342 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table I Cystine Oxides and Sulfur Acids Cystine Oxides R--SO--S--R Monoxide R--SO2--S--R Dioxide R--SO2--SO--R Trioxide R--SO2--SO2--R Tetroxide I R = NH•--CH--CH•--CO•H Sulfur Acids R t--S--OH Sulfcnic acid R•--SO•H Sulfinic acid R•--SOaH Sulfonic acid through C--S fission, then only one mole of sulfonic acid can be produced from each mole of disulfide that reacts. Nachtigal and Robbins (5) have shown this ratio to be greater than 1.6:1 for one sample of severely bleached hair, suggesting that this reaction is occurring largely by S--S fission. Secondly, if this reaction occurs through the C--S fission route, the alcohol produced would be a seryl residue which, on hydrolysis, would provide significantly larger quantities of serine in bleached hair hy- drolyzates than in hydrolyzates of unbleached hair. This is not the case (7). Therefore, the oxidative cleavage of the disulfide bond that occurs during the bleaching of human hair is predominantly an S--S fission process. Since the bleaching of human hair is carried out in an aqueous al- kaline oxidizing medium, hydrolysis of the cystine oxide intermediates (Fig. 3) should be competitive with oxidation. In fact, disproportiona- tion of the cystine oxides (20) may also occur, adding to the complexity of the total reaction scheme. R-S-S-R-•.,--R- SO- S- R-----.-.R-SO•-S-R---,--IR-SO•-SO- R•--R-SO•-SOu R.•,..-2 R-S%H R-SH + R-SO•H = R-SO2H+R-SOH R-S%H I__.. + R-SO2H - R-$%H + + R-$OH = R-S%H -- Figure 3. S--S fission of disulfides in aqueous alkaline oxidizing medium

BLEACHING HUMAN HAIR 343 Oxidation and hydrolysis reactions of the cystine oxides are sum- marized in Fig. 3. Disulfide trioxides have never been isolated, but may be inferred as possible intermediates in this scheme, since both disulfide dioxides (thiolsulfonates) and disulfide tetroxides (20) have been isolated. Cystine monoxide and dioxide are extremely sensitive to alkaline hy- drolysis (21, 22) but have been isolated from aqueous acidic oxidations (22) and the tetroxide should be even more sensitive to alkali (20). Al- though the importance of hydrolysis relative to oxidation for each of the cystine oxides is not known, it is certain that hydrolysis should be in- creasingly important with increasing pH, and at the pH of current bleach products (pH 9 to 11) hydrolysis of these species should be highly com- petitive with oxidation. In summary, sulfonic acid is the only established end product of the oxidative cleavage of the disulfide bond that occurs during the bleaching of human hair (3, 4). The mercaptan content of bleached hair, as one would predict, is lower than in unbleached hair (5), and neither the monoxide nor the dioxide occur as significant end products (5). Con- sidering all the species from the oxidation of disulfides described in Fig. 3, the sulfinic acid is the only species of even moderate stability (23) re- maining to be examined. Sulfenic acids are notoriously unstable (24) and trioxides and tetroxides are even more sensitive to alkali than diox- ides and monoxides. OXIDATION OF HAIR PIGMF, NTS The principal pigments of human hair are the brown-black melanins and the less prevalent red pigments, the trichosiderins. Hair pigments reside within the cortex and medulla (25) in ovoid or spherical granules that generally range in size from 0.2 to 0.8/• along their major axis (26). These particles generally comprise less than 3% of the total fiber mass as estimated by the residue weight after acid hydrolysis (27). Methods used for pigment granule isolation usually involve dissolving the hair from the granules (11, 26, 28-31). Funatsu (31) has found that the general composition of melanin granules consists of pigment, protein, and min- erals. Flesch (11) reports a similar general composition for the tri- chosiderin-containing granules. Schmidli et al. (32, 33), after acid or alkaline hydrolysis of hair, were able to isolate melanin combined with protein, suggesting that melanin exists in combination with protein in the granules (melanoprotein). Since the pigment granules are located in the cortical cells and in the medulla of human hair, it is reasonable to assume that pigment degrada-