Identifying the chloroperoxyl radical in acidified sodium chlorite solution.

The present study identified the active radical species in acidic sodium chlorite and investigated the feasibility of quantifying these species with the diethylphenylenediamine (DPD) method. Electron spin resonance (ESR) spectroscopy was used to identify the active species generated in solutions containing sodium chlorite (NaClO2). The ESR signal was directly observed in an acidified sodium chlorite (ASC) aqueous solution at room temperature. This ESR signal was very long-lived, indicating that the radical was thermodynamically stable. The ESR parameters of this signal did not coincide with previously reported values of the chlorine radical (Cl●) or chlorine dioxide radical (O = Cl●-O and O = Cl-O●). We refer to this signal as being from the chloroperoxyl radical (Cl-O-O●). Quantum chemical calculations revealed that the optimal structure of the chloroperoxyl radical is much more thermodynamically stable than that of the chlorine dioxide radical. The UV-visible spectrum of the chloroperoxyl radical showed maximum absorbance at 354 nm. This absorbance had a linear relationship with the chloroperoxyl radical ESR signal intensity. Quantifying the free chlorine concentration by the DPD method also revealed a linear relationship with the maximum absorbance at 354 nm, which in turn showed a linear relationship with the chloroperoxyl radical ESR signal intensity. These linear relationships suggest that the DPD method can quantify chloroperoxyl radicals, which this study considers to be the active species in ASC aqueous solution.

Theoretical study on CO2 reduction catalyzed by formate dehydrogenase using the cation radical of a bipyridinium salt with an ionic substituent as a co-enzyme.

Formate dehydrogenase from Candida boidinii (CbFDH; EC.1.2.1.2) is a useful enzyme for CO2 reduction to formate in the photoredox system of a visible-light sensitizer and an electron mediator in the presence of an electron donor. The electron mediator, cation radicals of 4,4'-bipyridinium salts (4,4'-BPs) act as the co-enzyme for CbFDH in the CO2 reduction to formate. We found that the CbFDH-catalyzed CO2 reduction to formate could be controlled by the ionic substituents introduced into the cation radical of 4,4'-BPs [Y. Amao, Sustainable Energy Fuels, 2018, 2, 1928-1950]. By using 1,1'-diaminoethyl-4,4'-bipyridinium salt (DABP), 1-aminoethyl-1'-methyl-4,4'-bipyridinium salt (AMBP), 1,1'-carboxymethyl-4,4'-bipyridinium salt (DCBP), and 1-carboxymethyl-1'-methyl-4,4'-bipyridinium salt (CMBP), the introduction of an amino-group into 4,4'-BP accelerates the CbFDH-catalyzed CO2 reduction to formate, while the introduction of a carboxy-group into 4,4'-BP slows the CO2 reduction to formate. This work clarified the direct interaction of the cation radicals of DABP, DCBP, AMBP, CMBP, and MV in the substrate-binding site of CbFDH by the docking simulation. In addition, a mechanistic investigation for the CbFDH-catalyzed CO2 reduction to formate with cation radicals of DABP, DCBP, AMBP, CMBP, and MV was carried out based on the energy of molecular orbitals calculated by density functional theory (DFT).

Influence of tryptic hydrolysis on the enzymatic function of the membrane-bound form of particulate methane monooxygenase from Methylosinus trichosporium OB3b.

Particulate methane monooxygenase (pMMO) is a membrane protein embedded in the intracytoplasmic membrane of methane-oxidizing bacteria. Structural analysis of pMMO showed the existence of a hydrophilic region exposed outside of the bacterial membrane. To obtain information regarding the role of this hydrophilic region in the enzymatic function of pMMO, trypsin proteolysis of the membrane-bound form of pMMO from Methylosinus trichosporium OB3b was performed at 4 °C. The polypeptides produced by this hydrolysis were analyzed by polyacrylamide gel electrophoresis and MALDI-TOF/TOF. Furthermore, the influence of this tryptic digestion on the methane hydroxylation and propene epoxidation enzymatic activities of pMMO was investigated. Among the three subunits of pMMO, PmoB and PmoC were hydrolyzed by trypsin, but PmoA was not. With 10 mg L-1 trypsin, both terminal regions or the C-terminal region of PmoC polypeptide was selectively hydrolyzed. Furthermore, the stability of pMMO was decreased by this digestion. These results indicate that PmoC plays a role in maintaining the stability of pMMO in vitro. On the other hand, the digestion of PmoB with 100 mg L-1 trypsin produced several polypeptides, indicating that trypsin digestion occurs at several sites of the hydrophilic region of PmoB. Hydrolysis led to a decrease in pMMO activity towards methane hydroxylation and propene epoxidation. These results indicate that the hydrophilic region of PmoB is critically important for the enzymatic function of pMMO, which is consistent with the models of the functional mechanism of pMMO proposed so far.

