Hydroquinone-Mediated Redox Cycling of Iron and Concomitant Oxidation of Hydroquinone in Oxic Waters under Acidic Conditions: Comparison with Iron–Natural Organic Matter Interactions

Abstract

Interactions of 1,4-hydroquinone with soluble iron species over a pH range of 3-5 in the air-saturated and partially deoxygenated solution are examined here. Our results show that 1,4-hydroquinone reduces Fe(III) in acidic conditions, generating semiquinone radicals (Q(•-)) that can oxidize Fe(II) back to Fe(III). The oxidation rate of Fe(II) by Q(•-)increases with increase in pH due to the speciation change of Q(•-) with its deprotonated form (Q(•-)) oxidizing Fe(II) more rapidly than the protonated form (HQ(•)). Although the oxygenation of Fe(II) is negligible at pH < 5, O2 still plays an important role in iron redox transformation by rapidly oxidizing Q(•-) to form benzoquinone (Q). A kinetic model is developed to describe the transformation of quinone and iron under all experimental conditions. The results obtained here are compared with those obtained in our previous studies of iron-Suwannee River fulvic acid (SRFA) interactions in acidic solutions and support the hypothesis that hydroquinone moieties can reduce Fe(III) in natural waters. However, the semiquinone radicals generated in pure hydroquinone solution are rapidly oxidized by dioxygen, while the semiquinone radicals generated in SRFA solution are resistant to oxidation by dioxygen, with the result that steady-state semiquinone concentrations in SRFA solutions are 2-3 orders of magnitude greater than in solutions of 1,4-hydroquinone. As a result, semiquinone moieties in SRFA play a much more important role in iron redox transformations than is the case in solutions of simple quinones such as 1,4-hydroquinone. This difference in the steady-state concentration of semiquinone species has a dramatic effect on the cycling of iron between the +II and +III oxidation states, with iron turnover frequencies in solutions containing SRFA being 10-20 times higher than those observed in solutions of 1,4-hydroquinone.

Towards identifying potent new hits for glioblastoma

“…activities of the individual phenols against the individual cell cultures (U87MG, U251MG, IN1472, IN1528 and IN1760) generally shows that there isn't much variation in activity, in a broader sense, with only modest activity observed, with the exception of the two hydroquinones (6 and 7) against the U251MG cell lines (EC 50 s = 17 and 57 μM respectively). Whilst this data suggests that no clear conclusion can be drawn for the relative activities of these phenols and hydroquinones against GBM, hydroquinones are known to be involved in redox cycling in other systems, 27,28 and so such compounds warrant further study in the area of brain tumour research. Especially, since it is known that such compounds can be toxic if not-targeted to the tumour site directly, and so further study would focus on finding analogues that could demonstrate selectivity over non-cancerous cells.…”

Decreased Electron Transfer between Cr(VI) and AH2DS in the Presence of Goethite

“…These decreases in absorbance of both quinone species could be indicative of radical formation. This hypothesis is supported by the fact that semiquinone radicals form during the reduction of Fe(III), as well as during microbial reduction of AQDS (Jiang et al, 2009; Jiang et al, 2015). If semiquinone radicals do form, cross reactions between semiquinone radicals and AH 2 DS and AQDS species occurs rapidly (Uchimiya and Stone, 2006, 2009), which would make less AH 2 DS available for electron transfer with either Fe(III) or Cr(VI).…”

Radionuclide uptake by colloidal and particulate humic acids obtained from 14 soils collected worldwide

“…1 ) may indicate only minor or no reduction of Fe(III) occurred during the incubation experiment. Such a difference between Fe and Np could be related to the fact that Fe(II) from HA reduction can be rapidly oxidized back to Fe(III) by O 2 under oxic conditions (i.e., redox cycling), especially when the pH is higher than 5 32 , while this may not happen to Np. For Pa, since it primarily existed as Pa(V) in oxic conditions and Pa(IV) typically has stronger particle-reactivity than Pa(V), the high activity of Pa in the particulate phase (i.e., strong Pa fixation by potentially immobile particles) may indicate HA-derived reduction of Pa during one-week incubation.…”