Generation and suppression of singlet oxygen in hair by photosensitization of melanin

We have studied the spectroscopic properties of hair (white, blond, red, brown, and black) under illumination with visible light, giving special emphasis to the photoinduced generation of singlet oxygen (1O2). Irradiation of hair shafts (λex > 400 nm) changed their properties by degrading the melanin. Formation of C3 hydroperoxides in the melanin indol groups was proven by 1H NMR. After 532-nm excitation, all hair shafts presented the characteristic 1O2 emission (λem = 1270 nm), whose intensity varied inversely with the melanin content. 1O2 lifetime was also shown to vary with hair type, being five times shorter in black hair than in blond hair, indicating the role of melanin as a 1O2 suppressor. Lifetime ranged from tenths of a nanosecond to a few microseconds, which is much shorter than the lifetime expected for 1O2 in the solvents in which the hair shafts were suspended, indicating that 1O2 is generated and suppressed inside the hair structure. Both eumelanin and pheomelanin were shown to produce and to suppress 1O2, with similar efficiencies. The higher amount of 1O2 generated in blond hair and its longer lifetime is compatible with the stronger damage that light exposure causes in blond hair. We propose a model to explain the formation and suppression of 1O2 in hair by photosensitization of melanin with visible light and the deleterious effects that an excess of visible light may cause in hair and skin.