How does methylviologen cation radical supply two electrons to the formate dehydrogenase in the catalytic reduction process of CO2 to formate?

Formate dehydrogenase from Candida boidinii (EC.1.2.1.2; CbFDH) is a commercially available enzyme and can be easily handled as a catalyst for the CO2 reduction to formate in the presence of NADH, single-electron reduced methylviologen (MV+˙) and so on. It was found that the formate oxidation to CO2 with CbFDH was suppressed using the oxidized MV as a co-enzyme and the single-electron reduced MV (MV+˙) was effective for the catalytic activity of CbFDH for the CO2 reduction to formate compared with that using the natural co-enzyme of NADH [Y. Amao, Chem. Lett., 2017, 46, 780-788]. The CO2 reduction to formate catalyzed by CbFDH requires two molecules of the MV+˙. In order to clarify the two-electron reduction process using MV+˙ in the CO2 reduction to formate catalyzed with CbFDH, we attempted enzyme reaction kinetics, electrochemical and quantum chemical analyses. Kinetic parameters obtained from the enzymatic kinetic analysis metric revealed an index of affinity of MV+˙ for CbFDH in the CO2 reduction to formate. From the results of the electrochemical analysis, it was predicted that only one molecule of MV+˙ was bound to CbFDH, and the MV bound to CbFDH was to be necessarily re-reduced by the electron source outside of CbFDH to supply the second electron in the CO2 reduction to formate. From the results of docking simulation and density functional theory (DFT) calculations, it was indicated that one molecule of MV bound to the position close to CO2 in the inner part of the substrate binding pocket of CbFDH contributed to the two-electron CO2 reduction to formate.

Influence of copper ions removal from membrane-bound form of particulate methane monooxygenase from Methylosinus trichosporium OB3b on its activity for methane hydroxylation.

The influence of metal removal with a chelating reagent, ethylenediaminetetraacetic acid (EDTA), and metal reconstitution on the activity of particulate methane monooxygenase (pMMO) from Methylosinus trichosporium OB3b toward methane oxidation to methanol was investigated. For this study, a membrane fraction containing pMMO and bacterial cell membrane components was prepared. pMMO activity was assessed with two different reductants, nicotinamide adenine dinucleotide (NADH) and 2,3,5,6-tetramethyl hydroquinone (duroquinol). The partial removal of metal ions with EDTA resulted in the selective inhibition of NADH-driven activity. The NADH-driven activity was restored by exogenous copper ions, but not by other divalent metal cations. Furthermore, both NADH- and duroquinol-driven activities were lost completely by increasing the amount of metal removed. Duroquinol-driven activity of the metal-deficient membrane fraction increased with increasing the amount of copper ions added, while NADH-driven activity increased when more than 5 mol of copper ions per mol of pMMO monomer was added. These results suggest that NADH-driven pMMO activity requires not only the catalytic copper center of pMMO but also copper ions outside the catalytic site.

Substantial evidence for the rhododendrol-induced generation of hydroxyl radicals that causes melanocyte cytotoxicity and induces chemical leukoderma.

