Tryptophan fluorescence in hair-examination of contributing factors

Various types of hair, including white (unpigmented), Piedmont (yellow-colored), blonde, brown, curly black of African origin, straight black of Chinese origin, and chemically processed (bleached) hair, were studied by using fluorescence spectroscopy. Fluorescence measurements were obtained by using a single- or double-grating fluorescence spectrophotometer and a bifurcated fiber optics accessory to measure the spectra directly from the surface of the hair. The results have shown that all types of hair share similar fluorescence characteristics, as recorded by excitation at 290 nm, 320 nm, 350 nm, and 380 nm, with emissions that could be ascribed to chromophores such as tryptophan (Trp), N-formylkynurenine, kynurenine, and 3-hydroxykynurenine. The relative intensities of fluorophore emissions were found to be dependent on factors such as melanin content and the history of UV light or thermal exposure. Trp fluorescence was also found to be dependent upon the state of the hair matrix. Softening of hair keratin by chemical reduction (breakage) of disulfide bonds or by hydration, leads to a 50%-100% increase in Trp emission intensity. Conversely, stiffening of the hair matrix by re-oxidation of reduced hair with hydrogen peroxide, or drying of wet hair, produces a decrease in fluorescence intensity. The results were interpreted by invoking the behavior of certain enzymes, which demonstrate Trp quenching by neighboring disulfide bonds or by hydrogen bonding with alanine residues, or with certain side-chain amino acids.