SULFENYL HALIDES AS MODIFYING REAGENTS FOR POLYPEPTIDES AND PROTEINS .3. AZOBENZENE-2-SULFENYL BROMIDE A SELECTIVE REAGENT FOR CYSTEINYL RESIDUES

Azobenzene-2-sulfenyl bromide (Burawoy, A., and Vellins, C. E. (1954), J. Chem. Soc., 90) is found to react selectively with cysteinyl residues in polypeptides and proteins giving unsymmetrical disulfides. The reagent is soluble in water and the reaction occurs selectively in buffer solutions close to neutrality (pH 5). Several model compounds were prepared using cysteine derivatives and glutathione and their physical and chemical properties were investigated. The thiol function is easily restored by using reducing agents such as β-mercaptoethanol, thioglycolic acid, or sodium borohydride. The mixed disulfides are stable in acidic solution, but are decomposed in alkaline media. The basicity of the azo group, determined spectrophotometrically, was found to be much less in the azobenzene-2-sulfenyl derivative of reduced lysozyme than in the simple model compounds. That the protein structure affects the basicity of the azo group was confirmed by the fact that the ionization equilibrium of azobenzene-2-sulfenyllyso- zyme is equal to that of azobenzene-2-sulfenylcysteine in 8 M urea solutions. © 1968, American Chemical Society. All rights reserved.