BACKGROUND: Rhododendrol (4-(4-hydroxyphenyl)-2-butanol) has been used as a lightening/whitening cosmetic but was recently reported to induce leukoderma. Although rhododendrol has been shown to be transformed by tyrosinase to hydroxyl-rhododendrol, which is cytotoxic to melanocytes, its detailed mechanism of action including the involvement of reactive oxygen species is not clearly understood. OBJECTIVE: To confirm the relationship of hydroxyl radical generation to melanocyte cytotoxicity induced by rhododendrol, this study was performed. METHODS: An electron spin resonance method with a highly sensitive detection system was utilized to monitor hydroxyl radicals generated from two distinct normal human epidermal melanocyte lines with different levels of tyrosinase activity after the addition of various amounts of rhododendrol. Cytotoxicity of rhododendrol was analyzed by AlamarBlue assay under the same condition. RESULTS: Hydroxyl radicals were generated depending on the amounts of rhododendrol and/or tyrosinase. After the correlation between hydroxyl radical generation with melanocyte viability was confirmed, an inhibitor of oxidative stress, N-acetyl cysteine, was shown to dramatically diminish rhododendrol-induced generation of hydroxyl radicals and melanocyte cytotoxicity by increasing glutathione levels. In contrast, buthionine sulfoximine, which depletes glutathione, augmented both of those parameters. CONCLUSION: Suppressing oxidative stress would prevent and/or mitigate some phenol derivative-induced leukoderma by avoiding hydroxyl radical-initiated melanocyte cytotoxicity.

Generation of hydroxyl radicals and singlet oxygen during oxidation of rhododendrol and rhododendrol-catechol.

The generation of hydroxyl radicals and singlet oxygen during the oxidation of 4-(4-hydroxyphenyl)-2-butanol (rhododendrol) and 4-(3,4-dihydroxyphenyl)-2-butanol (rhododendrol-catechol) with mushroom tyrosinase in a phosphate buffer (pH 7.4) was examined as the model for the reactive oxygen species generation via the two rhododendrol compounds in melanocytes. The reaction was performed in the presence of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) spin trap reagents for hydroxyl radical or 2,2,6,6-tetramethyl-4-piperidone (4-oxo-TEMP), an acceptor of singlet oxygen, and their electron spin resonances were measured. An increase in the electron spin resonances signal attributable to the adduct of DMPO reacting with the hydroxyl radical and that of 4-oxo-TEMP reacting with singlet oxygen was observed during the tyrosinase-catalyzed oxidation of rhododendrol and rhododendrol-catechol, indicating the generation of hydroxyl radical and singlet oxygen. Moreover, hydroxyl radical generation was also observed in the autoxidation of rhododendrol-catechol. We show that generation of intermediates during tyrosinase-catalyzed oxidation of rhododendrol enhances oxidative stress in melanocytes.

Determination of the positions of aluminum atoms introduced into SSZ-35 and the catalytic properties of the generated Brønsted acid sites.

The positions of aluminum (Al) atoms in SSZ-35 together with the characteristics of the generated protons were investigated by 27Al multiple quantum magic-angle spinning (MQ-MAS), 29Si MAS, and 1H MAS NMR data analyses accompanied by a variable temperature 1H MAS NMR analysis. The origin of the acidic -OH groups (Brønsted acid sites) generated by introducing Al atoms into the T sites was investigated and the T sites introduced into the Al atoms were revealed. To further determine the catalytic properties of the acidic protons generated in SSZ-35, the influence of the concentration of the Al atoms on the catalytic activity and selectivity during the transformation of toluene was examined.

Hydroxyl radical generation by dissociation of water molecules during 1.65 MHz frequency ultrasound irradiation under aerobic conditions.

The dissociation of water molecules by ultrasound irradiation under aerobic conditions was demonstrated experimentally. To be able to detect the dissociation of water molecules, we performed the ultrasound irradiation of 17O-labelled water (H217O) under aerobic conditions. The hydroxyl and hydrogen radicals generated during the ultrasound irradiation process were trapped with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), and electron spin resonance (ESR) spectroscopy was performed on the DMPO spin adducts. In the ESR spectrum, a 15-line signal attributable to the trapping of the hydroxyl radicals containing 17O (17OH radicals) by DMPO together with a 4-line signal attributable to the trapping of the hydroxyl radicals containing 16O (16OH radicals) by DMPO were observed. The generation of 17OH radicals indicated that H217O was dissociated by the sonolysis process under aerobic conditions. On the other hand, the ESR signal attributable to the trapping of hydrogen radicals by DMPO was not observed, suggesting that hydrogen radicals were not generated during the dissociation of water molecules.

Experimental and computational studies of the roles of MgO and Zn in talc for the selective formation of 1,3-butadiene in the conversion of ethanol.

