Elucidation of the mechanism of melamine adsorption on Pt(111) surface via density functional theory calculations.

The oxygen reduction reaction (ORR) activity of Pt catalysts in polymer electrolyte fuel cells (PEFCs) should be enhanced to reduce Pt usage. The adsorption of heteroaromatic ring compounds such as melamine on the Pt surface can enhance its catalytic activity. However, melamine adsorption on Pt and the consequent ORR enhancement mechanism remain unclear. In this study, we performed density functional theory calculations to determine the adsorption structures of melamine/Pt(111). Melamine was coordinated to Pt via two N lone pairs on NH2 and N- in the triazine ring, resulting in a chemisorption structure with slight electron transfer. Four types of adsorption structures were identified: three-point adsorption (two amino groups and a triazine ring: Type A), two-point adsorption (one amino group and a triazine ring: Type B), two-point adsorption (two amino groups: Type C), and one-point adsorption (one amino group: Type D). The most stable structure was Type B. However, multiple intermediate structures were formed owing to the conformational changes from the most stable to other stable adsorption structures. The resonance structures of the adsorbed melamine stabilise the adsorption, as increased resonance allows for more electron delocalisation. In addition, the lone-pair orbital of the amino group in the adsorbed melamine acquires the characteristics of an sp3 hybrid orbital, which prevents horizontal adsorption on the Pt surface. We believe that understanding these adsorption mechanisms will help in the molecular design of organic molecule-decorated Pt catalysts and will lead to the reduction of Pt usage in PEFCs.

Talin B regulates collective cell migration via PI3K signaling in Dictyostelium discoideum mounds.

Collective cell migration is a key process during the development of multicellular organisms, in which the migrations of individual cells are coordinated through chemical guidance and physical contact between cells. Talin has been implicated in mechanical linkage between actin-based motile machinery and adhesion molecules, but how talin contributes to collective cell migration is unclear. Here we show that talin B is involved in chemical coordination between cells for collective cell migration at the multicellular mound stage in the development of Dictyostelium discoideum. From early aggregation to the mound formation, talB-null cells exhibited collective migration normally with cAMP relay. Subsequently, talB-null cells showed developmental arrest at the mound stage, and at the same time, they had impaired collective migration and cAMP relay, while wild-type cells exhibited rotational cell migration continuously in concert with cAMP relay during the mound stage. Genetic suppression of PI3K activity partially restored talB-null phenotypes in collective cell migration and cAMP relay. Overall, our observations suggest that talin B regulates chemical coordination via PI3K-mediated signaling in a stage-specific manner for the multicellular development of Dictyostelium cells.

Theoretical investigation on the interaction between RhIII octaethylporphyrin and a graphite basal surface: a comparison study of DFT, DFT-D, and AFM.

Using density functional theory based calculations and atomic-force-microscopy observations, we investigated the interaction between [RhIII(OEP)(Cl)] (OEP = octaethylporphyrin) and a graphite basal surface, and the electronic structure of [RhIII(OEP)(Cl)]/graphite. The [RhIII(OEP)(Cl)] complex has an electronic structure effective for CO activation, possessing a closed singlet structure as its ground state; hence, both σ-donation from the CO molecule (anode-reaction reactant) to RhIII, and π-back-donation from RhIII to CO, occur, because the [RhIII(OEP)(Cl)] complex does not have a singlet occupied molecular orbital on the porphyrin ring, the π-π stacking interaction between porphyrin and graphite is not present and their interaction is dominated by dispersion forces. The [RhIII(OEP)(Cl)] complex easily diffused on the graphite basal surface, and an aggregated structure of [RhIII(OEP)(Cl)] was observed by atomic force microscopy. The difference of the electronic structures of [RhIII(OEP)(Cl)] before and after its adsorption is very small, the dispersion force being the dominant force for the adsorption. However, the lowest unoccupied molecular orbital of [RhIII(OEP)(Cl)]/graphite is a σ bonding orbital between RhIII and graphite that will cause fast electron transfer from [RhIII(OEP)(Cl)] to graphite during the CO electro-oxidation; this would be a reason why the carbon-supported [RhIII(OEP)(Cl)] has high catalytic activity for CO electro-oxidation.

Correction: Effects of p-substituents on electrochemical CO oxidation by Rh porphyrin-based catalysts.

