Cystine nucleophilic cleavage

Disulfide bonds present in the keratin of hair are potential binding sites for many nucleophilic molecules. Tolgyesi and Fang investigated the structural changes which occur in keratin as a result of alkaline treatments of hair. They reported that hydroxide ions initiated a p-ehmination reaction, resulting in cleavage of the disulfide bonds of cystine to produce dehydroalanine intermediates. Cysteine and lysine may react with dehydroalanine to form new cross links in keratin. Nucleophilic amines such as ethylamine and n-pentylamine may also react with dehydroalanine to form... 

Reese and Eyring [45] have demonstrated that the reaction of sulfite with hair is a pseudo-hrst-order reaction. In other words, the chemical reaction of sulfite with the disulfide bond of hair is slower than diffusion of sulfite into hair. Elsworth and Phillips [46, 47] and Volk [48] examined the sulfi-tolysis of keratin, demonstrating that the rate of cystine cleavage is optimal at acid pH. Wolfram and Underwood [49] found a broad optimum for cystine cleavage by sulhte at pH 4 to 6. The decrease in cystine cleavage at acidic pH (below pH 4) is due to a decrease in the concentration of the nucleophilic sulhte species. On the other hand, the decrease in cystine cleavage as pH is raised (alkaline pH) results from alkaline hydrolysis of the Bunte salt [50]. 

The mechanism of the nucleophilic attack in this reaction is not completely known it is still not clear whether species responsible for cleavage of the cystine are the sulfite (S03") or bisulfite (HS03 ) or disulfite (S2O5") ions. Products of the reaction are a free thiol residue and a cysteine—S—sulfonate residue, also known as Bunte salt. Though totally ionized at pH values above 1, Bunte salts are capable of a number of reactions, including reformation of disulfide bonds. 

Alternatively, cleavage of the cystine disulfide bond can occur by nucleophilic attack on sulfur, yielding a dehydroalanine residue via thiol and sulfinate intermediates ... 

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