Oxo Form Research Articles

Mechanisms of Mn(V)-Oxo to Mn(IV)-Oxyl Conversion: From Closed-Cubane Photosystem II to Mn(V) Catalysts and the Role of the Entering Ligands.

The low activation barrier for O-O coupling in the closed-cubane Oxygen-Evolving Centre (OEC) of Photosystem II (PSII) requires water coordination with the Mn4 'dangler' ion in the Mn(V)-oxo fragment. This coordination transforms the Mn(V)-oxo complex into a more reactive Mn4(IV)-oxyl species, enhancing O-O coupling. This study explains the mechanism behind the coordination and indicates that in the most stable form of the OEC, the Mn4 fragment adopts a trigonal bipyramidal geometry but needs to transition to a square pyramidal form to be activated for O-O coupling. This transition stabilizes the Mn4 dxy orbital, enabling electron transfer from the oxo ligand to the dxy orbital, converting the oxo ligand into an oxyl species. The role of the water is to coordinate with the square pyramidal structure, reducing the energy gap between the oxo and oxyl forms, thereby lowering the activation energy for O-O coupling. This mechanism applies not only to the OEC system but also to other Mn(V)-based catalysts. For other catalysts, ligands such as OH- stabilize the Mn(IV)-oxyl species better than water, improving catalyst activation for reactions like C-H bond activation. This study is the first to explain the Mn(V)-oxo to Mn(IV)-oxyl conversion, providing a new foundation for Mn-based catalyst design.

Synthesis, crystal structure, quantum chemical calculations, electrochemistry and electro-catalytical properties as cytochrome P-450 model of tetradentate Mn(III)-Schiff base complex

The tetradentate Schiff base ligand has been obtained from condensation with mixing ethylenediamine and 2 mmoles of 5-methoxy-2-hydroxybenzaldehyde in absolute ethanol H2L. To the ethanolic solution was added manganese(II)acetatetetrahydrated and lithium chloride (LiCl) to obtain the tetradentate manganese(III) Schiff base complex [Mn(III)(Cl)L]. Theprepared compounds have been characterized by several spectroscopic techniques such as elemental analyses, FT-IR, UV–vis., 1H NMR and HRMS. In this paper, the X-ray diffraction (XRD) and the computational studies (DFT) of the ligand(H2L) with its manganese(III)-Schiff base complex [Mn(III)(Cl)L] are described and confirmed the given molecularstructures. The crystallographic studies have been utilized toelucidate the kinetics, selectivity and stereochemistry of thetransferred oxygen atomsto the substrate molecules when the considered complex is used as catalyst accordingthecytochrome P450 model. In addition, the density functional theory (DFT) calculation with B3LYP/6-31G(d,p) level isperformed to obtain the optimized geometries and electronic properties of the prepared compounds. The global reactivityparameters have also been calculated using the energies of frontier molecular orbitals suggesting that the ligand H2L is morestable than its Mn(III) complex. This may be due to the presence of hydrogen bonds in the ligand and the weaker energies ofcoordination bonds in the complex. The electrochemical behaviour of Mn(III)(Cl)L has been studied by cyclic voltammetryin acetonitrile solutions at room temperature. The resulting cyclic voltammogram shows Mn(III)/Mn(II) couple at E1/2= -0.62V with glassy carbon (GC) electrode. This redox couple is involved in the electrocatalytic cycle where themanganese(III) cation is successively mono-electronated until the formation of superoxo intermediates and then the oxospecies, respectively. These oxo forms, generated in situ, transfer their oxygen atoms to the substrate giving the oxidizedproduct. So, the chemical and electrochemical reactions, implicated in this electrocatalytical process, obey to the biomimeticoxidation reactions as those of monooxygenase enzymes (Cytochrome P450).

Reversible Hydration of α-Dicarbonyl Compounds from Ab Initio Metadynamics Simulations: Comparison between Pyruvic and Glyoxylic Acids in Aqueous Solutions.