Correction for 'Effects of p-substituents on electrochemical CO oxidation by Rh porphyrin-based catalysts' by Shin-ichi Yamazaki et al., Phys. Chem. Chem. Phys., 2010, 12, 8968-8976.

Characterization of a Rh(III) porphyrin-CO complex: its structure and reactivity with an electron acceptor.

To analyse the electrocatalytic oxidation of carbon monoxide by Rh porphyrins, we isolated a CO-adduct of Rh octaethylporphyrin, and examined its properties and reactivity by IR, NMR, and X-ray crystallographic analyses. The results indicate that the CO adduct of Rh octaethylporphyrin is vulnerable to nucleophilic attack by H2O. The CO-adduct was easily oxidized by an electron acceptor (1,4-naphthoquinone) to generate CO2. This indicates that CO is sufficiently activated in the CO complex of Rh octaethylporphyrin to reduce an electron acceptor. This mechanism is in contrast to that for the CO oxidation by Pt-based electrocatalysts.

Electrochemical reduction of dioxygen by copper complexes with pyridylalkylamine ligands dissolved in aqueous buffer solution: the relationship between activity and redox potential.

The Cu(II)/Cu(I) redox properties and electrochemical O2 reduction activity of a series of Cu(II)-complexes with pyridylalkylamine ligands were investigated in a neutral buffer solution. The relationship between Cu(II)/Cu(I) redox properties and O2 reduction activity was clearly demonstrated by voltammetric analyses.

Electrocatalytic oxidation of alcohols by a carbon-supported Rh porphyrin.

A Rh porphyrin on carbon black was shown to catalyze the electro-oxidation of several aliphatic alcohols (ethanol, 1-propanol, and 2-propanol) and benzyl alcohols. The overpotentials for alcohol oxidation were very low. The reaction mechanism and substrate specificity are discussed.

Hydrogen peroxide as sustainable fuel: electrocatalysts for production with a solar cell and decomposition with a fuel cell.

Hydrogen peroxide was electrochemically produced by reducing oxygen in an aqueous solution with [Co(TCPP)] as a catalyst and photovoltaic solar cell operating at 0.5 V. Hydrogen peroxide thus produced is utilized as a fuel for a one-compartment fuel cell with Ag-Pb alloy nanoparticles as the cathode.

Electrospray ionization mass spectrometric analyses of rhodium tetraphenylporphyrin complexes as electrocatalysts for CO oxidation by tandem mass spectrometry and hyphenated method with capillary electrophoresis.

Carbon monoxide (CO) poisoning of platinum (Pt)-based electrocatalysts is a huge obstacle in the development of proton-exchange membrane fuel cells. Fuel cell performance can be improved by removing CO through electrolytic oxidation. Yamazaki et al. synthesized several rhodium (Rh) tetraphenylporphyrin complexes as electrocatalysts to reduce CO poisoning. Experimental results showed that these complexes display higher electrochemical CO oxidation activity than previously synthesized Rh octaethylporphyrin complex and that the differences in the structures of p-substituted groups influence their activity. To confirm the formation of these Rh tetraphenylporphyrin complexes, analyses were performed using electrospray ionization mass spectrometry (ESI-MS), electrospray ionization tandem mass spectrometry (ESI-MS/MS), and capillary electrophoresis-electrospray ionization mass spectrometry (CE-ESI-MS). In ESI-MS, peaks other than that of the Rh tetraphenylporphyrin complex were occasionally detected at different m/z values. The ESI-MS/MS and CE-ESI-MS results suggest that these peaks correspond to the complexes coordinating CO or other ions, or to remaining uncoordinated tetraphenylporphyrins. Coordination of CO is supported by the ESI-MS/MS results for the Rh octaethylporphyrin complex. Separation of the uncoordinated tetraphenylporphyrin from Rh tetraphenylporphyrin complex was achieved by CE-ESI-MS.

Effects of p-substituents on electrochemical CO oxidation by Rh porphyrin-based catalysts.

