A study of the aerial oxidation of L-cysteinyl-L-cysteine: purification of the product and equilibrium relationship involving the monomeric and dimeric cyclic derivatives

Abstract

The aerial oxidation of L-cysteinyl-L-cysteine in water at pH 8.5 has been investigated. The reaction products, monomeric cyclic disulphide and parallel and antiparallel cyclic dimers, were fractionated by gel and thin layer chromatography. The relative yields of the products are independent of the type of the catalyst used and strictly obey equilibrium relationships. The equilibrium constant for the reversible isomerization of the two cyclic dimers is of the order of magnitude expected for a random distribution of –S–S– systems at equilibrium (K ca. 1), whereas the high values of the equilibrium constants for the dimerization equilibria are in line with the free energy differences expected for a cis–trans transformation of two peptide groups.