Catalytic generation of chlorine dioxide from chlorite using a water-soluble manganese porphyrin

Aqueous Zinc-Chlorine Battery Modulated by a MnO2 Redox Adsorbent

Aqueous zinc-chlorine battery with high discharge voltage and attractive theoretical energy density is expected to become an important technology for large-scale energy storage. However, the practical application of Zn-Cl2 batteries has been restricted due to the Cl2 cathode with sluggish kinetics and low Coulombic efficiency (CE). Here, an aqueous Zn-Cl2 battery using an inexpensive and effective MnO2 redox adsorbent (referred to Zn-Cl2@MnO2 battery) to modulate the electrochemical performance of the Cl2 cathode is developed. Density functional theory calculations reveal that the existence of the intermediate state Clads free radical catalyzed by MnO2 on the Cl2 cathode contributes to the charge storage capacity, which is the key to modulate the electrode and improve the electrochemical performance. Further analysis of the Cl2 cathode kinetics discloses the adsorption and catalytic roles of the MnO2 redox adsorbent. The Zn-Cl2@MnO2 battery displays an enhanced discharge voltage of 2.0 V at a current density of 2.5 mA cm-2, and stable 1000 cycles with an average CE of 91.6%, much superior to the conventional Zn-Cl2 battery with an average CE of only 66.8%. The regulation strategy to the Cl2 cathode provides opportunities for the future development of aqueous Zn-Cl2 batteries.

Catalytic Degradation of Triphenylmethane Dyes with an Iron Porphyrin Complex as a Cytochrome P450 Model

The organic dyes used in printing and dyeing wastewater have complex components, diverse structures and strong chemical stability, which make them not suitable for treatment and difficult to degrade in the environment. Porphyrins are macromolecules with 18 π electrons formed by four pyrrole molecules connected with a methylene bridge that has a stable structure. Porphyrin combines with iron to form an active intermediate with a structure similar to the cytochrome P450 enzyme, so they are widely used in the biomimetic field. In the current study, 5,10,15,20-tetra (4-carboxyphenyl) porphine ferric chloride (III) (Fe(III)TCPP) was used as a catalyst and iodosobenzene was used as an oxidant to explore the catalytic degradation of triphenylmethane dyes, such as rhodamine B (RhB) and malachite green (MG). The results of UV-Vis spectral analysis have shown that the conversion rate of the rhodamine B was over 90% when the amount of Fe(III)TCPP was 0.027 mM and the amount of iodosobenzene was eight equivalents. When the catalyst was 0.00681 mM and the amount of the oxidant was five equivalents, the conversion rate of the malachite green reached over 95%. This work provides a feasible method for the degradation of triphenylmethane dyes.

Iron species activating chlorite: Neglected selective oxidation for water treatment

Chlorite (ClO₂ ^-) is the by-product of the water treatment process carried out using chlorine dioxide (ClO₂) as an effective disinfectant and oxidant; however, the reactivation of ClO₂ ^- has commonly been overlooked. Herein, it was unprecedentedly found that ClO₂ ^- could be activated by iron species (Fe_b: Fe⁰, Fe^II, or Fe^III), which contributed to the synchronous removal of ClO₂ ^- and selective oxidative treatment of organic contaminants. However, the above-mentioned activation process presented intensive H⁺-dependent reactivity. The introduction of Fe_b significantly shortened the autocatalysis process via the accumulation of Cl^- or ClO^- during the protonation of ClO₂ ^- driven by ultrasonic field. Furthermore, it was found that the interdependent high-valent-Fe-oxo and ClO₂, after identification, were the dominant active species for accelerating the oxidation process. Accordingly, the unified mechanisms based on coordination catalysis ([Fe ^N (H₂O) _a (ClO _x ^m-) _b ] ^{n +}-P) were putative, and this process was thus used to account for the pollutant removal by the Fe_b-activated protonated ClO₂ ^-. This study pioneers the activation of ClO₂ ^- for water treatment and provides a novel strategy for "waste treating waste". Derivatively, this activation process further provides the preparation methods for sulfones and ClO₂, including the oriented oxidation of sulfoxides to sulfones and the production of ClO₂ for on-site use.