The one-step conversion of ethanol to 1,3-butadiene was performed using talc containing Zn (talc/Zn) as a catalyst. The influence of the MgO and Zn in the talc on the formation rate and selectivity for 1,3-butadiene were investigated. MgO as a catalyst afforded 1,3-butadiene with a selectivity that was nearly the same as talc/Zn at ∼40% ethanol conversion at 673 K, although the rate of 1,3-butadiene formation over MgO was about 40 times lower than that over the talc/Zn. The introduced Zn cations were located in octahedral sites in place of Mg cations in the talc lattice. The Zn cations accelerated the rate of CH3CHO formation from ethanol, resulting in an increase in the rate of 1,3-butadiene formation. However, the rate of CH3CHO consumption to form crotonaldehyde was not influenced by Zn, although the distribution of crotonaldehyde was decreased with increasing Zn concentrations. X-ray photoelectron spectra of talc/Zn showed that the O1s binding energy was increased by increasing the concentration of Zn, while those of both Mg2p and Si2p were hardly influenced. DFT calculations were used to estimate the atomic charges on the O, Mg, Si, and Zn atoms when an atom of Zn per unit cell of talc was introduced into an octahedral site. On the basis of the results for the conversion of ethanol into 1,3-butadiene and the corresponding DFT calculations, the roles of the O, Zn, Mg, and Si atoms in the talc catalyst for the formation of 1,3-butadiene from ethanol were discussed.

Cascade Synthesis of Five-Membered Lactones using Biomass-Derived Sugars as Carbon Nucleophiles.

We report the cascade synthesis of five-membered lactones from a biomass-derived triose sugar, 1,3-dihydroxyacetone, and various aldehydes. This achievement provides a new synthetic strategy to generate a wide range of valuable compounds from a single biomass-derived sugar. Among several examined Lewis acid catalysts, homogeneous tin chloride catalysts exhibited the best performance to form carbon-carbon bonds. The scope and limitations of the synthesis of five-membered lactones using aldehyde compounds are investigated. The cascade reaction led to high product selectivity as well as diastereoselectivity, and the mechanism leading to the diastereoselectivity was discussed based on isomerization experiments and density functional theory (DFT) calculations. The present results are expected to support new approaches for the efficient utilization of biomass-derived sugars.

Direct Estimation of the Surface Location of Immobilized Functional Groups for Concerted Catalysis Using a Probe Molecule.

The location of active sites during concerted catalysis by a metal complex and tertiary amine on a SiO2 surface is discussed based on the interaction between the functionalized SiO2 surface and a probe molecule, p-formyl phenylboronic acid. The interactions of the probe molecule with the surface functionalities, diamine ligand, and tertiary amine, were analyzed by FT-IR and solid-state (13)C and (11)B MAS NMR. For the catalyst exhibiting high 1,4-addition activity, the diamine ligand and tertiary amine base exist in closer proximity than in the catalyst with low activity.

Singlet oxygen generation during the oxidation of L-tyrosine and L-dopa with mushroom tyrosinase.

The generation of singlet oxygen during the oxidation of tyrosine and L-dopa using mushroom tyrosinase in a phosphate buffer (pH 7.4), the model of melanin synthesis in melanocytes, was examined. The reaction was performed in the presence of 2,2,6,6-tetramethyl-4-piperidone (4-oxo-TEMP), an acceptor of singlet oxygen and the electron spin resonance (ESR) of the spin adduct, 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy (4-oxo-TEMPO), was measured. An increase in the ESR signal attributable to 4-oxo-TEMPO was observed during the oxidation of tyrosine and L-dopa with tyrosinase, indicating the generation of singlet oxygen. The results suggest that (1)O2 generation via tyrosinase-catalyzed melanin synthesis occurs in melanocyte.

Mechanistic Insight into a Sugar-Accelerated Tin-Catalyzed Cascade Synthesis of α-Hydroxy-γ-butyrolactone from Formaldehyde.

Applications of the formose reaction, which involves the formation of sugars from formaldehyde, have previously been confined to the selective synthesis of unprotected sugars. Herein, it is demonstrated that α-hydroxy-γ-butyrolactone (HBL), which is one of the most important intermediates in pharmaceutical syntheses, can be produced from paraformaldehyde. In the developed reaction system, homogeneous tin chloride exhibits high catalytic activity and the addition of mono- and disaccharides accelerates the formation of HBL. These observations suggest that the formose reaction may serve as a feasible pathway for the synthesis of important chemicals.

Discrimination of the prochiral hydrogens at the C-2 position of n-alkanes by the methane/ammonia monooxygenase family proteins.

