Kinetics of oxidation of <Emphasis Type="SmallCaps">d</Emphasis>(+)melibiose and cellobiose by <Emphasis Type="Italic">N</Emphasis>-bromoacetamide using a rhodium(III) chloride catalyst

The kinetics of oxidation of the sugars d(+)Melibiose (mel) and Cellobiose (cel) by N-bromoacetamide (NBA) in the presence of Rh(III) chloride as homogeneous catalyst in acidic medium at 45 °C have been investigated. The reactions are first-order with respect to [NBA], [Rh(III)] and [substrate]. The rate is proportional to [H⁺]. No effects of [Hg(II)], [NHA] or [Cl⁻] on the rates were observed. Ionic strength and dielectric constant also have little effect. The observed kinetic data, available literature and spectroscopic evidence lead us to conclude that NBAH⁺ and [RhCl₅(H₂O)]²⁻ are the reactive species of NBA and Rh(III) chloride, respectively. The rate-determining step of the proposed reaction path common for both sugars gives an activated complex by the interaction of a charged complex species and neutral sugar molecule, which in the subsequent steps disproportionates into the reaction products with the regeneration of catalyst. The reactions have been studied at four different temperatures and with the help of first-order rate constant values, various activation parameters have been calculated. The main oxidation products of the reactions were identified as arabinonic acid, formic acid and lyxonic acid in the case of mel and arabinonic acid and formic acid in the case of cel.     

Kinetics and mechanism of <Emphasis Type="Italic">L</Emphasis>-isoleucine oxidation by alkaline diperiodatoargentate(III)

The oxidation of L-isoleucine by alkaline diperiodatoargentate(III) (DPA) at 298 K and a constant ionic strength of 0.80 mol dm⁻³ was studied spectrophotometrically. The stoichiometry is [L-isoleucine]: [DPA] = 1:2. The reaction is first order in [DPA] and has less than unit order in both [L-isoleucine] and [alkali] and retarding effect in The oxidation reaction in alkaline medium has been shown to proceed via a L-isoleucine–DPA complex, which further reacts with one molecule of DPA in a rate determining step followed by other fast steps to give the products. Spot test and IR were used to identify the main products. The reaction constants involved in the different steps of the mechanism are calculated. The activation parameters with respect to the slow step of the mechanism are computed and discussed, and thermodynamic quantities are also determined. The probable active species of oxidant have been identified. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Kinetic study of ruthenium(III)-catalyzed oxidation of <Emphasis Type="SmallCaps">l</Emphasis>-alanine by diperiodatoargentate(III) in aqueous alkaline medium

The kinetics of Ru(III)-catalyzed oxidation of l-alanine (Ala) by diperiodatoargentate(III) (DPA) in alkaline medium at 25 °C and a constant ionic strength of 0.90 mol dm⁻³ was studied spectrophotometrically. The products are acetaldehyde, Ag(I), ammonia and bicarbonate. The [Ala] to [DPA] stoichiometry is 1:1. The reaction is first order in both [Ru(III)] and [DPA] and has less than unit order in both [Ala] and [alkali]. Addition of periodate has a retarding effect on the reaction. The effects of added products, ionic strength and dielectric constant of the reaction medium have been investigated. The reaction proceeds via a Ru(III)–Ala complex, which further reacts with one molecule of monoperiodatoargentate(III) in the rate-determining step. The reaction constants were calculated at different temperatures and the activation parameters have been evaluated.     

Bioconversion of <Emphasis Type="SmallCaps">d</Emphasis>-glucose into <Emphasis Type="SmallCaps">d</Emphasis>-glucosone by Glucose 2-oxidase from <Emphasis Type="Italic">Coriolus versicolor</Emphasis> at Moderate Pressures