Kinetic Role of Reactive Intermediates in Controlling the Formation of Chlorine Dioxide in the Hypochlorous Acid-Chlorite Ion Reaction

An advanced experimental protocol is reported for studying the kinetics and mechanism of the complex redox reaction between chlorite ion and hypochlorous acid under acidic condition. The formation of ClO₂ is followed directly by the classical two-component stopped-flow method. In sequential stopped-flow experiments, the target reaction is chemically quenched using NaI solution and the concentration of each reactant and product is monitored as a function of time by utilizing the principles of kinetic discrimination. Thus, in contrast to earlier studies, not only the formation of one of the products but the decay of the reactants was also directly followed. This approach provides a firm basis for postulating a detailed mechanism for the interpretation of the experimental results under a variety of conditions. The intimate details of the reaction are explored by simultaneously fitting 78 kinetic traces, i.e., the concentration vs. time profiles of ClO₂^-, HOCl, and ClO₂, to an 11-step kinetic model. The most important reaction steps were identified, and it was shown that two reactive intermediates have a pivotal role in the mechanism. While chlorate ion predominantly forms via the reaction of Cl₂O, chlorine dioxide is exclusively produced in reaction steps involving Cl₂O₂. This study leads to clear conclusions on how to control the stoichiometry of the reaction and achieve optimum conditions to produce chlorine dioxide and to reduce the formation of the toxic chlorate ion in practical applications.

Revealing the Generation of High-Valent Cobalt Species and Chlorine Dioxide in the Co3O4-Activated Chlorite Process: Insight into the Proton Enhancement Effect

A Co₃O₄-activated chlorite (Co₃O₄/chlorite) process was developed to enable the simultaneous generation of high-valent cobalt species [Co(IV)] and ClO₂ for efficient oxidation of organic contaminants. The formation of Co(IV) in the Co₃O₄/chlorite process was demonstrated through phenylmethyl sulfoxide (PMSO) probe and ¹⁸O-isotope-labeling tests. Both experiments and theoretical calculations revealed that chlorite activation involved oxygen atom transfer (OAT) during Co(IV) formation and proton-coupled electron transfer (PCET) in the Co(IV)-mediated ClO₂ generation. Protons not only promoted the generation of Co(IV) and ClO₂ by lowering the energy barrier but also strengthened the resistance of the Co₃O₄/chlorite process to coexisting anions, which we termed a proton enhancement effect. Although both Co(IV) and ClO₂ exhibited direct oxidation of contaminants, their contributions varied with pH changes. When pH increased from 3 to 5, the deprotonation of contaminants facilitated the electrophilic attack of ClO₂, while as pH increased from 5 to 8, Co(IV) gradually became the main contributor to contaminant degradation owing to its higher stability than ClO₂. Moreover, ClO₂^- was transformed into nontoxic Cl^- rather than ClO₃^- after the reaction, thus greatly reducing possible environmental risks. This work described a Co(IV)-involved chlorite activation process for efficient removal of organic contaminants, and a proton enhancement mechanism was revealed.

Chlorine Dioxide: Friend or Foe for Cell Biomolecules? A Chemical Approach

This review examines the role of chlorine dioxide (ClO₂) on inorganic compounds and cell biomolecules. As a disinfectant also present in drinking water, ClO₂ helps to destroy bacteria, viruses, and some parasites. The Environmental Protection Agency EPA regulates the maximum concentration of chlorine dioxide in drinking water to be no more than 0.8 ppm. In any case, human consumption must be strictly regulated since, given its highly reactive nature, it can react with and oxidize many of the inorganic compounds found in natural waters. Simultaneously, chlorine dioxide reacts with natural organic matter in water, including humic and fulvic acids, forming oxidized organic compounds such as aldehydes and carboxylic acids, and rapidly oxidizes phenolic compounds, amines, amino acids, peptides, and proteins, as well as the nicotinamide adenine dinucleotide NADH, responsible for electron and proton exchange and energy production in all cells. The influence of ClO₂ on biomolecules is derived from its interference with redox processes, modifying the electrochemical balances in mitochondrial and cell membranes. This discourages its use on an individual basis and without specialized monitoring by health professionals.

Iron Porphyrin as a Cytochrome P450 Model for the Degradation of Dye

Organic dyes are widely used in the textile, biological, medical and other fields. However, a serious environmental problem has appeared because of the presence of organic dyes in industrial aqueous effluents. Thus, the efficient treatment of organic dyes in industrial wastewaters is currently in real demand. The current study investigated the oxidative degradation of the organic dye gentian violet by meso-tetra(carboxyphenyl) porphyriniron(III), [Fe^III(TCPP)] as a cytochrome P450 model and iodosylbenzene (PhIO) as an oxidant at room temperature. The degradation reaction was monitored by UV-vis absorption spectroscopy via the observation of UV-vis spectral changes of the gentian violet. The results showed that the efficiency of catalyzed degradation reached more than 90% in 1 h, indicating the remarkable oxidative degradation capacity of the [Fe^III(TCPP)]/PhIO system, which provided an efficient approach for the treatment of dyeing wastewater.

