Superoxide radical scavenging by sodium 4,5-dihydroxybenzene-1,3-disulfonate dissolved in water: Experimental and quantum chemical studies

Abstract Tiron (sodium 4,5-dihydroxybenzene-1,3-disulfonate) is often used as a probe for detection of superoxide radicals. In this work, we have studied the interaction of Tiron with superoxide radicals dissolved in water solutions. The density functional theory (DFT) method was used for scrutinizing electron and energy characteristics of Tiron radicals in the systems modeling freshwater and salt water. In water solution at neutral pH, deprotonated form of superoxide radical, O2•-, dominates over the protonated form HO2•. Fully reduced (TH2) and fully oxidized (T) species of Tiron react with superoxide radicals, leading to the formation of the Tiron semiquinone radical. DFT calculations show that the reactions of O2•- with TH2 and T in water solution will advance if these reactions are catalyzed by hydronium ions: TH2 + O2•- + H3O+ → TH• + H2O2 + H2O and T + O2•- + H3O+ → TH• + O2 + H2O. Electron paramagnetic resonance (EPR) and DFT data show that Tiron radicals dissolved in water solution with neutral pH exist predominantly in deprotonated state T•-. The constants of the hyperfine coupling between an unpaired electron and the hydrogen atoms of deprotonated radicals T•-, computed for freshwater and seawater solutions, are in a fairly good agreement with experimental constants derived from the EPR spectra of Tiron radicals.