BLEACHING HUMAN HAIR Figure 1. Rate of cleavage of cystine crosslinks estimated from tensile properties 341 2I 2 + = ... (Dt/a2• The ter•n C•, is the 20% index at ti•ne (t), and represents the a•nount of cleaved disulfide at ti•ne (t) C, the 20% index at ti•ne zero represents the total amount of disulfide before oxidation, and a represents the fiber radius which is asstuned to be 40 v. Application of the Crank equation to the data in Fig. 1 provides an approxi•nate diffusion coefficient of 1.8 X 10 -9 cm2/•nin, which is of the anticipated •nagnitude, suggesting that this reaction (the oxidative cleavage of the disulfide bond) is diffusion controlled. Two types of mechanisms have been suggested for the oxidative deg- radation of disulfides (20), one involving S--S fission and the other C--S fission. The •nechanistic schemes for S--S and C--S fission preferred by this author are summarized in Fig. 2, and the nomenclature used for sev- eral of the structures involved in these two schemes is described in Table I. A more comprehensive discussion of the oxidative degradation of cystine is given by Savige and Maclaren (20). Two features that may be used to distinguish between these two reac- tion paths are the following: First of all, if the oxidation reaction pro- ceeds totally via S--S fission, then two •noles of sulfonic acid should be pro- duced per mole of reacted disulfide. However, if the reaction goes totally S-S FISSION R-S-$-R '•'" R- SO-S-R--R-S02-S-R '•'"" •- S02-S 0-R•"R- S02-S02-R '•"" 2 R-SO3H C-S FISSION R-S -S -R ---,.- R -S -S - OH ---m-R-S -SO•H '--"- R-S -S% H • R - S% H + H• SO 4 + R-OH Figure 2. Schemes for disulfide fission

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