Glyoxylic and pyruvic oxoacids are widely available in the atmosphere as gas-phase clusters and particles or in wet aerosols. In aqueous conditions, they undergo interconversion between the unhydrated oxo and gem-diol forms, where two hydroxyl groups replace the carbonyl group. We here examine the hydration equilibrium of glyoxylic and pyruvic acids with first-principles simulations in water at ambient conditions using ab initio metadynamics to reconstruct the corresponding free-energy landscapes. The main results are as follows: (i) our simulations reveal the high conformational diversity of these species in aqueous solutions. (ii) We show that gem-diol is strongly favored in water compared to its oxo counterpart by 29 and 16 kJ/mol for glyoxylic and pyruvic acids, respectively. (iii) From our atomic-scale simulations, we present new insights into the reaction mechanisms with a special focus on hydrogen-bond arrangements and the electronic structure of the transition state.

Hydrolysis of chiral organophosphorus compounds by phosphotriesterases and mammalian paraoxonase-1.

Some organophosphorus compounds (OPs), which are used in the manufacturing of insecticides and nerve agents, are racemic mixtures with at least one chiral center with a phosphorus atom. Acute exposure of humans to these mixtures induces the covalent modification of acetylcholinesterase (AChE) and neuropathy target esterase (NTE) and causes a cholinergic syndrome or organophosphate-induced delayed polyneuropathy syndrome (OPIDP). These irreversible neurological effects are due to the stereoselective interaction of the racemic OPs with these B-esterases (AChE and NTE) and such interactions have been studied in vivo, ex vivo and in vitro, using stereoselective hydrolysis by A-esterases or phosphotriesterases (PTEs) and the PTE from Pseudomonas diminuta, and paraoxonase-1 (PON1) from mammalian serum. PON1 has a limited hydrolytic potential of the racemic OPs, while the bacterial PTE exhibits a significant catalytic activity on the less toxic isomers P(+) of the nerve agents. Avian serum albumin also shows a hydrolyzing capacity of chiral OPs with oxo and thio forms. There are ongoing environmental and bioremediation efforts to design and produce recombinants as bio-scavengers of OPs.

Ascorbate Peroxidase Compound II Is an Iron(IV) Oxo Species.

The protonation state of the iron(IV) oxo (or ferryl) form of ascorbate peroxidase compound II (APX-II) is a subject of debate. It has been reported that this intermediate is best described as an iron(IV) hydroxide species. Neutron diffraction data obtained from putative APX-II crystals indicate a protonated oxygenic ligand at 1.88 Å from the heme iron. This finding, if correct, would be unprecedented. A basic iron(IV) oxo species has yet to be spectroscopically observed in a histidine-ligated heme enzyme. The importance of ferryl basicity lies in its connection to our fundamental understanding of C-H bond activation. Basic ferryl species have been proposed to facilitate the oxidation of inert C-H bonds, reactions that are unknown for histidine-ligated hemes enzymes. To provide further insight into the protonation status of APX-II, we examined the intermediate using a combination of Mössbauer and X-ray absorption spectroscopies. Our data indicate that APX-II is an iron(IV) oxo species with an Fe-O bond distance of 1.68 Å, a K-edge pre-edge absorption of 18 units, and Mössbauer parameters of ΔEq = 1.65 mm/s and δ = 0.03 mm/s.

Exploring reaction pathways for the structural rearrangements of the Mn cluster induced by water binding in the S3 state of the oxygen evolving complex of photosystem II

Photosynthetic oxidation of water to dioxygen is catalyzed by the Mn4CaO5 cluster in the protein-cofactor complex photosystem II. The light-driven catalytic cycle consists of four observable intermediates (S0, S1, S2, and S3) and one transient S4 state. Recently, using X-ray free-electron laser crystallography, two experimental groups independently observed incorporation of one additional oxygen into the cluster during the S2 to S3 transition, which is likely to represent a substrate. The present study implicates two competing reaction routes encountered during the structural rearrangement of the catalyst induced by the water binding and immediately preceding the formation of final stable forms in the S3 state. This mutually exclusive competition involves concerted versus stepwise conformational changes between two isomers, called open and closed cubane structures, which have different consequences on the immediate product in the S3 state. The concerted pathway involves a one-step conversion between two isomeric hydroxo forms without changes to the metal oxidation and total spin (Stotal = 3) states. Alternatively, in the stepwise process, the bound waters are oxidized and transformed into an oxyl–oxo form in a higher spin (Stotal = 6) state. Here, density functional calculations are used to characterize all relevant intermediates and transition structures and demonstrate that the stepwise pathway to the substrate activation is substantially favored over the concerted one, as evidenced by comparison of the activation barriers (11.1 and 20.9 kcal mol−1, respectively). Only after formation of the oxyl–oxo precursor can the hydroxo species be generated; this occurs with a slow kinetics and an activation barrier of 17.8 kcal mol−1. The overall thermodynamic driving force is likely to be controlled by the movements of two glutamate ligands, D1-Glu189 and CP43-Glu354, in the active site and ranges from very weak (+0.4 kcal mol−1) to very strong (–23.5 kcal mol−1).

