340 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS position of the peroxide. Additional details concerning bleaching for- mulae may be found in the book edited by Sagarin (14). ThE REACTION OF BLEACHING AGENTS WITH THE PROTEINS OF HUMAN HAIR The primary purpose in the bleaching of human hair is to lighten the hair however, due to the severe reaction conditions required for destruc- tion of the pigment chromophore, side reactions with the hair proteins occur simultaneously. Zahn (6) demonstrated that the reaction of oxidiz- ing agents with the proteins of human hair occurs primarily at cystine. Robbins and Kelly (7) have shown that rather small amounts of degrada- tion occur to the amino acid residues of tyrosine, threonine, and methio- nine during severe bleaching however, in accord with Zahn, the main site of attack is at the disulfide bonds of the cystyl residues in the fibers. These authors have also shown that whereas 15 to 25% of the disulfide bonds in human hair are degraded during "normal" bleaching, as high as 45% of the cystine crosslinks may be broken during severe "in practice" bleaching. Although a description of the kinetics of the reaction of human hair with bleaching agents could not be found, there is evidence to suggest that at least the oxidative cleavage of the disulfide bonds is diffusion con- trolled. Harris and Brown (15) have shown, by reduction and methyla- tion of keratin fibers, that the wet tensile properties decrease almost linearly with the disulfide content. Alexander et al. (16) have arrived at this same conclusion after studying wool fiber oxidized with peracetic acid. A similar phenomenon has been observed, by this author, for hair that had been oxidized with alkaline hydrogen peroxide. These observa- tions lead to the conclusion that the percentage loss of various parameters of the wet tensile properties of hair, that occur during bleaching, e.g., the 20% index (17), is a measure of the per cent cleavage of cystine cross- links. Edman and Marti (18) have described the change in the 20% index of hair fibers as a function of treatment time in 6% hydrogen peroxide, at 32øC, using a 25:1 solution:hair ratio at pH 9.52. Their data, plotted in Fig. 1, versus the square root of the time provides a straight line from which an approximate diffusion coefficient may be calculated. An equa- tion developed by Crank (19) which describes diffusion from a stirred so- lution of limited volume into a cylinder of infinite length was used in this instance.

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