Reactions of low valent transition-metal complexes with hydrogen peroxide. Are they "Fenton-like" or not? 3. The case of Fe(II) [N(CH2CO2)3](H2O)2-

Tailoring the Oxidizing Intermediate in the Fenton Reaction, OH⋅ or FeIV=Oaq, by Modifying the pKa of the (H2O)5FeII(H2O2) Complex, and the Case of PDS - a DFT Study

A DFT analysis of the Fenton and Fenton-like reactions points out that the pH effect on the nature of the oxidizing intermediate formed is due to a pKa of the peroxide when hydroperoxides are used. When S2O8 2- is used, the pH effect is due to the pKa of one of the water ligands of the central iron cation. The results suggest that the choice of the hydroperoxide and the ligands present affects the pH at which the transition from the formation of hydroxyl radicals to the formation of FeIV=Oaq occurs.

Comparative Stability Study of Polysorbate 20 and Polysorbate 80 Related to Oxidative Degradation

The surfactants polysorbate 20 (PS20) and polysorbate 80 (PS80) are utilized to stabilize protein drugs. However, concerns have been raised regarding the degradation of PSs in biologics and the potential impact on product quality. Oxidation has been identified as a prevalent degradation mechanism under pharmaceutically relevant conditions. So far, a systematic stability comparison of both PSs under pharmaceutically relevant conditions has not been conducted and little is known about the dependence of oxidation on PS concentration. Here, we conducted a comparative stability study to investigate (i) the different oxidative degradation propensities between PS20 and PS80 and (ii) the impact of PS concentration on oxidative degradation. PS20 and PS80 in concentrations ranging from 0.1 mg⋅mL-1 to raw material were stored at 5, 25, and 40 °C for 48 weeks in acetate buffer pH 5.5 and water, respectively. We observed a temperature-dependent oxidative degradation of the PSs with strong (40 °C), moderate (25 °C), and weak/no degradation (5 °C). Especially at elevated temperatures such as 40 °C, fast oxidative PS degradation processes were detected. In this case study, a stronger degradation and earlier onset of oxidation was observed for PS80 in comparison to PS20, detected via the fluorescence micelle assay. Additionally, degradation was found to be strongly dependent on PS concentration, with significantly less oxidative processes at higher PS concentrations. Iron impurities, oxygen in the vial headspaces, and the pH values of the formulations were identified as the main contributing factors to accelerate PS oxidation.

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

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.

Effect of some parameters on the rate of the catalysed decomposition of hydrogen peroxide by iron(III)-nitrilotriacetate in water

The decomposition rate of H(2)O(2) by iron(III)-nitrilotriacetate complexes (Fe(III)NTA) has been investigated over a large range of experimental conditions: 3 < pH < 11, [Fe(III)](T,0): 0.05-1 mM; [NTA](T,0)/[Fe(III)](T,0) molar ratios : 1-250; [H(2)O(2)](0): 1 mM-4 M) and concentrations of HO· radical scavengers: 0-53 mM. Spectrophotometric analyses revealed that reactions of H(2)O(2) with Fe(III)NTA (1 mM) at neutral pH immediately lead to the formation of intermediates (presumably peroxocomplexes of Fe(III)NTA) which absorb light in the region 350-600 nm where Fe(III)NTA and H(2)O(2) do not absorb. Kinetic experiments showed that the decomposition rates of H(2)O(2) were first-order with respect to H(2)O(2) and that the apparent first-order rate constants were found to be proportional to the total concentration of Fe(III)NTA complexes, were at a maximum at pH 7.95 ± 0.10 and depend on the [NTA](T,0)/[Fe(III)](T,0) and [H(2)O(2)](0)/[Fe(III)](T,0) molar ratios. The addition of increasing concentrations of tert-butanol or sodium bicarbonate significantly decreased the decomposition rate of H(2)O(2), suggesting the involvement of HO· radicals in the decomposition of H(2)O(2). The decomposition of H(2)O(2) by Fe(III)NTA at neutral pH was accompanied by a production of dioxygen and by the oxidation of NTA. The degradation of the organic ligand during the course of the reaction led to a progressive decomplexation of Fe(III)NTA followed by a subsequent precipitation of iron(III) oxyhydroxides and by a significant decrease in the catalytic activity of Fe(III) species for the decomposition of H(2)O(2).

