By The Cosmetic Chemist Staff
October 15, 2016
Photodamage of hair leads to morphological and chemical damage to the mature hair shaft.1-4 Significant evidence demonstrates that long-term exposure to UV radiation causes fusion of the lamellar components of the cuticle as well as damage to integral lipids of hair.5-7 In addition, UV irradiated hair experiences changes in pigmentation as well as damage to hair structural proteins.8-9 Not surprisingly, long-term exposure to UV light also results in diminished tensile strength properties of the fiber, resulting in a a greater degree of brittleness.
In previous studies, researchers demonstrated that prior chemical treatments could impact the effect of UV radiation on the hair structure. For example, bleached hair is more susceptible to UV damage than unbleached hair. More than likely, melanin in the unbleached hair absorbs solar radiation and prevents damaging light rays from degrading the fiber proteins.10
In a recent publication, Tang and coworkers investigated the effect of UV radiation on dyed hair.11 The focus of their study is on the effects of transition metals (iron and copper) from the dyeing process on photo-oxidative processes in hair. Until recently, little attention has been given to the presence of transition metals in hair.
Transition metals can increase the photosensitivity of hair and act as catalysts in the formation of the highly reactive hydroxyl radical (HO•), a reactive oxygen species that damages every type of biological molecule with which it comes in contact, including proteins, lipids, and DNA. The hydroxyl radical is formed by the Fenton reaction—a single electron transfer reaction in redox systems—in which the transition metal catalyzes the conversion of H2O2 to HO•.
It was shown in past studies that copper found in hair—more than likely obtained in tap water—can cause the formation of hydroxyl anion during the hair coloring process.12 In the more recent study by Tang and coworkers, transition metals found in hair were attributed to the dyeing process itself. By proteomic analysis of hair, they found a 1.6 fold increase in oxidized protein levels in dyed hair as compared to undyed hair. The photosensitization of hair proteins by permanent dye treatments challenges researchers to find novel ways of preventing or reducing the production of reactive oxygen species in hair during exposure to solar radiation, especially in the case of dyed hair where the levels of transition metals may be elevated.
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