HAIR PHOTODAMAGE 119 proteins such as keratin, the disulfide bond contributes strongly to the overall stability and integrity. Trp residue, besides being the most hydrophobic amino acid, can also participate in hydrogen bonding through the N-1 proton in the indole ring. The photodamage to Trp will, therefore, directly affect the stability of hair protein. It seems also very likely that Trp photodamage may not be just a local casualty, but may represent the earliest event in the photodamage process involving other residues, ulti- mately resulting in the observed deterioration of the hair condition. So far, there is fairly strong evidence linking Trp photodecomposition to yellowing of wool fibers (31). No other direct correlation between the Trp loss in hair and changes in other observables has been shown so far. PREVENTION OF PHOTODAMAGE An obvious application of these studies is in preventing, or at least reducing, hair photodamage. There are two main strategies. The first is to reduce the intensity of incident light by using photofilters. This can be accomplished by coating hair with suitable sunscreens characterized by absorption spectra overlapping the Trp absorption band. The UVB sunscreen molecules typically have extinction coefficients approxi- mately 60 times higher than that of Trp at 300 nm (9,32). The second approach would be to eliminate, or slow down, the photodecomposition through intervention in the photochemical pathway. This may involve changing the Trp microenvironment by using solvents that would retard the excited-state decomposition, or a somewhat com- plicated step such as treatment with non-chromophoric materials that would quench the photochemical intermediates. The non-chromophoric photoprotection route, although extremely promising, remains to be explored since it requires a detailed understanding of the mechanism of the photochemical reactions in hair keratin. As a starting point, we have evaluated the photoprotective ability of hair care formu- lations containing common UVB sunscreens. Due to the spectral overlap of such com- pounds with the Trp absorption spectrum, they are likely to filter off a significant fraction of the incident light. For example, we have calculated that --0.1 • film of a formulation containing 0.4% sunscreen (E -- 20,000 M-• cm-•), out of a total of --4% solids, would filter out --20% incident light at its •max (absorption maximum). These calculations, however, do not take into account the photodecomposition of the sunscreen itself. It is thus probable that the incident light would be initially signifi- cantly attenuated, with the photofiltering effect diminishing as a function of the irra- diation time as the sunscreen molecules are themselves gradually sacrificed. The experimental results obtained with a prototype leave-in hair treatment containing 0.2% octylmethoxy cinnamate are shown in Figure 8. The sample and the control were irradiated in a solar simulator. It is evident from these data that the product reduces the extent of Trp damage by about 40%. Similar experiments were performed for a variety of other cosmetic formulations such as fixatives, conditioners, and shampoos. We con- clude that consequential surface deposition of sunscreens, and photodamage protection of the order of tens of percent, can only be obtained with leave-in products. On the other hand, the presence of conventional sunscreen in shampoos does not produce measurable photoprotection if the treatment is performed in the usual manner, which includes short-term (30-60 s) lather followed by rinsing. This is not surprising since (i) com- pounds such as DEA methoxycinnamate or octyl methoxycinnamate do not possess

120 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ½__/ 310 3•0 3•z0 460 4•0 4•30 490 Wavelength (nm) Figure 8. Hair tryptophan photoprotection afforded by hairspray containing 0.2% octylmethoxy- cinnamate. The treated and the control samples were irradiated in a solar simulator for a total of 30 h. The sample was irradiated for 10 h each time, after which it was shampooed and retreated. The upper curve is due to unirradiated control, the middle curve is the treated sample, and the lowest curve is the irradiated control. The difference between the last two curves is the absolute photoprotection due to the treatment. The excitation was at 295 nm, and the excitation and the emission slits were 1 and 5 nm, respectively. strong affinity toward the hair surface, (ii) the contact time of the formulation with hair is too short to allow the penetration of sunscreen inside the fiber, and (iii) the application of the treatment is followed by rinsing, which removes weakly adsorbed species from the fiber. In contrast to this, measurable photoprotection can be imparted to hair soaked for two hours in a shampoo containing octyl-PABA and rinsed with water, while similar treatment in the absence of sunscreen has virtually no effect. This experiment (data not presented), although unrealistic from the point of view of practical application, never- theless confirms the notion that insufficient sunscreen deposition is the primary reason for the ineffectiveness in photoprotection of rinse-off type compositions. Formulating with UV absorbers characterized by high affinity to hair, brought about by incorpora- tion of cationic, hydrophobic, or reactive functions into the structure of a sunscreen, might enhance the performance of such treatments. CONCLUSIONS It has been shown that fluorescence spectroscopy can be used to monitor the decompo- sition of tryptophan by exposure to natural and artificial UV light. The application of light absorbers to the surface of hair was shown to significantly slow down the process of Trp destruction. In addition to this, the rate of tryptophan photodamage can be increased by absorption of water, which creates a polar environment surrounding Trp