Photoprotective action of melanin is commonly ascribed to its optical properties responsible for the efficient absorption of ultraviolet and visible light, ultrafast conversion of energy of the absorbed photons into heat, which minimizes the formation of reactive oxygen species, and the ability of melanin to scavenge oxidizing radicals and quench singlet oxygen. Although natural melanins were previously viewed as very stable pigments showing little metabolic turnover, it is now recognized the pigments can undergo with aging substantial physicochemical changes. Recently, we have demonstrated that in vitro photoaging of human hair melanosomes significantly modified their photochemical reactivity [1]. To address this issue comprehensively, a thorough analysis of selected physical and photochemical parameters of a synthetic DOPA-melanin, subjected to aerobic photolysis or treatment with hydrogen peroxide, was carried out [2]. Physical properties of the studied melanin were assessed by UV-vis absorption, EPR spectroscopy, and DLS. Molecular changes of the melanin undergoing photochemical or chemical bleaching were analyzed by alkaline hydrogen peroxide oxidation. Aerobic photoreactivity of the melanin was determined by EPR-oximetry, EPR-spin trapping, and time-resolved singlet oxygen phosphorescence. Changes in antioxidant properties of the melanin were monitored by DPPH EPR assay. The data show that photochemical and chemical treatment of the synthetic eumelanin caused its dose-dependent bleaching and irreversible modification of the melanin paramagnetic, electron- and ion-exchange properties. Bleached melanin exhibited enhanced efficiency in photogeneration of singlet oxygen and reduced antioxidant properties. It is postulated that oxidative modifications of melanin, accompanying aging, could affect its photoprotective functions and stimulate phototoxic behavior.
Supported by Poland National Science Centre (project OPUS-2021/43/B/ST4/03255).
References
[1] K. Mokrzynski et al., Journal of Photochemistry & Photobiology, B: Biology, 2023, 243: 1-16, 112704, doi: 10.1016/j.jphotobiol.2023.112704.
[2] K. Mokrzynski et al., Pigment Cell Melanoma Res 2024, 00: 1-14, doi:10.1111/pcmr.13176.