pH-Regulating Nanoplatform for the "Double Channel Chase " of Tumor Cells by the Synergistic Cascade between Chlorine Treatment and Methionine-Depletion Starvation Therapy

During rapid proliferation and metabolism, tumor cells show a high dependence on methionine. The deficiency of methionine exhibits significant inhibition on tumor growth, which provides a potential therapeutic target in tumor therapy. Herein, ClO₂-loaded nanoparticles (fluvastatin sodium&metformin&bupivacaine&ClO₂@CaSiO₃@MnO₂-arginine-glycine-aspatic acid (RGD) (MFBC@CMR) NPs) were prepared for synergistic chlorine treatment and methionine-depletion starvation therapy. After outer layer MnO₂ was degraded in the high glutathione (GSH) tumor microenvironment (TME), MFBC@CMR NPs released metformin (Me) to target the mitochondria, thus interfering with the tricarboxylic acid (TCA) cycle and promoting the production of lactate. In addition, released fluvastatin sodium (Flu) by the NPs acted on monocarboxylic acid transporter 4 (MCT4) in the cell membrane to inhibit lactate leakage and induce a decrease of intracellular pH, further prompting the NPs to release chlorine dioxide (ClO₂), which then oxidized methionine, inhibited tumor growth, and produced large numbers of Cl^- in the cytoplasm. Cl^- could enter mitochondria through the voltage-dependent anion channel (VDAC) channel, which was opened by bupivacaine (Bup). The disruption of Cl^- homeostasis promotes mitochondrial damage and membrane potential decline, leading to the release of cytochrome C (Cyt-C) and apoptosis inducing factor (AIF) and further inducing cell apoptosis. To sum up, the pH-regulating and ClO₂-loaded MFBC@CMR nanoplatform can achieve cascade chlorine treatment and methionine-depletion starvation therapy toward tumor cells, which is of great significance for improving the clinical tumor treatment effect.

Insights into the electronic structure and mechanism of norcarane hydroxylation by OxoMn(V) porphyrin complexes: A density functional theory study

Norcarane hydroxylation by neutral [PorMn(V)O-L] (L═OH^- , F^- ) and cationic [PorMn(V)O-L]⁺ (L═H₂ O, imidazole) oxoMn(V) porphyrin complex models has been investigated by density functional theory calculations to better understand the reaction mechanism and electronic structure. We found that the energy barriers of norcarane hydroxylation by cationic oxoMn(V) porphyrin complexes are lower than those by neutral oxoMn(V) porphyrin complexes. This indicates that cationic oxoMn(V) porphyrin complexes enhance norcarane hydroxylation compared with neutral oxoMn(V) porphyrin complexes. According to electronic structure analysis, in the C─H activation step, electron transfer occurs through initial interaction between the σ_CH and rich-oxygen π(Mn═O) orbitals to form real donor orbitals, followed by transfer to the acceptor π*(Mn═O) orbitals. Moreover, single electron shifts from norcarane (CH) to Mn atom during C─H activation. The positive charge of the cationic complex stabilizes the acceptor orbital more than the donor orbital, reducing the energy gap between these orbitals, thus lowering the reaction barrier.

Lignin-Derived Non-Heme Iron and Manganese Complexes: Catalysts for the On-Demand Production of Chlorine Dioxide in Water under Mild Conditions

A lignin-derived ligand, bis(2-hydroxy-3-methoxy-5-propylbenzyl)glycine (DHEG), was synthesized from 2-methoxy-4-propylphenol (dihydroeugenol (DHE)) and the amino acid glycine. Two mononuclear iron and manganese complexes of DHEG were prepared, characterized, and employed for the oxidation of chlorite to chlorine dioxide in aqueous solution. Peroxyacetic acid (PAA) was used as a "green" oxidant in the redox reactions to initiate the formation of high-valent Fe and Mn (IV)-OH intermediates. EPR studies verified the formation of a high-valent Mn^IV species. Both Fe and Mn complexes catalyzed chlorite oxidation with bimolecular rate constants of 32 and 144 M^-1 s^-1, respectively, at pH 4.0 and 25 °C. The Mn complex was found to be more efficient for chlorite oxidation with a turnover frequency of 17 h^-1 and remained active during subsequent additions of PAA. The rate of ClO₂ decomposition with PAA/Mn-DHEG was first order in PAA and increased significantly as pH increased. A mechanism that accounts for all observations is presented.

