ABSTRACT
Cystine is one of the several oxidized forms of cysteine (Figure 8.1). Cystine is a disulde; there are trisulde and tetrasulde forms of cysteine that are discussed at the end of this chapter. Early literature recognized cystine as a symmetrical amino acid distinct from cysteine.1 However, it was also recognized that cystine was formed from cysteine. While amino acid composition of a protein determined by amino acid analysis as a characteristic has been largely replaced by MS, early literature found such useful. Cystine and cysteine were expressed as 1/2 cystine (1/2Cys). Cysteine content of a protein was usually determined by a separate colorimetric procedure such as the nitroprusside reaction.2 The reduction of a protein was recognized early as a mild method for the characterization of a protein and determination of the importance of disulde bonds in biological activity.3 These early studies did report the difculty in the restoration of activity on oxidation of the reduced protein. These early reviews by Desnuelle,1 Tristram,2 and Putnam3 contain information on the early work on cystine. Cecil4,5 provides more recent consideration of the early work on sulfur in proteins. The reader is also directed to a more recent volume of Methods in Enzymology for a more recent discussion of redox chemistry and thiol/disuldes in proteins.6 There has been a long-standing interest in the analysis and modication of cystine in wool.7-19 There has also been interest in the modication of cystine in hair during permanent waving.20-22 The oxidation of wool is an interesting application of protein chemistry. Oxidation of fabric was a process used to reduce shrinkage of the nal product. Early work used chlorine/hypochloride solutions to shrinkproof nish fabric.23 Since the shrinkproong process was based on oxidation, there was a search for other more environmentally friendly oxidizing agents. Denning and coworkers24 reviewed the development of new oxidizing agents for wool resulting in the development of permonosulfate (Oxone®; potassium peroxymonosulfate; KHSO5) for the oxidation of wool followed by the addition of sodium sulte/bisulte yielding the S-sulfonate (Bunte salt; thiosulfates), cysteic acid, and cystine monoxide (Figure 8.2). Diz and coworkers25 were able to couple a cationic surfactant (Figure 8.2) with oxidized wool to yield a fabric with antibacterial properties. These investigators used the process of sultolysis (Figure 8.6) to generate cysteine-S-sulfonate in wool. Coupling the thiol compound could be accomplished either by reaction with the S-sulfonate derivate or by disulde exchange. Gao and Cranston26 showed that peroxymonosulfate/sulte treatment enabled productive binding of polyhexamethylene biguanide to wool fabric yielding a product with antimicrobial properties. Although not discussed, coupling of oxidized wool to a thiol-containing compound could also be accomplished via the cystine monoxide27 or cystine dioxide.28
