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-

344 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tion is a diffusion-controlled process however, the evidence supporting this contention is inconclusive at this time. In fact, determination of the rate-controlling step in this process is a large order task, since it is difficult to follow quantitatively the loss of pigment in hair, and since two impor- tant side reactions consume oxidizing agent--i.e., the previously described oxidation of amino acid residues (7), and dibasic amino acid residues as- sociate strongly with many oxidizing agents including hydrogen peroxide and persulfate (34, 35). The current knowledge of the structure of melanic pigments, the most preponderant pigments of human hair, has been described recently by Nicolaus (8) and Mason (9). Nicolaus has proposed that melanin is a complex random polymer formed from several species of the Raper scheme for the biological formation of 5,6-dihydroxyindole (36, 37) (Fig. 4), while Mason proposes that melanin is a homopolymer of 5,6-dihy- droxyindole. Regardless of the differences, both theories suggest that the indole quinone grouping (or its reduced form) is a major repeating structural unit of melanins (Fig. 5). TYROSINE DOPA H 2 H2 N/CH -- COzH • H 0 ,,,,.•...,•,• C H HO • • 2 HO HO•N•C H - C02H • I H H .5, 6 - DIHYDROXYINDOLE Figure 4. Rapcr's scheme for formation of 5,6-(lihydroxyindolc -•-co•, H H H }NDOLE QUINONE GROUPING m•E-2,•- DI•RBOXYLIC ACID PYRmE-2 •.D- TRI•RBOXYLIC AOD Figure 5. Repealing unit and fragmenls of melanins Binns and Swan (38) oxidized synthetic melanins with alkaline hy- drogen peroxide and identified only very small quantities of pyrole 2,3- dicarboxylic acid and pyrole 2,3,5-tricarboxylic acid as end products (Fig. 5). These same authors suggested that xnelanic acids are also products of this reaction however, no evidence was provided to support this sug- gestion.