Photochemical degradation of antioxidants: kinetics and molecular end products

Organic molecules in the ocean store as much carbon (C) as there exists in the atmosphere, thus their reactivity has important implications for the global C budget. Structurally complex and presumably recalcitrant molecules, rich in highly oxygenated, aliphatic functionalities, are believed to account for a substantial fraction of DOC, thereby acting as a C sink spanning millennial timescales. Understanding C turnover in aquatic systems can therefore be improved by constraining the formation fluxes of these molecules, which are thought to include photolysis and microbial alteration of organic matter. Recent work has shown that carotenoids and related compounds may serve as good models for testing the abiotic formation of recalcitrant DOC. Here, we aimed to characterize radical-assisted, solar degradation of known antioxidants as well as carotenoid-like compounds present in a variety of different organisms, including fungi and algae. The antioxidant activity of molecules harvested from culture media of five species of Ascomycota was up to 30-fold higher than that of the fungal biomass, as determined by a radical scavenging assay and normalized to C content. A preliminary screening of both biomass and medium samples via liquid chromatography-mass spectrometry (LC-MS) revealed known antioxidants, such as semiquinones, carotenoids (canthaxanthin), and b-carotene degradation products (beta-apo-4'-carotenal and retinal). Consequently, five representative antioxidant standards (b-carotene, zeaxanthin, fucoxanthin, coenzyme Q10 and vitamin K2) were selected and their organic solvent-water aggregates were treated under H₂O₂-assisted solar light. The irradiation time ranged up to 1 hour with a constant dose of H₂O₂ at 10 min intervals, mimicking OH-radical formation in the ocean’s euphotic zone. Gradual degradation of the aggregates and reaction kinetics were determined via UV-Vis spectroscopy. In addition, LC-QTOF (Quadrupole Time-of-Flight)-MS protocols were applied to reveal the structures of reaction products, which were hypothesized to represent short-chain, water-soluble, hydroxylated, alicyclic compounds, putative biomarkers of recalcitrant DOC.