Bleached hair, cystine

Cystine, which contains a disulfide bond, is reported to be the most numerous and reactive amino acid present in hair keratin. Disulfide bonds in cystine are reduced by mercaptans and phosphines, and oxidized by perborates, bromates, and bleach. These reactions result in structural rearrangements within keratin which may affect the physiochemical properties of hair, since disulfide bonds in cystine contribute to the stability of hair. For example, hydrogen peroxide bleaching of hair is an oxidative process which occurs readily in an alkaline medium. This results in the formation of perhydroxy anions which have been proposed to react with cystine to form cysteic acid residues. The process of bleaching results in the loss of approximately 15% of the cystine bonds originally present in keratin and may explain the increased permeability of bleached hair to chemicals. - ... 

The actual presence of large amounts of cysteic acid in bleached hair had at one time been in doubt [55, 56]. It had been theorized that the cysteic acid found in bleached hair hydrolysates was formed by decomposition of intermediate oxidation products of cystine during hydrolysis prior to the analytical procedure [55]. However, differential infrared spectroscopy [4] and electron spectroscopy for chemical analysis by Robbins and Bahl [5] on intact unhydrolyzed hair have conclusively demonstrated the existence of relatively large quantities of cysteic acid residues in chemically bleached hair. Evidence for other sulfur acids (e.g., sulfinic or sulfenic acids) in bleached hair has not been provided. It is unlikely that these amino acids exist in high concentrations in hair because these species are relatively unstable. 

Zahn and co-workers [2], using two-dimensional gel electrophoresis, separated up to 62 isolated protein spots from human hair. From the fluoro-gram of bleached hair, these scientists identified cystine oxides (monoxide and dioxide). Although the exact quantities of these intermediate oxidation products versus cysteic acid were not reported, the quantities were indicated to be small relative to the cysteic acid content [2]. 

To summarize, sulfonic acid is the principal established end product of the oxidative cleavage of the disulhde bond from the chemical bleaching of human hair with current hair bleach products [3,9]. The mercaptan content of bleached hair is lower than that of unbleached hair [4], The intermediate oxidation products of cystine (i.e., the disulhde monoxide, dioxide, trioxide, and tetroxide) do not exist as signihcant end products of hair bleaching using today s commercial bleach products [3,6,9]. Nevertheless, evidence has been presented demonstrating low levels of cystine oxides in bleached hair [2],... 

Robbins and KeUy [5] have examined bleached and unaltered hair by hydrolysis and amino acid analysis. Their results for severely bleached hair are summarized in Table 4-6 and suggest that methionine, tyrosine, lysine, and histidine, in addition to cystine, are degraded to the greatest extent (tryptophan could not be evaluated in this study). 

Since the bleaching of human hair is carried out in an aqueous alkaline oxidizing medium, hydrolysis of the cystine oxide intermediates (Figure... 

Table 5-14 describes the describes the influence of cross-link density in different keratin fibers on diffusion rate. These data clearly show that the rate of diffusion into keratin fibers decreases with increasing cystine content and therefore with increasing cross-hnk density. One may conclude that reactions that decrease the cross-link density of hair (e.g., bleaching) will lead to hair that is more rapidly penetrated, and its penetrability will increase with increased bleaching. Decreasing cross-link density obviously increases the rate of transcellular diffusion. 

Because bleaching increases the ratio of acidic to basic amino acids [122], the isoionic point should decrease with increasing oxidation. One might also anticipate a similar decrease in the isoelectric point of hair with bleaching because the A-layer of the cuticle cells is rich in cystine. 

The chemistry of bleaching shows that a major side reaction in the beaching of hair involves the oxidation of cystine cross-links to cysteic acid residues. This disruption of cross-links has a major influence on the wet tensile properties of hair. 

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