What Are the Oxidizing Intermediates in the Fenton and Fenton-like Reactions? A Perspective

Abstract

The Fenton and Fenton-like reactions are of major importance due to their role as a source of oxidative stress in all living systems and due to their use in advanced oxidation technologies. For many years, there has been a debate whether the reaction of FeII(H2O)62+ with H2O2 yields OH• radicals or FeIV=Oaq. It is now known that this reaction proceeds via the formation of the intermediate complex (H2O)5FeII(O2H)+/(H2O)5FeII(O2H2)2+ that decomposes to form either OH• radicals or FeIV=Oaq, depending on the pH of the medium. The intermediate complex might also directly oxidize a substrate present in the medium. In the presence of FeIIIaq, the complex FeIII(OOH)aq is formed. This complex reacts via FeII(H2O)62+ + FeIII(OOH)aq → FeIV=Oaq + FeIIIaq. In the presence of ligands, the process often observed is Ln(H2O)5−nFeII(O2H) → L•+ + Ln−1FeIIIaq. Thus, in the presence of small concentrations of HCO3− i.e., in biological systems and in advanced oxidation processes—the oxidizing radical formed is CO3•−. It is evident that, in the presence of other transition metal complexes and/or other ligands, other radicals might be formed. In complexes of the type Ln(H2O)5−nMIII/II(O2H−), the peroxide might oxidize the ligand L without oxidizing the central cation M. OH• radicals are evidently not often formed in Fenton or Fenton-like reactions.

Dynamic Intramolecular Cap for Preserving Metallodrug Integrity─A Case of Catalytic Fluoroquinolones

“…It is important to note however that in contrast to the Cu II/I couple, the high redox potential of the Co III/II couple precludes the possibility of a “Fenton-like” reaction . Rather, Co­(II) species have been shown to generate ROS via disproportionation of peroxide ligands without involving oxidation of the central cation. , …”

“… 44 Rather, Co(II) species have been shown to generate ROS via disproportionation of peroxide ligands without involving oxidation of the central cation. 44 , 45 …”

“…44 Rather, Co(II) species have been shown to generate ROS via disproportionation of peroxide ligands without involving oxidation of the central cation. 44,45 Interestingly, despite the different mechanisms of ROS formation, the Co(III) complexes of 7−14 show DNase activity that is comparable to that of the Cu(II) complexes, both in the presence of ascorbic acid (Supporting Figure S12) and DTT (Supporting Figure S13). However, under more celllike conditions (i.e., in the presence of enzyme buffer), 11− Cu(II) exhibits no oxidative activity in the presence of DTT, while 11−Co(III) retains significant oxidative activity (see Supporting Figure S14).…”

Enterococcus faecalis NADH Peroxidase-Defective Mutants Stain Falsely in Colony Zymogram Assay for Extracellular Electron Transfer to Ferric Ions

“…H 2 O 2 is known to react with Fe 2+ and reduced heme in Fenton and Fenton-like reactions [ 32 ]. The oxidation of Fe 2+ would negatively interfere in the ferric reductase activity assay by removing enzyme products.…”

“…Fenton-like reactions with H 2 O 2 can result in short-lived oxidative radicals that react with organic substances, causing, for example, damage to lipids and proteins in cells. The chemistry is complex and depends on the conditions, including the pH and how the iron ion is coordinated [ 32 ]. In E. faecalis , H 2 O 2 is mainly degraded by NADH peroxidase and also when heme is available by catalase, but other peroxidases also contribute [ 24 , 25 , 35 ].…”