Low‐Shear Extrusion of Grains Retains Resistant Starch to Increase Microbiome Saccharolysis and Fecal Hydroxy/Keto Redox Ratios of the Feline Gut

ObjectivesFood extrusion processes mixed ingredients to cook food through application of mechanical shear force and heat. This degrades higher‐order starch structure to improve carbohydrate digestibility in the upper gastrointestinal tract. However, evidence indicates the role that digestion resistant starch (RS) plays in promoting health through activity of the gut microbiome. We examined the impact of reducing temperature and shear force in extrusion‐based processing of feline foods to impact gut microbiome metabolites (postbiotics) related to cat gastrointestinal health.MethodsCats were housed in environments promoting social interaction, had access to natural light that varied with season, daily opportunity to exercise and interact with Caretakers and were fed to maintain healthy body weight. Health was confirmed through clinical indices. Collections were under IACUC approved protocols. Identically formulated diets (poultry meal, rice, corn gluten, corn, cellulose and micronutrients) were produced with either HIGH or LOW shear force. Specific Mechanical Energy (SME) for the HIGH shear was 52 W.h/kg, and the Specific Thermal Energy (STE) was 41 W.h/kg, for a Total Energy (TE) of 94 W.h/kg. The SME for the LOW shear parameters was 22 W.h/kg, and the STE was calculated to be 38 W.h/kg, for a TE of 61 W.h/kg. Dietary intakes of RS were 0.3 g/(kg BW^0.75) and 5.1 g/(kg BW^0.75) for the HIGH shear and LOW shear groups, respectively. Randomized design, 2 groups; High Shear extrusion (HIGH, n = 18) and Low Shear (LOW, n = 19). Cats fed for 8 weeks; fecal collections in week 4 and 8. Fecal analyses: metabolomics by multiplatform LC‐MS, short chain fatty acids (SCFA) by GC‐FID and ammonia by indophenol colorimetry. Data were analyzed by independent t‐test for group‐wise differences at the 4 and 8 week timepoints.ResultsThe primary finding was of heightened gut microbiome saccharolysis and fermentation in the cats fed the LOW shear food, indicated by increased RS‐derived fecal saccharides maltotriose, maltose and glucose as well as increased fecal butyrate and lactate. By 8 week there was also a reduction in gut microbial putrefaction as indicated by decreased fecal ammonia and branched SCFA derived from valine, leucine and isoleucine. Also notable was the global shift in NADH/NAD+ redox couples (hydroxy/oxo couples) toward a more reduced state, such that 5 observed couples all had ratios significantly favoring the hydroxy form over the oxo form in the LOW‐ vs HIGH‐shear foods.ConclusionsOptimizing food extrusion for cats can enable more robust saccharolysis and resultant fermentation by the gut microbiome to promote production of postbiotics known to provide health benefits. Additionally, processing of the food resulted in decreased putrefaction by the feline gut microbiome. Finally, we document evidence that providing RS to a gut microbiome shifts the microbiome NADH/NAD+ redox milieu to favor the hydroxy (vs oxo) form of common two‐electron couples. This work is expected to increase the awareness of the benefits that resistant starch can have for an animal typically considered an obligate carnivore.Support or Funding InformationHill's Pet Nutrition, Inc. Topeka, Kansas, USAThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Kinetics of β-Carotene Oxidation in the Presence of Highly Active Forms of µ-Carbido Diiron(IV) Tetraphenylporphyrinate

