Reduction of Hg²⁺by Individual Phenolics and Complex Samples and Its Application in Polarographic Antioxidant Assay

Abstract

Direct current (DC) polarographic antioxidant (AO) HPMC assay based on the decrease of anodic limiting current of [Hg(O 2 H)(OH)] (HydroxoPerhydroxoMercury(II) Complex-HPMC), formed in alkaline solution of H 2 O 2 , at potential of mercury oxidation, occurring upon AO addition, was validated and widely used recently. Here, a novel DC polarographic assay, MRAP (Mercury Reduction Antioxidant Power), based on the decrease of Hg 2+ cathodic current with addition of AOs was applied. Percentage of this decrease was plotted versus the amount of various individual phenolics or the volume of medicinal plant extracts, and the slope obtained was used to express AO activity. A high correlation between MRAP and HPMC assays was found for individual compounds (r = 0.974) and real samples (r = 0.968). MRAP was shown to be effective in the determination of AO activity of both synthetic mixtures of individual AOs and real samples containing various AOs. Agreement between the decrease of Hg 2+ cathodic current and HPMC anodic current lead to the assumption that Hg 2+ reduction causes a decrease in concentration of Hg 2+ available for HMPC complex formation, i.e. its anodic current decrease. The validity of MRAP assay was also confirmed through good correlation with FRAP assay.Behavior of H 2 O 2 in alkaline solution, at the potential of mercury oxidation, was investigated using direct current (DC) polarography by Sužnjević et al. 1 Formation of HydroxoPerhydroxoMercury(II) complex (HPMC) with 1:1 stoichiometry between Hg 2+ and H 2 O 2, which was first observed by Kikuchi and Murayama, 2 was proved by amperometric titration of H 2 O 2 with HgCl 2 and vice versa in Clark and Lubs buffer, pH 9.8. 1 The H 2 O 2 concentration, temperature and pH dependence of HPMC anodic current were comprehensively analyzed. The current response to antioxidant (AO) presence was employed to determine AO activity. Development and optimization of a DC polarographic AO assay based on HMPC current decrease was reported, 1 as was its wide application 3-5 (and papers cited therein). The slope of the plot of percentage of HPMC anodic current decrease vs the amount of individual or volume of complex samples was used to express AO activity.The polarographic and differential pulse (DP) voltammetric oxidation of peroxide on mercury electrodes was further examined by Palma et al. 6 Electrooxidation of peroxide involving hydroperoxide radicals and superoxide anions able to react with various AOs was suggested. 6 Anodic oxidation of mercury in the presence of H 2 O 2 in DP voltammetry was used to determine the AO character of radical scavengers, based on the decrease of the area under the observed anodic current peak. 7 Concentration of individual AOs or volume of real samples at which the area decreased by 10%, as well as their reciprocal values, 7 were used for expressing scavenging activity against hydroperoxide radical, proposed to react with AOs.Although assays developed by Sužnjević et al. 1 and Palma et al. 7 were successfully applied to various individual an...

Antioxidant Activity/Capacity Measurement. 1. Classification, Physicochemical Principles, Mechanisms, and Electron Transfer (ET)-Based Assays

“…In this DC polarographic antioxidant assay, the decrease in anodic limiting current of the [Hg­(O 2 H) (OH)] [hydroxoperhydroxo-mercury­(II) complex], formed in alkaline solution of H 2 O 2 , at the potential of mercury oxidation, varied with the concentration of added antioxidant capable of H 2 O 2 scavenging, and the method was validated against DPPH and Folin spectrophotometric assays for wines and for teas and herbal infusions . Although the signal decrease originated from the interaction of antioxidant with hydroperoxo radicals, the antioxidant did not directly scavenge H 2 O 2 in the absence of Hg, and the decrease of Hg 2+ cathodic current agreed with that of anodic current, leading to the assumption that mercury­(II) reduction caused a decrease in concentration of Hg 2+ available for hydroxoperhydroxo-mercury­(II) complex formation, bringing about the decrease in its anodic current …”