Electrochemical CO oxidation by several carbon-supported rhodium tetraphenylporphyrins with systematically varied meso-substituents was investigated. A quantitative analysis revealed that the p-substituents on the meso-phenyl groups significantly affected CO oxidation activity. The electrocatalytic reaction was characterized in detail based on the spectroscopic and X-ray structural results as well as electrochemical analyses. The difference in the activity among Rh porphyrins is discussed in terms of the properties of p-substituents along with a proposed reaction mechanism. Rhodium tetrakis(4-carboxyphenyl)porphyrin (Rh(TCPP)), which exhibited the highest activity among the porphyrins tested, oxidized CO at a high rate at much lower potentials (<0.1 V vs. a reversible hydrogen electrode, at 60 degrees C) than the present PtRu catalysts. This means that CO is electrochemically oxidized by this catalyst when a slight overpotential is applied during the operation of a proton exchange membrane fuel cell. This catalyst exhibited little H(2) oxidation activity, in contrast to Pt-based catalysts.

Electrochemical oxidation of sugars at moderate potentials catalyzed by Rh porphyrins.

In this communication, we demonstrate that certain kinds of Rh porphyrins on carbon black can electrochemically oxidize aldose at low potentials. The onset potential was much lower than those with the other complex-based catalysts. A product analysis suggested that this reaction involves 2-electron oxidation of the aldehyde group.

Efficient electrochemical conversion of carbon monoxide by rhodium octaethylporphyrin adsorbed on carbon black.

We have found efficient electrocatalytic removal of CO by rhodium octaethylporphyrin on carbon black at a wide potential range. Using carbon-supported rhodium octaethylporphyrin, we have separated the Rh(II) state participating reaction and the Rh(III) state participating reaction with CO. We have clearly demonstrated electrocatalytic CO oxidation by rhodium(III) porphyrin. The onset potential for CO oxidation is much lower than that for CO oxidation by conventional Pt/Ru catalysts and cobalt porphyrin.

Reversible electrochemical conversion between Rh(II) and Rh(III) states in Rh porphyrin adsorbed on carbon black.

We have clearly demonstrated reversible cyclic voltammograms for the redox reaction between the Rh(II) and Rh(III) states in rhodium octaethylporphyrin [Rh(OEP)] adsorbed on carbon black in an acidic aqueous solution. The emergence of the reversible wave can be ascribed to the suppression of the undesirable reactions between two molecules of [Rh(II)(OEP)] because of its strong interaction with the carbon black. The generated [Rh(II)(OEP)] exhibits a potent catalytic O(2) reduction activity.

Kinetic evaluation of catalase and peroxygenase activities of tyrosinase.

Tyrosinase is a copper monooxygenase containing a coupled dinuclear copper active site (type-3 copper), which catalyzes oxygenation of phenols (phenolase activity) as well as dehydrogenation of catechols (catecholase activity) using O(2) as the oxidant. In this study, catalase activity (conversion of H(2)O(2) to (1/2)O(2) and H(2)O) and peroxygenase activity (H(2)O(2)-dependent oxygenation of substrates) of mushroom tyrosinase have been examined kinetically by using amperometric O(2) and H(2)O(2) sensors. The catalase activity has been examined by monitoring the initial rate of O(2) production from H(2)O(2) in the presence of a catalytic amount of tyrosinase in 0.1 M phosphate buffer (pH 7.0) at 25 degrees C under initially anaerobic conditions. It has been found that the catalase activity of mushroom tyrosinase is three-order of magnitude greater than that of mollusk hemocyanin. The higher catalase activity of tyrosinase could be attributed to easier accessibility of H(2)O(2) to the dinuclear copper site of tyrosinase. Mushroom tyrosinase has also been demonstrated for the first time to catalyze oxygenation reaction of phenols with H(2)O(2) (peroxygenase activity). The reaction has been investigated kinetically by monitoring the H(2)O(2) consumption rate in 0.5 M borate buffer (pH 7.0) under aerobic conditions. Similarity of the substituent effects of a series of p-substituted phenols in the peroxygenase reaction with H(2)O(2) to those in the phenolase reaction with O(2) as well as the absence of kinetic deuterium isotope effect with a perdeuterated substrate (p-Cl-C(6)D(4)OH vs p-Cl-C(6)H(4)OH) clearly demonstrated that the oxygenation mechanisms of phenols in both systems are the same, that is, the electrophilic aromatic substitution reaction by a (micro-eta(2):eta(2)-peroxo)dicopper(II) intermediate of oxy-tyrosinase.

Kinetic evaluation of phenolase activity of tyrosinase using simplified catalytic reaction system.