The oxidative destruction of β-carotene in the presence of highly oxidized forms of µ-carbido-bis[(5,10,15,20-tetraphenyl-21H,23H-porphyrinato)iron(IV)] (1 → 3) or its analog with axially coordinated imidazole (2 → 4) obtained under the action of tert-butyl hydroperoxide tBuOOH was studied by spectrophotometry. It was found that compound 3 is the oxo form of compound 1 singly oxidized at the macrocyclic ligand (π radical cation) under the action of which β-carotene is oxidized with a rate constant k = 3.3 L2 mol–2 s–1. A conclusion is drawn that short-lived compound 4 has unique EAS and is capable of oxidizing tBuOOH to form O2, which makes it possible to consider it the model of peroxidase. The value of k for the reaction with the participation of β-carotene and compound 4 (k = 10.3 L2 mol–2 s–1) is three times higher than that with the participation of compound 3. If a new portion of β-carotene is added, the process of its oxidative destruction in the presence of compounds 3 or 4 occurs without additives of the dimeric complex and peroxide. A possible nature of compound 4 is discussed, as well as the influence of N-base in the coordination sphere of the complex on the nature of active intermediates and the rate of β-carotene decomposition.

Iridium-Catalyzed Asymmetric Hydrogenation of 4,6-Disubstituted 2-Hydroxypyrimidines.

An efficient iridium-catalyzed hydrogenation of 4,6-disubstituted 2-hydroxypyrimidines has been achieved, giving chiral cyclic ureas with excellent diastereoselectivities and up to 96% ee of enantioselectivities. In the presence of the in situ generated hydrogen halide, the equilibrium of the lactame-lactime tautomerism of 2-hydroxypyrimidine is more toward the oxo form with lower aromaticity, which effectively improves the reactivity to facilitate hydrogenation. Moreover, the cyclic ureas could be readily converted into chiral 1,3-diamine derivatives without loss of optical purity.

Сравнительное исследование эффективности применения таутомеров оротата магния для компенсации дефицита магния. Часть II. Влияние оксо- и гидроксиформы оротата магния на элементный состав крови и тканей органов лабораторных животных

Under fusemide-induced hypomagnesemia in animals, the hydroxy form of magnesium orotate shows a higher rate of compensation for magnesium deficiency in the blood compared to the oxo form. Furosemide load is accompanied not only by hypomagnesia, but also by the development diselementosis of the body. Changes in elemental status are different depending on the organ. The amount of magnesium in the furosemide load decreases in serum, does not change in the heart, rises in the spleen and thymus. The injection of the hydroxy form of magnesium orotate levels the ratio of micro- and macronutrients in blood plasma and organ tissues. Magnesium deficiency is accompanied by signs of immuno-inflammatory reaction, leuko- and lymphocytosis, eosinophilia, a decrease in the number of erythrocytes and a decrease in the level of hemoglobin. An earlier and complete restoration of cytological parameters is noted in the group of animals with the introduction of hydroxy form of magnesium orotate.

A thiocoumarin-based fluorescent probe for detection of hypochlorite with high selectivity over other typical desulfurizing agents (Hg2+/Ag+)

A thiocoumarin dye, Probe 1, has been developed for the detection of hypochlorite anion (ClO−) via the desulfurization of thiocoumarin. The heavy atom effect efficiently quenched the fluorescence of Probe 1. Upon the addition of ClO−, Probe 1 was instantaneously oxidized into its oxo form, resulting in a strong yellow fluorescence with an emission maximum at 552 nm and a large Stokes shift (141 nm). This probe exhibited excellent selectivity toward ClO− over other reactive oxygen species, common metal ions and anions. Gratifyingly, the typical desulfurizing agents such as Hg2+ and Ag+ had little interfering effect on the behavior of Probe 1 for the detection of ClO−. This probe dispalyed a low detection limit (1.7 × 10−7 M). Importantly, the application of this probe for the detection of endogenous ClO− in living cells had been successfully demonstrated.

Sensitive and Selective Detection of Oxo-Form Organophosphorus Pesticides Based on CdSe/ZnS Quantum Dots.

