Dioxygen activation at non-heme iron: insights from rapid kinetic studies

One of the common biochemical pathways of binding and activation of dioxygen involves non-heme iron centers. The enzyme cycles usually start with an iron(II) or diiron(II) state and traverse via several intermediates (detected or postulated) such as (di)iron(III)-superoxo, (di)iron(III)-(hydro)peroxo, iron(III)iron(IV)-oxo, and (di)iron(IV)-oxo species, some of which are responsible for substrate oxidation. In this Account, we present results of kinetic and mechanistic studies of dioxygen binding and activation reactions of model inorganic iron compounds. The number of iron centers, their coordination number, and the steric and electronic properties of the ligands were varied in several series of well-characterized complexes that provided reactive manifolds modeling the function of native non-heme iron enzymes. Time-resolved cryogenic stopped-flow spectrophotometry permitted the identification of kinetically competent intermediates in these systems. Inner-sphere mechanisms dominated the chemistry of dioxygen binding, intermediate transformations, and substrate oxidation as most of these processes were controlled by the rates of ligand substitution at the iron centers.     

Metalloporphyrines as active site analogues--lessons from enzymes and enzyme models

Research at the interface of enzyme chemistry and organic chemistry of metal complexes is particularly rewarding employing metal porphyrins as cofactor surrogates. Three examples are discussed: active site analogues of cytochrome P450 and chloroperoxidase (CPO), both heme-thiolate proteins, and enzyme models of beta-carotene monooxygenase, a non-heme iron protein. In all cases, catalytically active synthetic systems could be established displaying chemical reactivity close to the native proteins. Further, it is demonstrated that enzymatic reaction mechanisms can be elucidated by means of active site analogues (CPO) and information can be obtained from enzyme models that is useful to explain certain aspects of Nature's sophisticated approach to develop very efficient catalysts.     

Catalytic decomposition of nitrous oxide over Fe-BEA zeolites: Essential components of iron active sites

The catalytic decomposition of N₂O was investigated over Fe-BEA zeolites treated with various methods such as reduction, steaming and dissolution with potassium nitrate and nitric acid solutions in order to deduce the essential components of the active sites for the decomposition. The iron species were characterized by XPS, XANES, ESR, NO adsorption, and linear sweep voltammetry. The reduction-treated Fe-BEA zeolite with the large amounts of Fe(II) and Fe(III) species showed the highest activity. On the contrary, the dissolution treatment with the potassium nitrate solution seriously deteriorated the catalytic activity of the Fe-BEA zeolite by agglomerating iron oxide clusters and interaction between iron and potassium atoms. The catalytic roles of Fe(II)/Fe(III) species and the negative effect of potassium on the catalytic activity of the Fe-BEA zeolites were discussed.     

Type of hydrocarbon activation and nature of active sites of base catalysts in methane oxidative dehydrodimerization

Methane activation on magnesium-oxide-cased catalysts for methane dehydrodimerization has been studied. As found, the rate of dehydrodimerization changes proportionally with the concentration of base sites on the catalytic surface. The activation of methane results in the formation of the methyl anion, as ascertained by IR-Fourier spectroscopy. The dehydrodimerization rates have been compared with the concentration of one-electron sites and the rate of the CH₄---CD₄ isotope exchange. Based on the data obtained a new mechanism of methane dehydrodimerization has been proposed; at the first reaction step a fast heterolytic dissociation of methane on base sites takes place. The rate-determining step is that of the metal-methyl dissociation leading to the formation of the methyl radical which determines a high activation energy of methane dehydrodimerization on oxide catalysts.     

Ligand effects on dioxygen activation by copper and nickel complexes: reactivity and intermediates

Copper and nickel complexes having various active-oxygen species M n -O 2 ( n = 1 or 2), such as trans-(micro-1,2-peroxo)Cu (II) 2, bis(micro-oxo)M (III) 2, bis(micro-superoxo)Ni (II) 2, and ligand-based alkylperoxo-M (II) n , can be produced by a series of tetradentate tripodal ligands (TMPA analogues) containing sterically demanding 6-methyl substituent(s) on the pyridyl group(s), where TMPA = tris(2-pyridylmethyl)amine. Roles of the methyl substituent(s) for the formation of the active-oxygen species and their oxidation reactivities are reported.     