Glucose 2-oxidase (pyranose oxidase, pyranose:oxygen-2-oxidoreductase, EC 1.1.3.10) from Coriolus versicolor catalyses the oxidation of d-glucose at carbon 2 in the presence of molecular O₂ producing d-glucosone (2-keto-glucose and d-arabino-2-hexosulose) and H₂O₂. It was used to convert d-glucose into d-glucosone at moderate pressures (i.e. up to 150 bar) with compressed air in a modified commercial batch reactor. Several parameters affecting biocatalysis at moderate pressures were investigated as follows: pressure, [enzyme], [glucose], pH, temperature, nature of fluid and the presence of catalase. Glucose 2-oxidase was purified by immobilized metal affinity chromatography on epoxy-activated Sepharose 6B-IDA-Cu(II) column at pH 6.0. The rate of bioconversion of d-glucose increased with the pressure since an increase in the pressure with compressed air resulted in higher rates of conversion. On the other hand, the presence of catalase increased the rate of reaction which strongly suggests that H₂O₂ acted as inhibitor for this reaction. The rate of bioconversion of d-glucose by glucose 2-oxidase in the presence of either nitrogen or supercritical CO₂ at 110 bar was very low compared with the use of compressed air at the same pressure. The optimum temperature (55°C) and pH (5.0) of d-glucose bioconversion as well as kinetic parameters for this enzyme were determined under moderate pressure. The activation energy (E _a) was 32.08 kJ mol⁻¹ and kinetic parameters (V _max, K _m, K _cat and K _cat/K _m) for this bioconversion were 8.8 U mg⁻¹ protein, 2.95 mM, 30.81 s⁻¹ and 10,444.06 s⁻¹ M⁻¹, respectively. The biomass of C. versicolor as well as the cell-free extract containing glucose 2-oxidase activity were also useful for bioconversion of d-glucose at moderate pressures. The enzyme was apparently stable at moderate pressures since such pressures did not affect significantly the enzyme activity.     

Development of transition state analogue inhibitors for <Emphasis Type="Italic">N</Emphasis>-acetylglycosyltransferases bearing <Emphasis Type="SmallCaps">d</Emphasis>psico- or <Emphasis Type="SmallCaps">d</Emphasis>tagatofuranose scaffolds

New potential transition state analogue inhibitors for N-acetylglucosyltransferases (GnTs) were synthesised. These compounds based on psico- and tagatofuranose (structure) scaffold contained a 2-thiophenyl-1-O-diethylphosphate moiety mimicking the proposed model of the transition state of the enzymatic reaction catalysed by N-acetylglucosyltransferases. The synthesised compounds as well as their precursors were fully characterised by NMR, optical rotation and mass techniques. Anomeric configuration of tagatofuranose derivatives was confirmed by X-ray crystallography. Two types of potential human glycosyltransferase (GnTs) inhibitors representing donor UDP-GlcNAc, assigned for biological assays on human GnTs, were prepared.     

Kinetics of ruthenium(III) catalyzed and uncatalyzed oxidation of monoethanolamine by <Emphasis Type="Italic">N</Emphasis>-bromosuccinimide

Kinetics of uncatalyzed and ruthenium(III) catalyzed oxidation of monoethanolamine by N-bromosuccinimide (NBS) has been studied in an aqueous acetic acid medium in the presence of sodium acetate and perchloric acid, respectively. In the uncatalyzed oxidation the kinetic orders are: the first order in NBS, a fractional order in the substrate. The rate of the reaction increased with an increase in the sodium acetate concentration and decreased with an increase in the perchloric acid concentration. This indicates that free amine molecules are the reactive species. Addition of halide ions results in a decrease in the kinetic rate, which is noteworthy. Both in absence and presence of a catalyst, a decrease in the dielectric constant of the medium decreases the kinetic rate pointing out that these are dipole—dipole reactions. A relatively higher oxidation state of ruthenium i.e., Ru(V) was found to be the active species in Ru(III) catalyzed reactions. A suitable mechanism consistent with the observations has been proposed and a rate law has been derived to explain the kinetic orders.     

<Emphasis Type="SmallCaps">d</Emphasis>-Amino Acids in Animal Peptides

Summary. Secreted peptides from diverse sources have been found to contain a d-amino acid. From the sequence of cloned mRNAs coding for the precursors of such peptides it could be deduced that in all cases tested so far the d-amino acid in the final product is derived from the corresponding l-amino acid present in the primary product of translation. Enzymes catalyzing such an l- to d-isomerization in peptide linkage have been isolated from the venom of a spider and the skin secretions of frogs. Even though these are completely different proteins, the reaction mechanism is the same, namely a de-protonation/re-protonation of the α-carbon of an amino acid with concomitant inversion of the chirality. Sequences potentially coding for homologues of the frog enzyme are present in the genome of different vertebrate species.     

