Reaction of FeaqII with Peroxymonosulfate and Peroxydisulfate in the Presence of Bicarbonate: Formation of FeaqIV and Carbonate Radical Anions

Many advanced oxidation processes (AOPs) use Fenton-like reactions to degrade organic pollutants by activating peroxymonosulfate (HSO₅^-, PMS) or peroxydisulfate (S₂O₈^2-, PDS) with Fe(H₂O)₆²⁺ (Fe_aq^II). This paper presents results on the kinetics and mechanisms of reactions between Fe_aq^II and PMS or PDS in the absence and presence of bicarbonate (HCO₃^-) at different pH. In the absence of HCO₃^-, Fe_aq^IV, rather than the commonly assumed SO₄^•-, is the dominant oxidizing species. Multianalytical methods verified the selective conversion of dimethyl sulfoxide (DMSO) and phenyl methyl sulfoxide (PMSO) to dimethyl sulfone (DMSO₂) and phenyl methyl sulfone (PMSO₂), respectively, confirming the generation of Fe_aq^IV by the Fe_aq^II-PMS/PDS systems without HCO₃^-. Significantly, in the presence of environmentally relevant concentrations of HCO₃^-, a carbonate radical anion (CO₃^•-) becomes the dominant reactive species as confirmed by the electron paramagnetic resonance (EPR) analysis. The new findings suggest that the mechanisms of the persulfate-based Fenton-like reactions in natural environments might differ remarkably from those obtained in ideal conditions. Using sulfonamide antibiotics (sulfamethoxazole (SMX) and sulfadimethoxine (SDM)) as model contaminants, our study further demonstrated the different reactivities of Fe_aq^IV and CO₃^•- in the Fe_aq^II-PMS/PDS systems. The results shed significant light on advancing the persulfate-based AOPs to oxidize pollutants in natural water.

Targeted Decomplexation of Metal Complexes for Efficient Metal Recovery by Ozone/Percarbonate

Traditional methods cannot efficiently recover Cu from Cu(II)-EDTA wastewater and encounter the formation of secondary contaminants. In this study, an ozone/percarbonate (O₃/SPC) process was proposed to efficiently decomplex Cu(II)-EDTA and simultaneously recover Cu. The results demonstrate that the O₃/SPC process achieves 100% recovery of Cu with the corresponding k_obs value of 0.103 min^-1 compared with the typical ^•OH-based O₃/H₂O₂ process (81.2%, 0.042 min^-1). The carbonate radical anion (CO₃^•-) is generated from the O₃/SPC process and carries out the targeted attack of amino groups of Cu(II)-EDTA for decarboxylation and deamination processes, resulting in successive cleavage of Cu-O and Cu-N bonds. In comparison, the ^•OH-based O₃/H₂O₂ process is predominantly responsible for the breakage of Cu-O bonds via decarboxylation and formic acid removal. Moreover, the released Cu(II) can be transformed into stable copper precipitates by employing an endogenous precipitant (CO₃^2-), accompanied by toxic-free byproducts in the O₃/SPC process. More importantly, the O₃/SPC process exhibits excellent metal recovery in the treatment of real copper electroplating wastewater and other metal-EDTA complexes. This study provides a promising technology and opens a new avenue for the efficient decomplexation of metal-organic complexes with simultaneous recovery of valuable metal resources.

One- and Two-Electron Oxidations of β-Amyloid25-35 by Carbonate Radical Anion (CO3•-) and Peroxymonocarbonate (HCO4-): Role of Sulfur in Radical Reactions and Peptide Aggregation