Atomically dispersed Pt-N4 sites as efficient and selective electrocatalysts for the chlorine evolution reaction

Chlorine evolution reaction (CER) is a critical anode reaction in chlor-alkali electrolysis. Although precious metal-based mixed metal oxides (MMOs) have been widely used as CER catalysts, they suffer from the concomitant generation of oxygen during the CER. Herein, we demonstrate that atomically dispersed Pt-N4 sites doped on a carbon nanotube (Pt1/CNT) can catalyse the CER with excellent activity and selectivity. The Pt1/CNT catalyst shows superior CER activity to a Pt nanoparticle-based catalyst and a commercial Ru/Ir-based MMO catalyst. Notably, Pt1/CNT exhibits near 100% CER selectivity even in acidic media, with low Cl- concentrations (0.1 M), as well as in neutral media, whereas the MMO catalyst shows substantially lower CER selectivity. In situ electrochemical X-ray absorption spectroscopy reveals the direct adsorption of Cl- on Pt-N4 sites during the CER. Density functional theory calculations suggest the PtN4C12 site as the most plausible active site structure for the CER.

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal-oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal-oxo species, are the basis for the various biological functions of O2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and transport, degradation of reactive oxygen species, redox signaling, and biological oxygenation, etc., but also have driven the development of bioinorganic chemistry and biomimetic catalysis. In this review, we survey the range of O2 activation processes mediated by heme proteins and model compounds with a focus on recent progress in the characterization and reactivity of important iron-oxygen intermediates. Representative reactions initiated by these reactive intermediates as well as some context from prior decades will also be presented. We will discuss the fundamental mechanistic features of these transformations and delineate the underlying structural and electronic factors that contribute to the spectrum of reactivities that has been observed in nature as well as those that have been invented using these paradigms. Given the recent developments in biocatalysis for non-natural chemistries and the renaissance of radical chemistry in organic synthesis, we envision that new enzymatic and synthetic transformations will emerge based on the radical processes mediated by metalloproteins and their synthetic analogs.

Biomimetic Reactivity of Oxygen-Derived Manganese and Iron Porphyrinoid Complexes

Heme proteins utilize the heme cofactor, an iron porphyrin, to perform a diverse range of reactions including dioxygen binding and transport, electron transfer, and oxidation/oxygenations. These reactions share several key metalloporphyrin intermediates, typically derived from dioxygen and its congeners such as hydrogen peroxide. These species are composed of metal-dioxygen, metal-superoxo, metal-peroxo, and metal-oxo adducts. A wide variety of synthetic metalloporphyrinoid complexes have been synthesized to generate and stabilize these intermediates. These complexes have been studied to determine the spectroscopic features, structures, and reactivities of such species in controlled and well-defined environments. In this Review, we summarize recent findings on the reactivity of these species with common porphyrinoid scaffolds employed for biomimetic studies. The proposed mechanisms of action are emphasized. This Review is organized by structural type of metal-oxygen intermediate and broken into subsections based on the metal (manganese and iron) and porphyrinoid ligand (porphyrin, corrole, and corrolazine).

Immobilized Lignin Peroxidase-Like Metalloporphyrins as Reusable Catalysts in Oxidative Bleaching of Industrial Dyes

Synthetic and bioinspired metalloporphyrins are a class of redox-active catalysts able to emulate several enzymes such as cytochromes P450, ligninolytic peroxidases, and peroxygenases. Their ability to perform oxidation and degradation of recalcitrant compounds, including aliphatic hydrocarbons, phenolic and non-phenolic aromatic compounds, sulfides, and nitroso-compounds, has been deeply investigated. Such a broad substrate specificity has suggested their use also in the bleaching of textile plant wastewaters. In fact, industrial dyes belong to very different chemical classes, being their effective and inexpensive oxidation an important challenge from both economic and environmental perspective. Accordingly, we review here the most widespread synthetic metalloporphyrins, and the most promising formulations for large-scale applications. In particular, we focus on the most convenient approaches for immobilization to conceive economical affordable processes. Then, the molecular routes of catalysis and the reported substrate specificity on the treatment of the most diffused textile dyes are encompassed, including the use of redox mediators and the comparison with the most common biological and enzymatic alternative, in order to depict an updated picture of a very promising field for large-scale applications.

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

Proton-coupled electron transfers (PCETs) are unconventional redox processes in which both protons and electrons are exchanged, often in a concerted elementary step. While PCET is now recognized to play a central a role in biological redox catalysis and inorganic energy conversion technologies, its applications in organic synthesis are only beginning to be explored. In this chapter, we aim to highlight the origins, development, and evolution of the PCET processes most relevant to applications in organic synthesis. Particular emphasis is given to the ability of PCET to serve as a non-classical mechanism for homolytic bond activation that is complimentary to more traditional hydrogen atom transfer processes, enabling the direct generation of valuable organic radical intermediates directly from their native functional group precursors under comparatively mild catalytic conditions. The synthetically advantageous features of PCET reactivity are described in detail, along with examples from the literature describing the PCET activation of common organic functional groups.