New mononuclear CuII and ZnII complexes capable of stabilizing phenoxyl radicals as models for the active form of galactose oxidase

Herein, we describe the synthesis and crystal structure of mononuclear Cu^II and Zn^II complexes containing the new pentadentate H₂L ligand (H₂L = N-(methyl)-N^′-(2-pyridylmethyl)-N,N^′-bis(3,5-di-tert-butyl-2-hydroxybenzyl)-1,3-propanediamine. The [Cu^II(L)] and [Zn^II(L)] complexes and their one-electron oxidized [Cu^II]⁺ ([1]⁺) and [Zn^II]⁺ ([2]⁺) compounds exhibit electrochemical and spectroscopic (UV–Vis and EPR) features with relevant insight into the oxidizing half-reaction of the metalloenzyme galactose oxidase.     

Molecular description of active sites in oxidation reactions: Acid-base and redox properties, and role of water

Oxidation reactions in heterogeneous catalysis usually involve a Mars and van Krevelen mechanism which includes activation of the substrate on a metallic cation, insertion of oxygen from lattice oxygen ions, a redox mechanism on the catalyst surface, and the transfer of several electrons. It follows that such a reaction necessitates both acid-base and redox properties of a catalyst the acid site being of Lewis type (cations) and the basic sites being the surface O^2- or OH^- species which could exhibit electrophilic or nucleophilic properties.

The active site should be able to fulfil the following requirements: H abstraction from the substrate, oxygen insertion, and electron transfer. It has been shown to correspond to an ensemble of atoms of limited size in an inorganic molecular complex. It could correspond to local structural defects including steps, kinks, coordinatively unsaturated cations or to clusters of atoms on the surface. Some examples are described namely:

1. (i) n-butane oxidation to maleic anhydride on (VO)₂P₂O₇ catalyst where four dimers of vanadyl cations on the (100) face were suggested to form the active site;

2. (ii) isobutyric acid oxidative dehydrogenation to methacrylic acid on iron hydroxy phosphates where trimers of iron oxide octahedra were shown to constitute the most efficient and selective catalytic site while water was observed to be absolutely necessary to facilitate the reaction which corresponds to hydroxylated surface sites ensuring the redox mechanism;

3. (iii) propane oxidative dehydrogenation to propene on VMgO samples which was shown to depend both on VO_x arrangements with respect to MgO and on the basicity of the material induced by MgO while vanadium cations induced acidic features.

     

MoS2光电催化剂活性位点的优化和效能研究进展

MoS₂是一类典型的后石墨烯二维材料,具有优异的光电性能及良好的化学稳定性,是近年来被广泛研究的一类新型光电催化剂,有关其催化活性位点的构效关系和反应机理是目前的研究热点。本文介绍了MoS₂的结构特性和活性位点分布,重点归纳分析了近年来有关MoS₂活性位点的构筑方法,包括利用降低维度、晶相调控、特殊形貌设计和非晶化等方法对MoS₂本体进行改造,以及采用原子掺杂、缺陷工程的方式对MoS₂进行协同修饰。通过对催化性能及反应机理的研究,表明这些方法能有效提高MoS₂的催化性能。最后,结合研究现状对目前存在的挑战和研究方向进行了分析总结,指出在MoS₂上构造有序分布的活性位点并平衡其稳定性以及催化性能的构效关系是未来MoS₂在光电催化领域中的研究重点。     

Synthetic mutants of Clostridium pasteurianum ferredoxin: open iron sites and testing carboxylate coordination

The entire polypeptide chains for two new Clostridium pasteurianum ferredoxin (Fd) mutants were prepared with the following site-specific substitutions: Cys11Asp and Cys11 alpha-aminobutyric acid (Cys11 alpha- Aba), the latter being a non-naturally occurring amino acid. Standard t- Boc procedures were used for the synthesis and the peptides. The two apoproteins were reconstituted to the 2[4Fe-4S] holoprotein and their spectroscopic, redox and thermal properties were compared with those of native C.pasteurianum Fds. The fully reconstituted Cys11Asp and Cys11 alpha-Aba mutants were initially found to have both clusters intact, i.e. they were 2[4Fe-4S] ferredoxins. The unconventional ligands of Asp and alpha-Aba led to holo-Fds that were not very stable and easily released an iron to form the [3Fe-4S] cluster, presumably through oxidation. The Cys11 alpha-Aba mutant was somewhat more thermally stable than Cys11Asp. In contrast, while both mutants were less stable than the native protein upon exposure to oxygen, the Cys11 alpha-Aba mutant was less stable than Cys11Asp. The Cys11Gly mutant was also prepared, but all attempts, despite repeated and varied experimental conditions, at reconstitution to the Cys11Gly holo 2[4Fe-4S] Fd were unsuccessful, probably because a Gly-Gly sequence is known to break structure. This work, when compared with molecular biological site- specific mutagenesis, shows some of the advantages of chemical/in vitro reconstitution: certain mutants which cannot be detected as holoproteins by site-specific mutagenesis can be formed after all in vitro. Nonetheless, it seems apparent that altering any of the Cys coordination sites of the Fd clusters results in fundamentally more unstable ferredoxins.      