A rapid, sensitive and enzyme-based optical biosensor was applied for the determination of seven organophosphorus pesticides (OPPs), including the oxo forms (malaoxon, paraoxon, dibrom, and dichlorvos), the thio forms (malathion and parathion) and the mixed form (demeton) in Panax ginseng. The principal of the proposed method is that the fluorescence quenching effect of quantum dots (QDs) can be observed by enzyme-generated H2O2. The active centers of acetylcholinesterase (AChE) could be inhibited in the presence of pesticides, which caused decrease of the generated H2O2. Then, the inhibition efficiency of pesticide to AChE activity could be evaluated by measuring the fluorescence changes. Different from biosensors based on immobilized enzyme or self-assembling technique, the proposed biosensor demonstrated a good selectivity for the detection of oxo forms of OPPs. In the present study, the important experimental conditions of the proposed biosensor were investigated. Under the optimized conditions (incubation temperature, 35 °C; incubation time, 20 min; pH value, 8.0; detection time, 30 min; AChE concentration, 40.9 U/L; and choline oxidase (ChOx) concentration, 637.5 U/L), the limit of detection for the investigated oxo-form OPPs was no more than 0.05 μM, which suggested that the proposed method could be used for sensitive and selective determination of trace amounts of OPPs residues in real samples with complex matrices.

Amino Acid Oxidation: A Combined Study of Cysteine Oxo Forms by IRMPD Spectroscopy and Simulations.

The redox activity of cysteine sulfur allows numerous post-translational protein modifications involved in the oxidative regulation of metabolism, in metal binding, and in signal transduction. A combined approach based on infrared multiple photon dissociation spectroscopy at the Centre Laser Infrarouge d'Orsay (CLIO) free electron laser facility, calculations of IR frequencies, and finite temperature ab initio molecular dynamics simulations has been employed to characterize the gas-phase structures of deprotonated cysteine sulfenic, sulfinic, and sulfonic acids, [cysSOx ]- (x=1, 2, 3, representing the number of S-bound oxygen atoms), which are key intermediates in the redox-switching chemistry of proteins. The ions show different structural motifs owing to preferential binding of the proton to either the carboxylate or sulfur-containing group. Due to the decreasing basicity of the sulfenic, sulfinic, and sulfonic terminals, the proton bound to SO- in [cysSO]- migrates to the carboxylate in [cysSO3 ]- , whereas it turns out to be shared in [cysSO2 ]- . Evidence is gathered that a mixture of close-lying low-energy conformers is sampled for each cysteine oxo form in a Paul ion trap at room temperature.

Toxicological characterisation of a thio-arsenosugar-glycerol in human cells

Arsenosugars are water-soluble arsenic species predominant in marine algae and other seafood including mussels and oysters. They typically occur at levels ranging from 2 to 50mg arsenic/kg dry weight. Most of the arsenosugars contain arsenic as a dimethylarsinoyl group (Me2As(O)-), commonly referred to as the oxo forms, but thio analogues have also been identified in marine organisms and as metabolic products of oxo-arsenosugars. So far, no data regarding toxicity and toxicokinetics of thio-arsenosugars are available. This in vitro-based study indicates that thio-dimethylarsenosugar-glycerol exerts neither pronounced cytotoxicity nor genotoxicity even though this arsenical was bioavailable to human hepatic (HepG2) and urothelial (UROtsa) cells. Experiments with the Caco-2 intestinal barrier model mimicking human absorption indicate for the thio-arsenosugar-glycerol higher intestinal bioavailability as compared to the oxo-arsenosugars. Nevertheless, absorption estimates were much lower in comparison to other arsenicals including arsenite and arsenic-containing hydrocarbons. Arsenic speciation in cell lysates revealed that HepG2 cells are able to metabolise the thio-arsenosugar-glycerol to some extent to dimethylarsinic acid (DMA). These first in vitro data cannot fully exclude risks to human health related to the presence of thio-arsenosugars in food.