The selectivity of ammonia monooxygenase from Nitrosomonas europaea (AMO-Ne) for the oxidation of C4-C8n-alkanes to the corresponding alcohol isomers was examined to show the ability of AMO-Ne to recognize the n-alkane orientation within the catalytic site. AMO-Ne in whole cells produces 1- and 2-alcohols from C4-C8n-alkanes, and the regioselectivity is dependent on the length of the carbon chain. 2-Alcohols produced from C4-C7n-alkanes were predominantly either the R- or S-enantiomers, while 2-octanol produced from n-octane was racemic. These results indicate that AMO-Ne can discriminate between the prochiral hydrogens at the C-2 position, with the degree of discrimination varying according to the n-alkane. Compared to the particulate methane monooxygenase (pMMO) of Methylococcus capsulatus (Bath) and that of Methylosinus trichosporium OB3b, AMO-Ne showed a distinct ability to discriminate between the orientation of n-butane and n-pentane in the catalytic site.

Mechanistic studies on the cascade conversion of 1,3-dihydroxyacetone and formaldehyde into α-hydroxy-γ-butyrolactone.

The chemical synthesis of commercially and industrially important products from biomass-derived sugars is absolutely vital to establish biomass utilization as a sustainable alternative source of chemical starting materials. α-Hydroxy-γ-butyrolactone is a useful synthetic intermediate in pharmaceutical chemistry, and so novel biomass-related routes for its production may help to validate this eco-friendly methodology. Herein, we report the specific catalytic activity of homogeneous tin halides to convert the biomass-derived triose sugar 1,3-dihydroxyacetone and formaldehyde into α-hydroxy-γ-butyrolactone. A detailed screening of catalysts showed the suitability of tin catalysts for this reaction system, and isotope experiments using [D2]paraformaldehyde, substrate screening, and time profile measurements allowed us to propose a detailed reaction pathway. In addition, to elucidate the activated species in this cascade reaction, the effect of additional water and the influence of additional Brønsted acids on the reaction preferences for the formation of α-hydroxy-γ-butyrolactone, lactic acid, and vinyl glycolate were investigated. The active form of the Sn catalyst was investigated by (119)Sn NMR spectroscopy.

Influence of zeolite pore structure on product selectivities for protolysis and hydride transfer reactions in the cracking of n-pentane.

The conversion of n-pentane was carried out to examine the effects of reaction conditions on changes in product selectivities at 823 K, using zeolites with 10- and 12-membered rings. We also investigated the influence of the pore structure of these zeolites on their catalytic activities for both protolysis and hydride transfer reactions. In the first half of this work, we examined the influence of acidic proton concentration and n-pentane pressure on the reaction rates for protolysis and hydride transfer reactions using ZSM-5 zeolites. The rates of hydride transfer reactions were more influenced by pentane pressure compared to protolysis reactions, and were proportional to the square of n-pentane pressure and the concentration of acidic protons. In the second half of this work, the influence of the zeolite pore structure on changes in product selectivities with n-pentane conversion and that on the rates of protolysis and the hydride transfer reactions were revealed using various zeolites with 10- and 12-membered rings. The catalytic activities of zeolites for the protolysis and hydride transfer reactions were influenced more by the spatial volume of the zeolite cavity than the acid strength of protons on the zeolite.

Tin-catalyzed conversion of biomass-derived triose sugar and formaldehyde to α-hydroxy-γ-butyrolactone.

The direct conversion of biomass-derived 1,3-dihydroxyacetone (DHA) and formaldehyde to α-hydroxy-γ-butyrolactone (HBL) was achieved through the use of tin(iv) chloride and a small amount of water and the yield reached up to 70%. The reaction mechanism was also investigated by incorporating d2-formaldehyde into the reaction mixtures.

Highly active and selective catalysis of copper diphosphine complexes for the transformation of carbon dioxide into silyl formate.

Copper diphosphine complexes have been found to be highly active and selective homogeneous catalysts for the hydrosilylation of CO2. The structure of the phosphine ligands strongly affects their catalytic activity. Turnover number (TON) reaches 70,000 after 24 hours with 1,2-bis(diisopropylphosphino)benzene as a ligand under 1 atmosphere of CO2. (1)H and (13)C NMR spectra, carried out under the reaction conditions, showed the reaction mechanism through insertion of CO2 into Cu-H to afford Cu/formate species.