Synthesis and studies of spectroscopic and electrochemical properties of dinuclear ruthenium(<Emphasis Type="SmallCaps">II</Emphasis>) and manganese(<Emphasis Type="SmallCaps">II</Emphasis>) complexes

New dinuclear ruthenium manganese complexes of general composition (bpy)₂Ru(L)MnCl_x(H₂O)₂ (L is 1,10-phenanthroline-5,6-dione, 3,3′-dicarboxy-2,2′-bipyridyl, or bis(pyrazolyl); x = 2 or 4) were synthesized by the reaction of (bpy)₂Ru(L) with MnCl₂ · 4H₂O. These compounds and the starting mononuclear ruthenium complexes were studied by spectrophotometric and electrochemical methods in MeCN. The position of the charge-transfer band Ru^II → L in the spectra depends on the donor-acceptor characteristics of the ligand L. For the dinuclear complex under study, the formal potentials of reversible one-electron oxidation of Ru^II are in the range of 0.9–1.2 V (vs. the standard hydrogen electrode), whereas oxidation of Mn^II occurs at more positive (by 0.1–0.2 V) potentials. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2281–2285, October, 2005.     

Ferrocene-containing tri- and tetranuclear cyclic copper(<Emphasis Type="SmallCaps">i</Emphasis>) and silver(<Emphasis Type="SmallCaps">i</Emphasis>) pyrazolates

New tri- and tetranuclear macrocyclic silver(i) and copper(i) 3-ferrocenyl-5-(trifluoromethyl)pyrazolates were prepared: [{(3-((η⁵-C₅H₄)Fe(η⁵-C₅H₅))-5-(CF₃)-Pz}M]₃ (M = Cu (1), Ag (2)) and [{(3-(( η⁵-C₅H₄)Fe(η⁵-C₅H₅))-5-(CF₃)-Pz}Cu]₄ (3). The structures of compounds were established by X-ray diffraction analysis. In the crystalline state, a planar trinuclear silver-containing macrocycliс pyrazolate and a saddle-shaped tetranuclear copper-containing macrocycle are formed. The introduction of a bulky substituent, ferrocene, into the pyrazole ligand results in complete shielding of the acidic metal sites, which precludes the coordination of base molecules.     

Investigation of the mechanism of oxidation of <Emphasis Type="SmallCaps">l</Emphasis>-cystine by diperiodatoargentate(III) in aqueous alkaline medium by stopped flow technique

The kinetics of oxidation of l-cystine by diperiodatoargentate(III) (DPA) in alkaline medium at a constant ionic strength of 0.10 mol dm⁻³ was studied spectrophotometrically. The reaction exhibits a 1:2 stoichiometry (l-cys:DPA) and is first order in [DPA]. The order in both [l-cystine] and [alkali] changes from first to zero order as their concentrations increase. Added periodate retards the rate of reaction. The effects of added products have been investigated. The active species of silver(III) is identified as monoperiodatoargentate(III) (MPA). The oxidation is thought to proceed via an MPA–l-cystine complex, which decomposes in a rate-determining step to give a free radical followed by a fast step to give the products. The products were identified by spot test, IR and GC–MS. The reaction constants involved in different steps of the mechanism were evaluated. The activation parameters with respect to the slow step of the mechanism were computed and discussed.     

Kinetics and mechanism of the ruthenium(III)-catalysed oxidation of aminoalcohols by alkaline hexacyanoferrate(III)

Summary The kinetics of the ruthenium(III)-catalysed oxidation of aminoalcoholsviz. 2-aminoethanol and 3-aminopropanol by alkaline hexacyanoferrate(III) has been studied spectrophotometrically. The reactions are rapid initially, then follow a second order rate dependence with respect to each of the catalyst and the oxidant. The second order rate dependence with respect to ruthenium(III) was observed for the first time. The order in [Aminoalcohol] and [OH^–] is unity in each case. A suitable mechanism, consistent with the observed kinetic data is postulated.     

New tin(<Emphasis Type="SmallCaps">iv</Emphasis>) <Emphasis Type="Italic">o</Emphasis>-iminosemiquinone complexes

New tin(iv) mono- and bis-o-iminosemiquinone complexes were obtained by the exchange reaction of radical anion lithium salt of 4,6-di-tert-butyl-N-(2,6-diisopropylphenyl)-o-imino-benzoquinone with tin(iv) organochlorides. The compounds synthesized were characterized by EPR spectroscopy and X-ray diffraction analysis. Substituents on the tin atom were found to affect stability of paramagnetic metal derivatives formed.     

