Connecting antioxidant depletion and reactive oxygen species production by aerosol-borne quinones

Abstract

Exposure to particulate matter (PM) is known to induce oxidative stress. In particular, quinones, a class of chemical compounds found in PM, are known to induce oxidative stress via redox cycling where the quinone can catalyze reactions that deplete antioxidants and produce oxidants. The first objective of this study is to determine if the rate of antioxidant depletion and oxidant production correspond for a number of quinones. Given that quinones can have different numbers of benzene rings and hydroxyl (OH) groups (due to atmospheric oxidation), the second objective of the study is to determine the effect of functional groups on the ability of quinones to deplete antioxidants and produce oxidants. To answer these questions, the dithiothreitol (DTT) assay and 2’-7’-dichlorofluorescein. (DCFH) assay were performed on three series of homologous quinones (1, 2, and 3 benzene rings) with increasing OH functionality. The DTT and DCFH assays measure the rate of antioxidant decay (represented by DTT decay) and rate of oxidant production (specifically H2O2 production), respectively. Out of all the quinones tested, HNQ had the fastest second order DTT decay rate constant, followed by 5,8-HNQ, NQ and BQ, respectively. Where 5,8-HNQ had the fastest second order H2O2 production rate constant followed by HNQ, NQ and lastly, BQ did not significantly produce any H2O2. Overall, it was found that the rate of DTT decay and H2O2 production do not correspond for all quinones, and that characterizing both the potential to deplete antioxidant and produce oxidants are required towards accurately representing the potential toxicity of aerosol due to induction of oxidative stress. Using Density Function Theory (DFT) it was found that semiquinone radicals can also undergo subsequent protonation and reduction reactions. These reactions are predicted to affect the overall observed rates of both DTT decay and H2O2 production based on calculated ΔGred values. It has also been predicted that BQ undergoes a reductive addition reaction with DTT instead of forming a semiquinone. This mechanism could explain the significant reduction of DTT but no production of H2O2.