NMR evidence of formation of cyclocystine loops in peptide models of the high sulfur proteins from wool.

We report that in peptide models of the high sulfur proteins of the matrix from wool a disulfide bond forms between two sequential Cys-residues as a result of a simple oxidation procedure. This tiny cyclocystine loop manifests itself in many ways in 1H NMR spectra. The formation of the loop is accompanied, as expected, by conversion of the Cys-Cys peptide bond from its usual trans-configuration into the energetically less favorable cis-configuration. Possible consequences of the formation of cyclocystine loops for the elasticity of the network of the matrix are discussed.