An educational overview of the chemistry, biochemistry and therapeutic aspects of Mn porphyrins--From superoxide dismutation to H2O2-driven pathways

Most of the SOD mimics thus far developed belong to the classes of Mn-(MnPs) and Fe porphyrins(FePs), Mn(III) salens, Mn(II) cyclic polyamines and metal salts. Due to their remarkable stability we have predominantly explored Mn porphyrins, aiming initially at mimicking kinetics and thermodynamics of the catalysis of O2(-) dismutation by SOD enzymes. Several MnPs are of potency similar to SOD enzymes. The in vivo bioavailability and toxicity of MnPs have been addressed also. Numerous in vitro and in vivo studies indicate their impressive therapeutic efficacy. Increasing insight into complex cellular redox biology has been accompanied by increasing awareness of complex redox chemistry of MnPs. During O2(-) dismutation process, the most powerful Mn porphyrin-based SOD mimics reduce and oxidize O2(-) with close to identical rate constants. MnPs reduce and oxidize other reactive species also (none of them specific to MnPs), acting as reductants (antioxidant) and pro-oxidants. Distinction must be made between the type of reactions of MnPs and the favorable therapeutic effects we observe; the latter may be of either anti- or pro-oxidative nature. H2O2/MnP mediated oxidation of protein thiols and its impact on cellular transcription seems to dominate redox biology of MnPs. It has been thus far demonstrated that the ability of MnPs to catalyze O2(-) dismutation parallels all other reactivities (such as ONOO(-) reduction) and in turn their therapeutic efficacies. Assuming that all diseases have in common the perturbation of cellular redox environment, developing SOD mimics still seems to be the appropriate strategy for the design of potent redox-active therapeutics.

Manganese Catalyzed C-H Halogenation

The remarkable aliphatic C-H hydroxylations catalyzed by the heme-containing enzyme, cytochrome P450, have attracted sustained attention for more than four decades. The effectiveness of P450 enzymes as highly selective biocatalysts for a wide range of oxygenation reactions of complex substrates has driven chemists to develop synthetic metalloporphyrin model compounds that mimic P450 reactivity. Among various known metalloporphyrins, manganese derivatives have received considerable attention since they have been shown to be versatile and powerful mediators for alkane hydroxylation and olefin epoxidation. Mechanistic studies have shown that the key intermediates of the manganese porphyrin-catalyzed oxygenation reactions include oxo- and dioxomanganese(V) species that transfer an oxygen atom to the substrate through a hydrogen abstraction/oxygen recombination pathway known as the oxygen rebound mechanism. Application of manganese porphyrins has been largely restricted to catalysis of oxygenation reactions until recently, however, due to ultrafast oxygen transfer rates. In this Account, we discuss recently developed carbon-halogen bond formation, including fluorination reactions catalyzed by manganese porphyrins and related salen species. We found that biphasic sodium hypochlorite/manganese porphyrin systems can efficiently and selectively convert even unactivated aliphatic C-H bonds to C-Cl bonds. An understanding of this novel reactivity derived from results obtained for the oxidation of the mechanistically diagnostic substrate and radical clock, norcarane. Significantly, the oxygen rebound rate in Mn-mediated hydroxylation is highly correlated with the nature of the trans-axial ligands bound to the manganese center (L-Mn(V)═O). Based on the ability of fluoride ion to decelerate the oxygen rebound step, we envisaged that a relatively long-lived substrate radical could be trapped by a Mn-F fluorine source, effecting carbon-fluorine bond formation. Indeed, this idea led to the discovery of the first Mn-catalyzed direct aliphatic C-H fluorination reactions utilizing simple, nucleophilic fluoride salts. Mechanistic studies and DFT calculations have revealed a trans-difluoromanganese(IV) species as the key fluorine transfer intermediate. In addition to catalyzing normal (19)F-fluorination reactions, manganese salen complexes were found to enable the incorporation of radioactive (18)F fluorine via C-H activation. This advance represented the first direct Csp(3)-H bond (18)F labeling with no-carrier-added [(18)F]fluoride and facilitated the late-stage labeling of drug molecules for PET imaging. Given the high reactivity and enzymatic-like selectively of metalloporphyrins, we envision that this new Heteroatom-Rebound Catalysis (HRC) strategy will find widespread application in the C-H functionalization arena and serve as an effective tool for forming new carbon-heteroatom bonds at otherwise inaccessible sites in target molecules.

