176 JOURNAL OF COSMETIC SCIENCE 12 10 0 a b c d Figure 4. HTA production from human hair: 0.2 g hair (1-mm long) in 2 ml of 2 mM TA at pH 7.6, irradiated for 90 min. a. Brown hair. b. Blond hair. c. Red hair. d. Bleached hair. cysteinyldopa and its isomers (present in smaller quantities) (34). It would appear from photobleaching studies that the sulfur-containing benzothiazine unit of pheomelanin is more resistant to oxidation than the indolic units of eumelanin (35). Therefore, eumela- nin seems to be more efficient than pheomelanin as a scavenger of reduced oxygen species. Hence the amount of 02 ß -/OH' that is available to reach the spin traps (DMPO or TA) (Figures 4, 6 reference 25) is greater for red hair and red hair melanin. The greater ability to scavenge free radicals for eumelanin suggests that the eumelanin has a better photoprotective effect with regard to hair structure than pheomelanin. This is consistent with the fact that brown hair is more easily photobleached than either red (35) or black hair (36). As shown from the experiments with wool keratin (Figure 3), the structural component of hair produces oxyradicals even in the absence of melanin. In addition to carbon- and oxygen-centered radicals, it has been reported that sulfur-centered radicals were also produced from irradiated wool keratin (17). This involves thiyl radicals, generated via

OXYRADICALS FROM PHOTOIRRADIATED HAIR 177 12 10 a b c Figure 5. Effects of additives on HTA production from irradiated bleached hair: 0.2 g bleached hair (1-mm long) in 2 mM TA at pH 7.6, irradiated for 90 min. a. Ethanol added (1 M). b. Azide added (10 mM). c. No additives. metal-sulfur complexes, which have considerable reactivity, and thus provide another pathway for oxyradical production (20,37). The sequence given in Scheme 2 has been suggested to explain the fate of thiyl radicals produced during the autoxidation of cysteine (38,39). In our spin trapping studies, irradiation was not carried out in the ESR cavity, and we find no evidence for the presence of transient sulfur radicals. Among the free radicals involved in these reactions, superoxide, hydroxyl, and hydrogen peroxide are most cytotoxic species induced by ionizing radiation and photosensitized