Gasification of active carbons of different texture impregnated with nickel,cobalt and iron

Various series of activated carbons of different texture were impregnated with nickel, cobalt and iron and their reactivities determined for gasification with carbon dioxide. It was found that nickel and cobalt were better catalysts than iron, but in any case high conversions could not be obtained due to catalyst deactivation. The same kinetics were found for nickel and cobalt, suggesting a common mechanism. The texture of the activated carbons was found to be the key factor in determining metal dispersion and, therefore, the reactivity of the impregnated carbons.     

Comparison of N2O emissions from soils at three temperate agricultural sites: simulations of year-round measurements by four models

Nitrous oxide (N₂O) flux simulations by four models were compared with year-round field measurements from five temperate agricultural sites in three countries. The field sites included an unfertilized, semi-arid rangeland with low N₂O fluxes in eastern Colorado, USA; two fertilizer treatments (urea and nitrate) on a fertilized grass ley cut for silage in Scotland; and two fertilized, cultivated crop fields in Germany where N₂O loss during the winter was quite high. The models used were daily trace gas versions of the CENTURY model, DNDC, ExpertN, and the NASA-Ames version of the CASA model. These models included similar components (soil physics, decomposition, plant growth, and nitrogen transformations), but in some cases used very different algorithms for these processes. All models generated similar results for the general cycling of nitrogen through the agro-ecosystems, but simulated nitrogen trace gas fluxes were quite different. In most cases the simulated N₂O fluxes were within a factor of about 2 of the observed annual fluxes, but even when models produced similar N₂O fluxes they often produced very different estimates of gaseous N loss as nitric oxide (NO), dinitrogen (N₂), and ammonia (NH₃). Accurate simulation of soil moisture appears to be a key requirement for reliable simulation of N₂O emissions. All models simulated the general pattern of low background fluxes with high fluxes following fertilization at the Scottish sites, but they could not (or were not designed to) accurately capture the observed effects of different fertilizer types on N₂O flux. None of the models were able to reliably generate large pulses of N₂O during brief winter thaws that were observed at the two German sites. All models except DNDC simulated very low N₂O fluxes for the dry site in Colorado. The US Trace Gas Network (TRAGNET) has provided a mechanism for this model and site intercomparison. Additional intercomparisons are needed with these and other models and additional data sets; these should include both tropical agro-ecosystems and new agricultural management techniques designed for sustainability.     

Polymer-supported enzymes: Achievements,problems and perspectives

Over the last two decades, intense research activity in the area of enzyme and cell immobilisation techniques has fostered the industrial application of several new enzyme-catalysed processes for chemical and pharmaceutical production, as well as for the food industry. Biosensors, based on immobilised enzymes or whole cells, are finding wide-spread applications in the analytical and biomedical fields. The recent remarkable progress attained in the development of recombinant DNA techniques promises to provide any enzyme having properties tailored to meet the specific demands of any user.     

Ammoxidation of methylaromatics over vanadium phosphate catalysts II. Catalyst formation, active sites and reaction mechanism

The interactions of VOHPO₄· 0.5H₂O and (VO)₂P₂O₇ with the ammoxidation feed and the single components such as ammonia, oxygen, water and component mixtures were studied in detail using XRD and temperature-programmed reaction spectroscopy. The aim of this work was to improve the knowledge of the formation of the active phases or active sites of the catalysts from their precursors under the condition of the ammoxidation reaction. Similar catalytic properties of various applied VPO materials were discussed in terms of the presence of similar structure elements (domains of adjacent edge-sharing VO₆ octahedra-units and P-O-NH₄ groups).     

The nature, role and fate of surface active sites in Li/MgO oxidative coupling catalysts

Drastic changes in nature and surface composition of lithium promoted magnesia are shown to occur in the course of the oxidative coupling of methane, together with changes in kinetics. Thus, the fresh material with a large and heterogeneous surface promotes the total oxidation of methane into CO, CO₂ while the aged catalyst, sintered, partially decarbonated and coated by an alkali film leads preferentially to dimerization products. On the basis of the above features, the nature, role and fate of the active sites for methane conversion are discussed, aiming at possible catalyst improvements.     

Nature,activity and stability of active sites during alkali metal carbonate-catalysed gasification reactions of coal char

The activity pattern of potassium carbonate-containing coal char during gasification with low pressures of steam, is shown to consist of at least three stages of activity. The relative importance of these stages, that occur at different burn-off levels, is influenced by the pretreatment temperature, the catalyst loading and the sequence of catalyst addition and coal pyrolysis. An explanation for this is given, based on the assumption of three processes occurring simultaneously during pretreatment: oxygen retention by potassium, intercalation of potassium and potassium carbonate crystallization. Comparison with activated carbon results supports the interpretation of the results. It follows that the maximum activity of alkali metals correlates with the oxygen content of carbon and that intercalation depends on the degree of graphitization of carbon.     