Mechanistic study of a manganese porphyrin catalyst for on-demand production of chlorine dioxide in water

A water-soluble manganese porphyrin complex was examined for the catalytic formation of chlorine dioxide from chlorite under ambient temperature at pH 5.00 and 6.90. Quantitative kinetic modeling allowed for the deduction of a mechanism that accounts for all experimental observations. Catalysis is initiated via an OAT (Oxygen Atom Transfer) reaction resulting in formation of a putative manganese(V) oxo species, which undergoes ET (Electron Transfer) with chlorite to form chlorine dioxide. As chlorine dioxide accumulates in solution, chlorite consumption slows down and ClO 2 reaches a maximum as the system reaches equilibrium. In phosphate buffer at pH 6.90, manganese(IV) oxo accumulates and its reaction with ClO 2 gives ClO 3-. However, at pH 5.00 acetate buffer proton coupled electron transfer (PCET) from chlorite to manganese(IV) oxo is fast and irreversible leading to chlorate formation only via the putative manganese(V) oxo species. These differences underscore how PCET rates affect reaction pathways and mechanism. The ClO 2 product can be collected from the aqueous reaction mixture via purging with an inert gas, allowing for the preparation of chlorine dioxide on-demand.

Production of dioxygen in the dark: dismutases of oxyanions

O₂-generating reactions are exceedingly rare in biology and difficult to mimic synthetically. Perchlorate-respiring bacteria enzymatically detoxify chlorite (ClO₂(-) ), the end product of the perchlorate (ClO(4)(-) ) respiratory pathway, by rapidly converting it to dioxygen (O₂) and chloride (Cl(-)). This reaction is catalyzed by a heme-containing protein, called chlorite dismutase (Cld), which bears no structural or sequence relationships with known peroxidases or other heme proteins and is part of a large family of proteins with more than one biochemical function. The original assumptions from the 1990s that perchlorate is not a natural product and that perchlorate respiration might be confined to a taxonomically narrow group of species have been called into question, as have the roles of perchlorate respiration and Cld-mediated reactions in the global biogeochemical cycle of chlorine. In this chapter, the chemistry and biochemistry of Cld-mediated O₂generation, as well as the biological and geochemical context of this extraordinary reaction, are described.

The kinetics and mechanism of the oxidation of pyruvate ion by hypochlorous acid

Kinetic experiments and DFT calculations confirm a concerted oxygen atom transfer mechanism for the oxidation of pyruvic acid by HOCl.

Selective hydroxylation of benzene derivatives and alkanes with hydrogen peroxide catalysed by a manganese complex incorporated into mesoporous silica–alumina

Selective hydroxylation of benzene to phenol with hydrogen peroxide was efficiently catalysed by a manganese complex incorporated into mesoporous silica–alumina.

Non-heme manganese catalysts for on-demand production of chlorine dioxide in water and under mild conditions

Two non-heme manganese complexes are used in the catalytic formation of chlorine dioxide from chlorite under ambient temperature at pH 5.00. The catalysts afford up to 1000 turnovers per hour and remain highly active in subsequent additions of chlorite. Kinetic and spectroscopic studies revealed a Mn(III)(OH) species as the dominant form under catalytic conditions. A Mn(III)(μ-O)Mn(IV) dinuclear species was observed by EPR spectroscopy, supporting the involvement of a putative Mn(IV)(O) species. First-order kinetic dependence on the manganese catalyst precludes the dinuclear species as the active form of the catalyst. Quantitative kinetic modeling enabled the deduction of a mechanism that accounts for all experimental observations. The chlorine dioxide producing cycle involves formation of a putative Mn(IV)(O), which undergoes PCET (proton coupled electron-transfer) reaction with chlorite to afford chlorine dioxide. The ClO2 product can be efficiently removed from the aqueous reaction mixture via purging with an inert gas, allowing for the preparation of pure chlorine dioxide for on-site use and further production of chlorine dioxide.

Rapid healing of cutaneous leishmaniasis by high-frequency electrocauterization and hydrogel wound care with or without DAC N-055: a randomized controlled phase IIa trial in Kabul

Background

Anthroponotic cutaneous leishmaniasis (CL) due to Leishmania (L.) tropica infection is a chronic, frequently disfiguring skin disease with limited therapeutic options. In endemic countries healing of ulcerative lesions is often delayed by bacterial and/or fungal infections. Here, we studied a novel therapeutic concept to prevent superinfections, accelerate wound closure, and improve the cosmetic outcome of ACL.

