-bonded conformations. The related peptide in which Gly-2 is replaced by α-aminoisobutyric acid has been shown to adopt a similar backbone conformation based on NMR and CD data. Based on the known stereochemical preferences of α-aminoisobutyric acid residues, a consecutive β-turn conformation involving two intramolecular 4 → 1 hydrogen bonds is proposed for both disulfides. Vicinal coupling constants and CD data are discussed with reference to the side-chain conformation of the cysteine residues. Large structural differences have been established between the thioredoxin active-site model disulfide and its acyclic precursor.