Current-time behaviour for copper electrodeposition. II. Experimental examination of surface diffusion and direct incorporation models at hemispherical sites

The surface diffusion model has been examined by the measurement of current-time transients for copper electrodeposition on to a polycrystalline copper electrode in acidic copper sulphate solution. The results did not satisfy the surface diffusion model criteria. The current-time profiles showed a linear dependence on 1/t ^1/2 except at high overpotentials. This behaviour could be explained by a direct incorporation model at hemispherical sites through a hemispherical diffusion layer. A parameter characterizing this model, the number of sites, was estimated to be of the order of 10⁴ cm^–2.     

Investigations of the roles of the distal heme environment and the proximal heme iron ligand in peroxide activation by heme enzymes via molecular engineering of myoglobin

To pursue structure-function relationships of heme enzymes in the activation of peroxides, we have chosen to use myoglobin as the framework for our molecular engineering studies. Comparison of the crystal structures of myoglobin and peroxidases reveals differences in the arrangement of amino acid residues in heme active sites. On the basis of these structural differences and the reaction mechanisms of peroxidases, we have converted myoglobin into a peroxidase-like enzyme by alternation of the heme distal pocket via site-directed mutagenesis. The replacement of the proximal histidine with cysteine and the exogenous substituted imidazoles slightly accelerates the peroxide O-O bond cleavage due to the electron donor characteristics. However, we have not observed an enhancement in the activation of peroxide by the proximal mutant with tyrosine, the exogenous phenolate, and benzoate. A clear understanding of the absolute role of the proximal ligand remains elusive.     

Predicted ligand interactions for 3',5'-cyclic nucleotide-gated channel binding sites: comparison of retina and olfactory binding site models

Cyclic nucleotide-gated channels (CNGC) open in response tothe binding of 3',5'-cyclic nucleotides. Members of the CNGCfamily vary as much as 100-fold in their ability to respondto cAMP and cGMP. Molecular models of the nucleotide bindingdomains of the bovine retina and catfish and rat olfactory CNGCswere built from the crystal structure of cAMP bound to catabolitegene activator protein (CAP) with AMMP, a program for molecularmechanics and dynamics. The nucleotide conformation can be predictedfrom the number of strong and weak interactions between thepurine ring and the binding site. The amino acids predictedto be important for determining the nucleotide affinity andspecificity are residues 61, 83 (mediated through a water molecule),119 and 127 (CAP sequence numbers) which interact with the purinering. These residues also dictate the conformation of the ligandin the binding pocket cGMP is preferentially bound in the synconformation in bovine retina, bovine olfactory and rat olfactoryCNGCs due to Thr83, while either conformation can bind in catfisholfactory CNGC. cAMP is predicted to bind either in syn or anticonformation, depending on the interaction with residue 119:the anti conformation is preferentially bound in olfactory CNGCs.     

Photocatalytic oxidation of toluene at indoor air levels (ppbv): Towards a better assessment of conversion, reaction intermediates and mineralization

We report here a new analytical methodology for the investigation of toluene photocatalytic removal at indoor-relevant concentration level (ppbv). Experiments were performed using an annular flow-through reactor with TiO₂ as photocatalyst, toluene as a model VOC and under different ranges of relative humidity (RH: 0–70%), inlet concentration (20–400 ppbv) and flow rate (70–350 mL min⁻¹). Analysis of reaction intermediates was conducted using an automated thermal desorption technique coupled to GC–MS instrument (ATD–GC–MS) whereas a GC coupled to pulsed discharge helium ionization detector (GC–PDPID) was used for the first time for on-line measurements of CO and CO₂ at ppbv level.Under these conditions, toluene conversion was up to 90–100% with a slight influence of inlet concentration and RH, whereas flow rate was found to be a prevalent factor. Mineralization (%) varied from 55 to 95% and has shown to be strongly inhibited by the increase of RH whereas flow rate and inlet concentration exhibited a negligible effect. The reaction intermediates were found to be different according to the RH level: in absence of water vapor, traces of low molecular weight carbonyls (formaldehyde, methyl glyoxal, etc.) were detected and quantified in the gas phase whereas at RH 40%, hydroxylated intermediates such as cresols and benzyl alcohol were observed. On the basis of identification results, a reaction mechanism was proposed involving mainly direct hole oxidation at dry conditions and hydroxylation by OH radicals at high RH level.