The β-amyloid (Aβ) peptide plays a key role in the pathogenesis of Alzheimer’s disease. The methionine (Met) residue at position 35 in Aβ C-terminal domain is critical for neurotoxicity, aggregation, and free radical formation initiated by the peptide. The role of Met in modulating toxicological properties of Aβ most likely involves an oxidative event at the sulfur atom. We therefore investigated the one- or two-electron oxidation of the Met residue of Aβ_25-35 fragment and the effect of such oxidation on the behavior of the peptide. Bicarbonate promotes two-electron oxidations mediated by hydrogen peroxide after generation of peroxymonocarbonate (HCO₄⁻, PMC). The bicarbonate/carbon dioxide pair stimulates one-electron oxidations mediated by carbonate radical anion (CO₃^•−). PMC efficiently oxidizes thioether sulfur of the Met residue to sulfoxide. Interestingly, such oxidation hampers the tendency of Aβ to aggregate. Conversely, CO₃^•− causes the one-electron oxidation of methionine residue to sulfur radical cation (MetS^•+). The formation of this transient reactive intermediate during Aβ oxidation may play an important role in the process underlying amyloid neurotoxicity and free radical generation.

Cu,Zn-superoxide dismutase-driven free radical modifications: copper- and carbonate radical anion-initiated protein radical chemistry

The understanding of the mechanism, oxidant(s) involved and how and what protein radicals are produced during the reaction of wild-type SOD1 (Cu,Zn-superoxide dismutase) with H2O2 and their fate is incomplete, but a better understanding of the role of this reaction is needed. We have used immuno-spin trapping and MS analysis to study the protein oxidations driven by human (h) and bovine (b) SOD1 when reacting with H2O2 using HSA (human serum albumin) and mBH (mouse brain homogenate) as target models. In order to gain mechanistic information about this reaction, we considered both copper- and CO3(*-) (carbonate radical anion)-initiated protein oxidation. We chose experimental conditions that clearly separated SOD1-driven oxidation via CO(*-) from that initiated by copper released from the SOD1 active site. In the absence of (bi)carbonate, site-specific radical-mediated fragmentation is produced by SOD1 active-site copper. In the presence of (bi)carbonate and DTPA (diethylenetriaminepenta-acetic acid) (to suppress copper chemistry), CO(*-) produced distinct radical sites in both SOD1 and HSA, which caused protein aggregation without causing protein fragmentation. The CO(*-) produced by the reaction of hSOD1 with H2O2 also produced distinctive DMPO (5,5-dimethylpyrroline-N-oxide) nitrone adduct-positive protein bands in the mBH. Finally, we propose a biochemical mechanism to explain CO(*-) production from CO2, enhanced protein radical formation and protection by (bi)carbonate against H2O2-induced fragmentation of the SOD1 active site. Our present study is important for establishing experimental conditions for studying the molecular mechanism and targets of oxidation during the reverse reaction of SOD1 with H2O2; these results are the first step in analysing the critical targets of SOD1-driven oxidation during pathological processes such as neuroinflammation.

Intrastrand G-U cross-links generated by the oxidation of guanine in 5'-d(GCU) and 5'-r(GCU)

It has been suggested that carbonate radical anions are biologically important because they may be produced during the inflammatory response. The carbonate radicals can selectively oxidize guanine in DNA and RNA by one-electron transfer mechanisms and the guanine radicals thus formed decay by diverse competing pathways with other free radicals or nucleophiles. Using a photochemical method to generate CO(3)(-) radicals in vitro, we compare the distributions of products initiated by the one-electron oxidation of guanine in the trinucleotides 5'-r(GpCpU) and 5'-d(GpCpU) in aqueous buffer solutions (pH 7.5). Similar distributions of stable end products identified by LC-MS/MS methods were found in both cases. The guanine oxidation products include the diastereomeric pair of spiroiminodihydantoin (Sp) and 2,5-diamino-4H-imidazolone (Iz). In addition, intrastrand cross-linked products involving covalent bonds between the G and the U bases (GCU) were also found, although with different relative yields in the 2'-deoxy- and the ribotrinucleotides. The positive-ion MS/MS spectra of the 5'-r(GpCpU) and 5'-d(GpCpU) products clearly indicate the presence of covalently linked G-U products that have a mass smaller by 2 Da than the sum of the G and U bases in both types of trinucleotides. The 5'-d(GCU) cross-linked product was further characterized by 1D and 2D NMR methods that confirm its cyclic structure in which the guanine C8 atom is covalently linked to the uracil N3 atom.