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Photo‐degradation of melatonin: influence of argon, hydrogen peroxide, and ethanol

Abstract:  When organic compounds are irradiated with UV light at 254 nm, part of their covalent bonds can dissociate if the compound absorbs light at that wavelength. Therefore, photo‐degradation depends strongly on the wavelength used. The energy of a light quanta at 254 nm amounts to approximately 110 kcal/mol quanta, which is in many cases higher than the binding energy of a variety of covalent bonds. As a consequence, the absorbing molecule is degraded. As melatonin absorbs light at 254 nm, this compound is vulnerable to UV light. In order to minimize undesired effects of other absorbing substances, we used as solvent mostly pure water and analyzed the influence of λirr = 254 nm on the disappearance of the educt (melatonin) as well as on the appearance of products derived from melatonin in the presence of oxygen, argon, hydrogen peroxide, and ethanol by UV–VIS spectroscopy and high‐performance liquid chromatography (HPLC) technique. N1‐acetyl‐N2‐formyl‐5‐methoxykynuramine (AFMK) appears to be the main, but obviously not the only product of photo‐degradation of melatonin, independently of whether the system contains oxygen or not. If the system contains additionally hydrogen peroxide, a very strong oxidant, the hydroxyl radical (•OH), is formed. Under such conditions, melatonin is not solely photo‐degraded but also attacked by the formed •OH which interact similarly with the formed main photo‐product AFMK. Ethanol, as a potent scavenger of •OH, efficiently blocks the effect of this aggressive radical even at low concentrations of that scavenger (0.1% v/v) but is less effective in preventing photo‐degradation of melatonin.