BLEACHING OF HAIR 187 of sepia eumelanin in polyvinyl alcohol films in one case (I) the eumelanin is in particu- late form (intact melanosomes) in the other (II) the eumelanin is in its water-soluble form. The film of intact sepia is blackish grey in color. Its appearance is the result of two optical phenomena: light absorption and light scattering. In this case, the latter makes the major contribution to appearance. Because of the dimensions of the indi- vidual melanosomes or its aggregates ( 1 I•m), the wavelength dependence of optical constants is greatly suppressed (Mie scattering regimen), and thus the scattering is relatively uniform throughout the spectral region (achromatic). On the other hand, the deep brown color of the film of solubilized melanin is almost totally dominated by the absorbance characteristics of the eumelanin, with negligible scattering effects. It is very impressive how a simple change in the state of dispersion brings about a profound transformation in the optical characteristics. Figure 8 displays the reflectance spectra of these melanin films together with one of pheomelanin. It can be seen that the solubili- zation of eumelanin brings its reflectance curve to almost coincide with that of pheomel- anin. Extrapolating this to hair implies that reddish and warm hues may be generated in situ not only by the pheomelanin (which, by the way, unlike eumelanin, is water-sol- uble) but also by affecting the degree of aggregation and dispersion of the eumelanin pigment. It is reasonable to assume that photochemical degradation of the hair pigment that occurs during solar radiation involves some degree of solubilization (water accel- 2O 4• 15 10 I i i I i 400 500 600 700 Wavelength (nm) Figure 8. Reflectance spectra of intact sepia melanin ([•), solubilized sepia melanin (A), and pheomelanin (0).

188 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS erates the photo-bleaching), and thus, a spectral shift of the type observed is to be anticipated. Being water-soluble, the pheomelanin is not likely to show a spectral shift indeed, even eumelanin, when totally water-soluble, does not exhibit such a shift (Figure 6). It is natural, at this stage, to inquire as to the mechanism responsible for the photo- bleaching phenomenon and the reasons for the apparent photostability of pheomelanin. Of particular value in this regard is the finding (4) that irradiation of pheo- and eumel- anin solutions produce optical changes that are wavelength-invariant and depend on the presence of oxygen. The uniform decrease in absorbance over the entire spectrum suggests a lack of selectivity of the bleaching step. Such a selectivity would be antici- pated if bleaching was the primary photolyric phenomenon. This reaction pattern, com- bined with the need for oxygen, points to a two-stage process: a) Excitation of the melanin moiety to a state in which it reacts with molecular oxygen to produce highly reactive species such as superoxide anion 02 =, and b) the dismutation of the superoxide anion in the presence of water to yield hydrogen peroxide (Eq. 1) which is the active bleaching agent: 2 H20 + 2 O2=--• H202 + 02 q- 2 OH- (1) Strong support for this view comes from the TLC pattern of photo-bleached melanin 25 20 D 40 D 10 i i i i 400 500 600 Wavelength (nm) Figure 9. Reflectance spectra of red hair before (O) and after (O) bleaching with H202. ! 700