DNA Damage Photoinduced by Cosmetic Pigments and Sunscreen Agents under Solar Exposure and Artificial UV Illumination

Abstract

Nanostructured ZnO particles present in skin-care cosmetics and UVB/UVA sunscreen products generate strong oxidizing species (free radicals) when illuminated with UV radiation that can damages human skin and the horny layer. Damage to DNA by ZnO and other pigmentary ingredients in sunscreen formulations under artificial and solar UV exposure has been examined by Agarose gel electrophoresis using pUC 18 DNA plasmids (2686 base-pairs). Initial photoinduced oxidative damage done to DNA plasmids have been probed by nicking assays under in vitro conditions for ZnO. The effects of nanosize ZnO and CeO₂ particles, and the newly developed CaO-doped and SiO₂-coated CeO₂ pigment are compared when subjected to artificial (75-W Hg-lamp) and solar UV radiation. Supercoiled DNA plasmids undergo one nick to produce the relaxed form, followed by a second nick yielding the linear form of the plasmids. The DNA constituents deoxyadenosine-5’-monophosphate (dAMP), guanosine-5’-monophosphate (GMP) and cytidine-5’-monophosphate (CMP) have been examined to assess the photooxidative damage done to these nucleotides under photocatalytic conditions using the cosmetic/sunscreen ZnO pigment. Adsorption of the nucleotide through the phosphate on the positively charged ZnO surface, followed by attack of the ribose/phosphate backbone by photogenerated · OH (and/or · OOH) radicals on the ZnO surface lead to the degradation of the dAMP’s ribose moiety and subsequently to decomposition of the adenine base residue. About 90% mineralization of the ribose/phosphate backbone occurred as evidenced by formation of H₂PO₄^- ions after only 30 min of UV irradiation. The nitrogen atoms of the adenine base were converted to NO₃^- and NH₄⁺ ions. About 45% of the organic carbons constituting the dAMP ribose backbone was mineralized to CO₂ within 8 h of UV irradiation occurring through formation of carboxylic acid intermediates (succinic, acetic and formic), with 85% of the remaining nucleobase ultimately mineralized after 48 h of UV irradiation. Similar occurrences were seen for the GMP and CMP nucleotides.