Methodology/Principal Findings

From 2004 to 2008 we performed a two-armed, randomized, double-blinded, phase IIa trial in Kabul, Afghanistan, with patients suffering from L. tropica CL. The skin lesions were treated with bipolar high-frequency electrocauterization (EC) followed by daily moist-wound-treatment (MWT) with polyacrylate hydrogel with (group I) or without (group II) pharmaceutical sodium chlorite (DAC N-055). Patients below age 5, with facial lesions, pregnancy, or serious comorbidities were excluded. The primary, photodocumented outcome was the time needed for complete lesion epithelialization. Biopsies for parasitological and (immuno)histopathological analyses were taken prior to EC (1^st), after wound closure (2^nd) and after 6 months (3^rd). The mean duration for complete wound closure was short and indifferent in group I (59 patients, 43.1 d) and II (54 patients, 42 d; p = 0.83). In patients with Leishmania-positive 2^nd biopsies DAC N-055 caused a more rapid wound epithelialization (37.2 d vs. 58.3 d; p = 0.08). Superinfections occurred in both groups at the same rate (8.8%). Except for one patient, reulcerations (10.2% in group I, 18.5% in group II; p = 0.158) were confined to cases with persistent high parasite loads after healing. In vitro, DAC N-055 showed a leishmanicidal effect on pro- and amastigotes.

Conclusions/Significance

Compared to previous results with intralesional antimony injections, the EC plus MWT protocol led to more rapid wound closure. The tentatively lower rate of relapses and the acceleration of wound closure in a subgroup of patients with parasite persistence warrant future studies on the activity of DAC N-055.

Trial Registration

ClinicalTrails.gov NCT00947362

In many countries of the Middle East such as Afghanistan, cutaneous leishmaniasis is a highly prevalent, chronic and stigmatizing skin disease. Poor hygiene conditions frequently aggravate the lesions due to bacterial and fungal superinfections. Classical treatments with injections of pentavalent antimony are hampered by costs, side effects, resistance development, supply and manufactural quality problems. In the present study on Afghan patients with Leishmania tropica-induced skin lesions we evaluated the clinical effect of an initial removal of lesion tissue by electrocoagulation using a bipolar high-frequency electrosurgery instrument, followed by daily moist wound treatment with or without a preparation of pharmaceutical sodium chlorite (DAC N-055). DAC N-055 is a compound with anti-infective, immunomodulatory and tissue repair-promoting effects. Our analysis revealed that the carefully performed moist wound treatment led to a rapid healing of the wounds within an average period of 6 weeks, even in the absence of the sodium chlorite preparation. This is considerably faster than the time spans previously reported for local or systemic antimony treatment. We believe that the current standard for local care of chronic wounds should also be applied to Leishmania skin lesions. If combined with an initial single high-frequency electrocoagulation, it is a highly effective, inexpensive and well-tolerated treatment option for cutaneous leishmaniasis.

Efficient water oxidation catalyzed by homogeneous cationic cobalt porphyrins with critical roles for the buffer base

A series of cationic cobalt porphyrins was found to catalyze electrochemical water oxidation to O2 efficiently at room temperature in neutral aqueous solution. Co-5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrin, with a highly electron-deficient meso-dimethylimidazolium porphyrin, was the most effective catalyst. The O2 formation rate was 170 nmol · cm(-2) · min(-1) (k(obs) = 1.4 × 10(3) s(-1)) with a Faradaic efficiency near 90%. Mechanistic investigations indicate the generation of a Co(IV)-O porphyrin cation radical as the reactive oxidant, which has accumulated two oxidizing equivalents above the Co(III) resting state of the catalyst. The buffer base in solution was shown to play several critical roles during the catalysis by facilitating both redox-coupled proton transfer processes leading to the reactive oxidant and subsequent O-O bond formation. More basic buffer anions led to lower catalytic onset potentials, extending below 1 V. This homogeneous cobalt-porphyrin system was shown to be robust under active catalytic conditions, showing negligible decomposition over hours of operation. Added EDTA or ion exchange resin caused no catalyst poisoning, indicating that cobalt ions were not released from the porphyrin macrocycle during catalysis. Likewise, surface analysis by energy dispersive X-ray spectroscopy of the working electrodes showed no deposition of heterogeneous cobalt films. Taken together, the results indicate that Co-5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrin is an efficient, homogeneous, single-site water oxidation catalyst.

