The generation of excess reactive oxygen species (ROS) by hydroxyphenols that become ‘trapped’ in a redox cycle with their corresponding quinones, resulting in oxidative stress, is associated with toxicity  and can be detected in in vitro
tests . Briefly, the pathway involves reversible chemically- and enzymatically-mediated steps which include the formation of deprotonated hydroxyphenol ion and semiquinone radical ion intermediates. The positive control from in vitro
tests for this phenomenon (1,1'-ethylene-2,2'-bipyridyldiylium dibromide, the herbicide Diquat dibromide) and four hydroxyphenols known to cause this cycling (2-hydroxyphenol, 4-hydroxyphenol, 2,3,5-trimethyl-4-hydroxyphenol and 2-tert
-butyl-4-hydroxyphenol)  were investigated in order to test the hypothesis that intermediates along the pathway are of comparable energy and can thus exist in equilibrium, thus there is little driving force to escape the cycle. In the absence of hydroxyphenols known not to undergo this process - other than 3-hydroxyphenols (resorcinols) which are unable to form quinones - the electronic scope of the reaction was investigated by considering a series of electron-poor hydroxyphenols substituted with nitro groups and electron-rich ones substituted with dimethylamino groups. A ligand-based method was developed, using methanol-methanoate as a comparative acid-base pair for deprotonations and molecular oxygen-superoxide as an electron sink, and Density Functional Theory (DFT) calculations were performed at the B3LYP/6-311G** level of theory [4,5]. For all four hydroxyphenols, the semiquinone radical anion is comparatively stable relative to other species in the pathway, potentially slowing formation of the quinone and allowing the ROS-forming cycle to occur. In addition, the deprotonated hydroxyphenol is of comparable energy to the quinone, meaning that the two can exist in equilibrium, prolonging the cycle. This energy gap is particularly small (<20 kJ mol-1
) in the case of the ortho-substituted species where the remaining hydroxyl group in the deprotonated species can form a strong intramolecular hydrogen bridge between the two oxygens, stabilising the negative charge, yet the two electron-rich oxygens in the ortho-quinone repel each other, destabilising it. In conclusion, a method has been developed which allows characterisation of the Gibbs free energy profiles of the hydroxyphenol-quinone equilibrium. In the absence of negative examples, this method will assist in confirming the suitability of category membership (i.e.
that they are energetically similar to known compounds) for specific hydroxyphenols in respect of Read-Across analyses.
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2) Gutierrez PL, “The metabolism of quinone-containing alkylating agents: free radical production and measurement” (2000), Front. Biosci. 5, D629-D638.
3) Studies in RepDose (http://fraunhofer-repdose.de) and ToxRef (https://cfpub.epa.gov/si/si_public_record_report.cfm?dirEntryId=227139) databases.
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