HAIR PHOTODAMAGE 115 100 o 80 c) 6O 0 4O 2O I I _ -V I I I 0 50 100 150 200 Irradiation Time (rain) Figure 4. Wavelength dependence of tryptophan destruction. The sample was irradiated in the fluorimeter for the indicated times at either 295 (O), 305 (W), or 315 nm (•') and a 10-nm band-pass. Fluorescence emission was then measured by excitation at 295 nm with a nominal 1-nm band-pass. The % tryptophan damage was then calculated from the emission intensities at 340 nm. Note that the sample is assumed to contain 100% tryptophan (0% destruction) prior to laboratory irradiation. Although this is unlikely, it is inconsequential in the present context. nation of the irradiation dose, quantitative comparisons can be made only for the hair samples subjected to side-by-side weathering. Laboratory experiments with simulated solar irradiation, on the other hand, do not have these limitations and are expected to be fully reproducible. We have attempted to compare the Trp photodamage rates in the dry and wet fibers, since water is known to play an important role in photolytic reactions. The rate of photodamage was found to be only slightly faster in wet fibers (data not shown). It should, however, be noted that water binds to keratin with high affinity, and it is likely that significant amounts of moisture may have remained in the fibers as a result of sample manipulation at ambient conditions after the drying process. Furthermore, as noted above, the data obtained with dry fibers have a relatively large associated error due to overwhelming background scattering. Further insight into the effect of the medium on Trp photolysis was gained by comparing Trp photodamage rates in hair wetted with water with those in hair saturated with mineral oil. The data, shown in Figure 6, suggest that the hydrophilic, polar medium increases both the rate and extent of Trp photodamage. For example, while 50% measurable hair Trp was lost in eight minutes in the presence of water, it took almost 28 minutes for the same relative damage in the presence of mineral oil. In addition, Trp emission in hydrophobically treated hair was found to be shifted toward shorter wavelength (hypsochromic effect) by about 5 nm,

116 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 300 3•5 3•0 3•z5 460 4•5 450 Wavelength (nrn) Figure 5. Tryptophan photodamage in yak hair due to solar irradiation during June 1991. The fluorescence was excited at 295 nm, and the excitation and emission slits were 1 and 3 nm, respectively. The spectra from the top represent fluorescence emission after irradiation times of 0, 18 h, 49 h, and 72 h, respectively. It should be noted that both the solar wavelength spectrum, as well as the intensity, show significant day-to-day variation. compared to that observed for the sample containing water (data not shown). These results are in agreement with the known dependence of the wavelength of emission maximum, as well as the photostability, of Trp on the polarity of its environment (24). In order to identify other degradative processes associated with weathering besides Trp damage, we have measured IR spectra of pigmented and weathered (blond) hair samples from the same source as in Figure 1. The most significant difference appears to be in the S=O vibration region of the spectra. The data, shown in Figure 7, reveal that weathered hair contains a significantly larger amount of cysteic acid, as judged by the relative intensity of the 1041 cm-• band. This is in agreement with reports in the literature that show that photodamage of both hair and wool results in the oxidation of cystine (disulfide) to cysteic acid (5,25). While weathered hair is characterized by reduced tryptophan and increased cysteic acid, it has not been determined whether these two processes are correlated. It would be very important to establish this relationship in future research, especially in view of the fact that the reduced fiber strength of weathered hair appears to be the consequence of the scission of disulfide bonds and their conversion to cysteic acid residues. The likely involvement of Trp damage with other aspects of hair photodamage can be inferred from the existing literature on photochemistry of proteins and amino acids (9). It has been shown that besides being photolabile, Trp sensitizes the photolysis of other chromophoric and non-chromophoric amino acids. Using synthetic peptide hormones,