A very simple tyrosinase reaction system has been developed using borate anion as a trapping agent of catechols and hydroxylamine as an external reductant to evaluate the phenolase activity without the interference of catecholase activity. Reactivities of variously para-substituted phenols in this system were compared directly to those of the phenols in the model reactions, demonstrating that the enzymatic oxygenation reaction of phenols proceeds via the same mechanism as the model reaction, that is, electrophilic aromatic substitution mechanism.

Glucose metabolism of lactic acid bacteria changed by quinone-mediated extracellular electron transfer.

It can be expected that extracellular electron transfer to regenerate NAD+ changes the glucose metabolism of the homofermentative lactic acid bacteria. In this work, the glucose metabolism of Lactobacillusplantarum and Lactococcus lactis was examined in resting cells with 2-amino-3-carboxy-1,4-naphthoquinone (ACNQ) as the electron transfer mediator and ferricyanide (Fe(CN)6(3-)) as the extracellular electron acceptor. NADH in the cells was oxidized by ACNQ with the aid of diaphorase, and the reduced ACNQ was reoxidized with Fe(CN)6(3-). The extracellular electron transfer system promoted the generation of pyruvate, acetate, and acetoin from glucose, and restricted lactate production. Diaphorase activity increased when cultivation was aerobic, and this increased the concentrations of pyruvate, acetate, and acetoin relative to the concentration of lactate to increase in the presence of ACNQ and Fe(CN)6(3-)

Escherichia coli and its application in a mediated amperometric glucose sensor.

Escherichia coli cells, which contain apo-glucose dehydrogenase, were used in constructing a mediated amperometric glucose sensor. The E. coli modified glucose sensor, which was prepared by immobilizing E. coli cells behind a dialysis membrane on a carbon paste electrode containing 2,3-dimethoxy-5-methyl-1,4-benzoquinone (Q(0)), produced a current for the electrocatalytic oxidation of glucose with Q(0) as an electron transfer mediator only after the addition of a trace amount of pyrroloquinoline quinone (PQQ), the cofactor of the enzyme. This allows a novel method of glucose measurements free from the interference of the redox active substances, if contained, in a sample solution. The glucose sensor was insensitive to dioxygen; the currents measured under anaerobic and aerobic conditions, and even under dioxygen saturated conditions were almost the same in magnitude at a given concentration of glucose over the range of 0.2-10 mM. Response time of the glucose sensor was 2 min to attain 90% level of the steady-state current. The E. coli modified glucose sensor was reusable when treated with ethylenediaminetetraacetic acid (EDTA). When E. coli cells were lyophilized, they could be stored at room temperature in a dry box for more than six months without loss of the catalytic activity.

Kinetic analysis and mechanistic aspects of autoxidation of catechins.

A peroxidase-based bioelectrochemical sensor of hydrogen peroxide (H(2)O(2)) and a Clark-type oxygen electrode were applied to continuous monitoring and kinetic analysis of the autoxidation of catechins. Four major catechins in green tea, (-)-epicatechin, (-)-epicatechin gallate, (-)-epigallocatechin, and (-)-epigallocatechin gallate, were used as model compounds. It was found that dioxygen (O(2)) is quantitatively reduced to H(2)O(2). The initial rate of autoxidation is suppressed by superoxide dismutase and H(+), but is independent of buffer capacity. Based on these results, a mechanism of autoxidation is proposed; the initial step is the one-electron oxidation of the B ring of catechins by O(2) to generate a superoxide anion (O(2)(*-)) and a semiquinone radical, as supported in part by electron spin resonance measurements. O(2)(*-) works as a stronger one-electron oxidant than O(2) against catechins and is reduced to H(2)O(2). The semiquinone radical is more susceptible to oxidation with O(2) than fully reduced catechins. The autoxidation rate increases with pH. This behavior can be interpreted in terms of the increase in the stability of O(2)(*-) and the semiquinone radical with increasing pH, rather than the acid dissociation of phenolic groups. Cupric ion enhances autoxidation; most probably it functions as a catalyst of the initial oxidation step of catechins. The product cuprous ion can trigger a Fenton reaction to generate hydroxyl radical. On the other hand, borate ion suppresses autoxidation drastically, due to the strong complex formation with catechins. The biological significance of autoxidation and its effectors are also discussed.