Determination of pKa and Hydration Constants for a Series of α-Keto-Carboxylic Acids Using Nuclear Magnetic Resonance Spectrometry

The determination of the acid-base dissociation constants, and thus the pKa values, of α-keto acids such as pyruvic acid is complex because of the existence of these acids in their hydrated and nonhydrated or oxo state. Equilibria involved in the hydration and dehydration of the α-keto group of pyruvic acid and three other α-keto acids, 3-methyl-2-oxobutanoic acid, 4-methyl-2-oxopentanoic acid, and 2-oxo-2-phenylacetic acid, were investigated by proton and carbon nuclear magnetic resonance spectrometry, at constant ionic strength, 0.15, and 25°C. Dissociation constants for the oxo (pKaoxo) and hydrated (pKahyd) acids of each compound were estimated from the change in the degree of hydration with changes in pH and directly from the changes in chemical shifts of various hydrogen and carbons nuclei with pH. α-Keto acids showed greater hydration in their acidic forms than their carboxylate forms. The degree of hydration was sensitive to steric and electronic/resonance factors. As expected, the oxo forms of the acids were stronger acids compared with their hydrated analogs, and their dissociation constants were also sensitive to steric and electronic factors.

Voltammetric and flow-injection determination of carbohydrates using composite electrodes based on carbon nanotubes and Nickel(II) hexacyanoferrate

The catalytic activities of oxo and hydroxo forms of nickel(II) on the surface of a nickel deposit and an inorganic film of nickel(II) hexacyanoferrate(III) (NiHCF) electrodeposited on a glassy-carbon electrode both unmodified and modified by multi-wall or single-wall (also functionalized) carbon nanotubes in the electrooxidation of glucose, sucrose, and maltose are compared. A more pronounced catalytic effect was obtained in the electrooxidation of these carbohydrates on an electrode modified with functionalized singlewall carbon nanotubes and a NiHCF film. Methods are proposed for the voltammetric and flow-injection determination of carbohydrates on this composite electrode. A linear dependence of the analytical signal on the analyte concentration was observed in the range from 5 × 10−7 to 5 × 10−2 M under stationary conditions and from 0.003 to 0.3 μmol under flow conditions.

Solvent influence on the excited states of the oxo form of barbituric acid and the mechanisms of the out-of-plane non-radiative elongation of the N[sbnd]H bond: A comparative theoretical and experimental study

The solvent influence on the excited states, emission and absorption energies of the oxo for of barbituric acid was studied with experimental (UV and fluorescence spectra) and theoretical methods. The excited-state reaction paths of the out-pf-plane elongation of the NH bond of the oxo form of barbituric acid were also investigated (TD DFT level) to the conical intersections mediating internal conversions to the ground state. The 1nσ* excited state was found to be the driven electronic state. We found that the increase of the polarity of the solvent reduces the 1nσ* excited state decay rate through a non-planar conical intersection and increases its energy. Thus, solvents with higher polarity disfavor the non-radiative decay through conical intersections.

Multi-component symmetry-projected approach for molecular ground state correlations

The symmetry-projected Hartree-Fock ansatz for the electronic structure problem can efficiently account for static correlation in molecules, yet it is often unable to describe dynamic correlation in a balanced manner. Here, we consider a multi-component, systematically improvable approach, that accounts for all ground state correlations. Our approach is based on linear combinations of symmetry-projected configurations built out of a set of non-orthogonal, variationally optimized determinants. The resulting wavefunction preserves the symmetries of the original Hamiltonian even though it is written as a superposition of deformed (broken-symmetry) determinants. We show how short expansions of this kind can provide a very accurate description of the electronic structure of simple chemical systems such as the nitrogen and the water molecules, along the entire dissociation profile. In addition, we apply this multi-component symmetry-projected approach to provide an accurate interconversion profile among the peroxo and bis(μ-oxo) forms of [Cu2O2](2+), comparable to other state-of-the-art quantum chemical methods.