Comprehensive pharmacokinetic studies and oral bioavailability of two Mn porphyrin-based SOD mimics, MnTE-2-PyP5+ and MnTnHex-2-PyP5+

The cationic, ortho Mn(III) N-alkylpyridylporphyrins (alkyl=ethyl, E, and n-hexyl, nHex) MnTE-2-PyP(5+) (AEOL10113, FBC-007) and MnTnHex-2-PyP(5+) have proven efficacious in numerous in vivo animal models of diseases having oxidative stress in common. The remarkable therapeutic efficacy observed is due to their: (1) ability to catalytically remove O2(•-) and ONOO(-) and other reactive species; (2) ability to modulate redox-based signaling pathways; (3) accumulation within critical cellular compartments, i.e., mitochondria; and (4) ability to cross the blood-brain barrier. The similar redox activities of both compounds are related to the similar electronic and electrostatic environments around the metal active sites, whereas their different bioavailabilities are presumably influenced by the differences in lipophilicity, bulkiness, and shape. Both porphyrins are water soluble, but MnTnHex-2-PyP(5+) is approximately 4 orders of magnitude more lipophilic than MnTE-2-PyP(5+), which should positively affect its ability to pass through biological membranes, making it more efficacious in vivo at lower doses. To gain insight into the in vivo tissue distribution of Mn porphyrins and its impact upon their therapeutic efficacy and mechanistic aspects of action, as well as to provide data that would ensure proper dosing regimens, we conducted comprehensive pharmacokinetic (PK) studies for 24h after single-dose drug administration. The porphyrins were administered intravenously (iv), intraperitoneally (ip), and via oral gavage at the following doses: 10mg/kg MnTE-2-PyP(5+) and 0.5 or 2mg/kg MnTnHex-2-PyP(5+). Drug levels in plasma and various organs (liver, kidney, spleen, heart, lung, brain) were determined and PK parameters calculated (Cmax, C24h, tmax, and AUC). Regardless of high water solubility and pentacationic charge of these Mn porphyrins, they are orally available. The oral availability (based on plasma AUCoral/AUCiv) is 23% for MnTE-2-PyP(5+) and 21% for MnTnHex-2-PyP(5+). Despite the fivefold lower dose administered, the AUC values for liver, heart, and spleen are higher for MnTnHex-2-PyP(5+) than for MnTE-2-PyP(5+) (and comparable for other organs), clearly demonstrating the better tissue penetration and tissue retention of the more lipophilic MnTnHex-2-PyP(5+).

Design, mechanism of action, bioavailability and therapeutic effects of mn porphyrin-based redox modulators

Based on aqueous redox chemistry and simple in vivo models of oxidative stress, Escherichia coli and Saccharomyces cerevisiae, the cationic Mn(III) N-substituted pyridylporphyrins (MnPs) have been identified as the most potent cellular redox modulators within the porphyrin class of drugs; their efficacy in animal models of diseases that have oxidative stress in common is based on their high ability to catalytically remove superoxide, peroxynitrite, carbonate anion radical, hypochlorite, nitric oxide, lipid peroxyl and alkoxyl radicals, thus suppressing the primary oxidative event. While doing so MnPs could couple with cellular reductants and redox-active proteins. Reactive species are widely accepted as regulators of cellular transcriptional activity: minute, nanomolar levels are essential for normal cell function, while submicromolar or micromolar levels impose oxidative stress, which is evidenced in increased inflammatory and immune responses. By removing reactive species, MnPs affect redox-based cellular transcriptional activity and consequently secondary oxidative stress, and in turn inflammatory processes. The equal ability to reduce and oxidize superoxide during the dismutation process and recently accumulated results suggest that pro-oxidative actions of MnPs may also contribute to their therapeutic effects. All our data identify the superoxide dismutase-like activity, estimated by log k(cat)O2-*), as a good measure for the therapeutic efficacy of MnPs. Their accumulation in mitochondria and their ability to cross the blood-brain barrier contribute to their remarkable efficacy. We summarize herein the therapeutic effects of MnPs in cancer, central nervous system injuries, diabetes, their radioprotective action and potential for imaging. Few of the most potent modulators of cellular redox-based pathways, MnTE2-PyP5+, MnTDE-2-ImP5+, MnTnHex-2-PyP5+ and MnTnBuOE-2-PyP5+, are under preclinical and clinical development.

Water-soluble ionic benzoporphyrins

Novel ionic water-soluble tetrabenzoporphyrins have been successfully synthesized via a cascade reaction based on the Heck reaction. The UV-Vis spectra of these porphyrins displayed red-shifted and broadened Soret bands, and significantly enhanced Q bands. These porphyrins are highly water soluble.