Free radicals, hair melanin, and human hair. Terephthalate anion: A probe for hydroxyl radicals
Abstract
Terephthalate dianion (TA) has been used in this study as a hydroxyl radical probe to quantitatively measure the production of hydroxyl radicals from irradiated melanin, human hair, autoxidation of ascorbic acid, and the Fenton reaction.^ The production of radicals from irradiated human hair melanin under UVA-Visible light has been studied by the combination of two techniques: Electron spin resonance and fluorescence spectrometry. As a comparison, sepia melanin which is pure natural eumelanin has also been studied. The fluorescent probe for hydroxyl radicals has allowed us to compare the amount of radicals from different types of melanin under UVA-Visible irradiation $(\rm \lambda > 320nm).$ The ESR experiments show the superoxide is the initial product from irradiated melanin, and undergoes further dismutation to form hydrogen peroxide and produce hydroxyl radicals either through the Fenton reaction or the photolysis of hydrogen peroxide. Hydroxyl radical production is proportional to the irradiation time. Red hair melanin produces more radicals than both sepia and brown hair melanin. The effects of hydroxyl radical scavengers and other additives have also studied.^ ESR spin trapping experiments show that hydroxyl radicals are produced when solid human hair is irradiated at wavelengths above 320 nm. Fluorescent results show that red hair and bleached hair produce more radicals than brown and blond hair. Ammonium thioglycolate extraction shows that bleached hair contains much more extractable iron than brown hair and white hair presumably due to the presence of more negative charges on the surface of bleached hair.^ 2-hydroxyterephthalate (HTA) is produced from TA when ascorbic acid undergoes autoxidation in the presence of trace amounts of transition metals. Reverse-phase HPLC was used to monitor the concentration of ascorbic acid. During this process, hydroxyl radicals are produced via the Fenton cycle. In the presence of trace amounts of melanin, the oxidation of ascorbic acid is largely inhibited. When the concentration of melanin increases to a certain value, the inhibition starts to decrease because melanin is also a radical producer. More melanin is needed to obtain the same percentage of inhibition when iron is added to the ascorbic acid solution. Melanin inhibits the oxidation of ascorbic acid by acting as a metal chelator. For comparison, the effect of deferoxamine was also studied. The inhibition increases with the deferoxamine concentration. The decomposition of hydrogen peroxide is inhibited in the presence of melanin. Melanin competes with EDTA for iron ions. DOPA melanin is a weaker chelator than natural melanins. ^
Subject Area
Chemistry, Analytical
Recommended Citation
Xinhua Qu,
"Free radicals, hair melanin, and human hair. Terephthalate anion: A probe for hydroxyl radicals"
(1998).
Dissertations and Master's Theses (Campus Access).
Paper AAI9902574.
http://digitalcommons.uri.edu/dissertations/AAI9902574