Understanding the Oxidative Relationships of the Metal Oxo, Hydroxo, and Hydroperoxide Intermediates with Manganese(IV) Complexes Having Bridged Cyclams: Correlation of the Physicochemical Properties with Reactivity

Multiple transition metal functional groups including metaloxo, hydroxo, and hydroperoxide groups play significant roles in various biological and chemical oxidations such as electron transfer, oxygen transfer, and hydrogen abstraction. Further studies that clarify their oxidative relationships and the relationship between their reactivity and their physicochemical properties will expand our ability to predict the reactivity of the intermediate in different oxidative events. As a result researchers will be able to provide rational explanations of poorly understood oxidative phenomena and design selective oxidation catalysts. This Account summarizes results from recent studies of oxidative relationships among manganese(IV) molecules that include pairs of hydroxo/oxo ligands. Changes in the protonation state may simultaneously affect the net charge, the redox potential, the metal-oxygen bond order (M-O vs M═O), and the reactivity of the metal ion. In the manganese(IV) model system, [Mn(IV)(Me(2)EBC)(OH)(2)](PF(6))(2), the Mn(IV)-OH and Mn(IV)═O moieties have similar hydrogen abstraction capabilities, but Mn(IV)═O abstracts hydrogen at a more than 40-fold faster rate than the corresponding Mn(IV)-OH. However, after the first hydrogen abstraction, the reduction product, Mn(III)-OH(2) from the Mn(IV)-OH moiety, cannot transfer a subsequent OH group to the substrate radical. Instead the Mn(III)-OH from the Mn(IV)═O moiety reforms the OH group, generating the hydroxylated product. In the oxygenation of substrates such as triarylphosphines, the reaction with the Mn(IV)═O moiety proceeds by concerted oxygen atom transfer, but the reaction with the Mn(IV)-OH functional group proceeds by electron transfer. In addition, the manganese(IV) species with a Mn(IV)-OH group has a higher redox potential and demonstrates much more facile electron transfer than the one that has the Mn(IV)═O group. Furthermore, an increase in the net charge of the Mn(IV)-OH further accelerates its electron transfer rate. But its influence on hydrogen abstraction is minor because charge-promoted electron transfer does not enhance hydrogen abstraction remarkably. The Mn(IV)-OOH moiety with an identical coordination environment is a more powerful oxidant than the corresponding Mn(IV)-OH and Mn(IV)═O moieties in both hydrogen abstraction and oxygen atom transfer. With this full understanding of the oxidative reactivity of the Mn(IV)-OH and Mn(IV)═O moieties, we have clarified the correlation between the physicochemical properties of these active intermediates, including net charge, redox potential, and metal-oxygen bond order, and their reactivities. The reactivity differences between the metal oxo and hydroxo moieties on these manganese(IV) functional groups after the first hydrogen abstraction have provided clues for understanding their occurrence and functions in metalloenzymes. The P450 enzymes require an iron(IV) oxo form rather than an iron(IV) hydroxo form to perform substrate hydroxylation. However, the lipoxygenases use an iron(III) hydroxo group to dioxygenate unsaturated fatty acids rather than an iron(III) oxo species, a moiety that could facilitate hydroxylation reactions. These distinctly different physicochemical properties and reactivities of the metal oxo and hydroxo moieties could provide clues to understand these elusive oxidation phenomena and provide the foundation for the rational design of novel oxidation catalysts.

Indirect electrochemical oxidation of organophosphorous pesticides for efficient detection via acetylcholinesterase test

Organothiophosphorous pesticides diazinon, malathion, chlorpyrifos, azinphos-methyl and phorate, have been indirectly electrochemically oxidized in aqueous media using anodically evolved Cl2, Br2 or I2 as a pre-step for their detection via acetylcholinesterase-based test. The presence of single oxidation product, corresponding oxo-form, was confirmed by UPLC analysis, as well as its stability with respect to hydrolysis. Comparing different halogens, the best results were obtained using Br2 as the oxidant due to high reactivity of HOBr, which is formed upon chemical reaction of anodically formed Br2 with water. Limits of detection of five analyzed pesticides were lowered upon indirect electrochemical oxidation with Br2 for two orders of magnitude or more, comparing to unoxidized parental thio-forms. In fact, the lowest possible detection limits for all five pesticides using proposed analytical procedure were achieved, as being determined by detection limits of corresponding oxo forms. Comparison of here proposed electrochemical oxidation pre-step with earlier reported ones is provided and discussed.