Hydroperoxide-induced DNA damage and mutations

Hydroperoxides (ROOH) are believed to play an important role in the generation of free radical damage in biology. Hydrogen peroxide (R=H) is produced by endogenous metabolic and catabolic processes in cells, while alkyl hydroperoxides (R=lipid, protein, DNA) are produced by free radical chain reactions involving molecular oxygen (autooxidation). The role of metal ions in generating DNA damage from hydroperoxides has long been recognized, and several distinct, biologically relevant mechanisms have been identified. Identification of the mechanistic pathways is important since it will largely determine the types of free radicals generated, which will largely determine the spectrum of DNA damage produced. Some mechanistic aspects of the reactions of low valent transition metal ions with ROOH and their role in mutagenesis are reviewed with a perspective on their possible role in the biological generation of DNA damage. A survey of hydroperoxide-induced mutagenesis studies is also presented. In vitro footprinting of DNA damage induced by hydroperoxides provides relevant information on sequence context dependent reactivity, and is valuable for the interpretation of mutation spectra since it represents the damage pattern prior to cellular repair. Efforts in this area are also reviewed.

Reactions of low valent transition metal complexes with hydrogen peroxide. Are they "Fenton-like" or not? 4. The case of Fe(II)L, L = edta; hedta and tcma

The question whether hydroxyl free radicals are formed in the reactions of divalent iron complexes Fe(II)L; L = edta; hedta; tcma (tcma = 1-acetato-1,4,7-triazacyclononane) with hydrogen peroxide in neutral and slightly acidic solutions was studied by using the beta elimination reaction as an assay for the formation of hydroxyl free radicals, OH. The results show that at pH < 5.5 the iron(II)peroxide intermediate complex decomposes rapidly to yield free hydroxyl radicals for L = edta and hedta. This is in contrast to the mechanism of the corresponding Fe(II)nta peroxide complex, which probably decomposes to form Fe(IV)nta which then reacts with organic substrates to yield aliphatic free radicals. Thus, the non-participating ligand L has an appreciable effect on the mechanism of reaction of the metal center with hydrogen peroxide. Blank experiments using ionizing radiation as the source of .CH2CR(CH3)OH, R = H or CH3 radicals indicate that when L = tcma intermediates of the type LFeIII-CH2CR(CH3)OHaq are formed, but their major mode of decomposition is not the beta elimination reaction. Thus, the present assay for the formation of hydroxyl free radicals by the Fenton Reaction does not fit the latter system.

The Fenton reagents

Numerous transition metal ions and their complexes in their lower oxidation states (LmMn+) were found to have the oxidative features of the Fenton reagent, and, therefore, the mixtures of these metal compounds with H2O2 were named "Fenton-like" reagents. Using the Marcus theory and the experimental data in the literature, it is shown that in most cases the reaction of these metal complexes with H2O2 is unlikely to occur via an outer-sphere electron-transfer mechanism. It is suggested that the first step in this process is the formation of a transient complex LmM-H2O2n+, which may decompose to an .OH radical or a higher oxidation state of the metal, LmM(n + 2)+, or it may yield an organic free radical in the presence of organic substrates. Thus, the question whether free .OH radicals are being formed or not via the Fenton reaction depends on the relative rates of the decomposition reactions of the metal-peroxide complex and that of its reaction with organic substrates. Contradictory conclusions described from the study of different systems might only indicate that these relative rates are different in these systems.

Factors that influence the deoxyribose oxidation assay for Fenton reaction products

The mechanism of oxidation of deoxyribose to thiobarbituric acid-reactive products by Fenton systems consisting of H2O2 and either Fe2+ or Fe2+ (EDTA) has been studied. With Fe2+ (EDTA), dependences of product yield on reactant concentrations are consistent with a reaction involving OH.. With Fe2+ in 5-50 mM phosphate buffer, yields of oxidation products were much higher and increased with increasing deoxyribose concentration up to 30 mM. The product yield varied with H2O2 and Fe2+ concentrations in a way to suggest competition between deoxyribose and both reactants. Deoxyribose oxidation by Fe2+ and H2O2 was enhanced 1.5-fold by adding superoxide dismutase, even though superoxide generated by xanthine oxidase increased deoxyribose oxidation. These results are not as expected for a reaction involving free OH. or site localized OH. product on the deoxyribose. They can be accommodated by a mechanism of deoxyribose oxidation involving an iron(IV) species formed from H2O2 and Fe2+, but the overall conclusion is that the system is too complex for definitive identification of the Fenton oxidant.