HAIR PHOTODAMAGE 117 100l 80 c- 20 I I I I \ \ \ \ \ \ " Irczter 0 20 40 60 80 100 rrcdiction Time (min) Figure 6, Amount oftryptophan photodestruction in hair upon irradiation at 295 ñ 5 nm for the indicated times. The hair samples were either soaked in water or in mineral oil. The % tryptophan damage was calculated as explained in the caption to Figure 4. Note that the half-life (50% destruction time) in an aqueous environment is almost thrice that in mineral oil. it has been shown that basic amino acids flanking Trp in the primary sequence undergo Trp-sensitized photodamage (26-28). Electron transfer from an excited Trp to a neigh- boring Tyr in model peptides has been documented (29). Literature reports also suggest Trp-sensitized damage to the disulfide bond (30). It should be noted here that although the disulfide bond itself has a much weaker absorption in the UVB as compared to Trp (tE290 = 40 and 4500 M-• cm-•, respectively) (9,11), the net absorption, due to the high disulfide concentration in keratins, may in fact be non-negligible. Preliminary experiments (data not shown) using FTIR microscopy, which is very sensitive in de-

118 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS B-x A B I 1000 1075 1150 Wavenumbers (1/cm) Figure 7. FTIR absorption spectra of virgin brown (T.J.) and naturally weathered (blond) hair from the same source. The spectra were measured in the IR-Plan TM microscope. The data were flattened with a curved baseline function between 1368, 1142, and 1000 cm- 1 and enhanced with a 13-point smoothing routine. tecting disulfide oxidation products, however, did not reveal any oxidation under the irradiation conditions that resulted in significant Trp loss in hair. Tryptophan photo- damage, therefore, appears to kinetically precede disulfide photoxidation. The strength and structural stability of hair is essentially governed by the integrity of its protein constituents. Besides the all-pervasive peptide bonds, the protein stability also derives from the amino acid side-chain interactions. These include the electrostatic forces between basic and acidic amino acids (salt linkages), the hydrophobic interactions between the aromatic and alkyl side groups, the hydrogen bonding, and the isopeptidic interactions between various groups in the protein structure. In cystine-containing