Synthesis of metallacarborane ruthenium(<Emphasis Type="SmallCaps">ii</Emphasis>) and ruthenium(<Emphasis Type="SmallCaps">iii</Emphasis>) complexes with chelate 1,3-bis(diphenylphosphino)propane ligand and their mutual transformation in one-electron redox reactions

The first example of 12-vertex metallapolyhedra, namely, tris-phosphorus-containing closo-metallacarborane ruthenium(II) cluster Open image in new window , was synthesized and structurally studied. A principal possibility of the use of aliphatic amines as one-electron reducing agents converting carborane ruthenium(III) complexes to structurally close ruthenium(II) compounds was experimentally demonstrated.     

Coordination polymers based on zinc(<Emphasis Type="SmallCaps">ii</Emphasis>) and manganese(<Emphasis Type="SmallCaps">ii</Emphasis>) with 1,4-cyclohexanedicarboxylic acid

Three new metal-organic coordination polymers were obtained namely, [Mn₃(chdc)₃-(NMP)₂(DMF)₂] (1, chdc^2– is trans-1,4-cyclohexanedicarboxylate, NMP is N-methylpyrrolidone, DMF is N,N-dimethylformamide), [Zn₃(chdc)₃(NMP)₂]?2NMP (2), and [Zn₃(chdc)₃(ur)-(DMF)_0.5]?DMF (3, ur is the urotropine). The crystal structures of polymers 1, 2, and 3 were determined by single-crystal X-ray crystallography. All three compounds were found to contain a trinuclear secondary building unit {M₃(OOC)₆}. Coordination polymers 1 and 2 have a layered structure, while polymer 3 has a three-dimensional coordination framework with isolated pores formed due to the presence of urotropine bridging molecules. Compounds 1 and 3 were characterized by IR spectroscopy, thermogravimetric and elemental analysis data, powder X-ray diffraction. Compound 3 was also characterized by UV-Vis diffuse reflectance spectrum.     

Temperature- and pressure-responsive properties of <Emphasis Type="SmallCaps">l</Emphasis>- and<Emphasis Type="SmallCaps"> dl</Emphasis>-forms of poly(<Emphasis Type="Italic">N</Emphasis>-(1-hydroxymethyl)propylmethacrylamide) in aqueous solutions

The structural transition of the l- and dl forms of poly(N-(1- hydroxymethyl)propylmethacrylamide (PHMPMA) in aqueous solution was studied by measuring the pressure dependence of the apparent scattering intensity, differential scanning calorimetry (DSC), and circular dichroism (CD). The thermodynamic implications of the results are discussed in relation to the chiral structure of the side chain, and differences in the thermal and barometric transitions. T-P diagrams of the transition showed characteristic ellipsoid features. Antagonism of the temperature and pressure effects was observed only for P(dl-HMPMA). For P(l-HMPMA), the transition temperature (T _tr) decreased with increasing pressure, and the highest T _tr was observed at atmospheric pressure (0.1 MPa). For both polymers, the highest P _trs were observed at the lowest temperatures. The l polymer showed a specific negative peak in its CD spectrum at around 220 nm in the lower temperature region and the temperature dependence was reproduced by a single-step transition, with the midpoint corresponding to the T _tr obtained from the scattering measurements. Coupled with the results from the DSC, the different behavior between the P(l-HMPMA) and P(dl-HMPMA) could be explained in terms of the chain states before and after the transition. The cooperative factors derived from the DSC measurement revealed that about 4 to 5 polymers of the present size were necessary to perform a thermal transition for P(l-HMPMA), and that P(dl-HMPMA) underwent its transition as an almost single molecular event.This revised version was published online in June 2005 with correction to the article category.     

Kinetics and mechanism of oxidation of <Emphasis Type="Italic">N</Emphasis>-methylethylamine by bis(hydrogenperiodato)argentate(III) complex anion

Oxidation of N-methylethylamine by bis(hydrogenperiodato)argentate(III) ([Ag(HIO₆)₂]⁵⁻) in alkaline medium results in demethylation, giving rise to formaldehyde and ethylamine as the oxidation products. The oxidation kinetics has been followed spectrophotometrically in the temperature range of 20.0–35.0 °C, and shows an overall second-order character: being first-order with respect to both Ag(III) and N-methylethylamine. The observed second-order rate constants k′ increase with increasing [OH⁻] of the reaction medium, but decrease with increasing the total concentration of periodate. An empirical rate expression for k′ has been derived as: k′ = (k _a + k _b[OH⁻])K ₁/{f([OH⁻])[IO₄ ⁻]_tot + K ₁}, where k _a and k _b are rate parameters, and K ₁ is an equilibrium constant. These parameters have been evaluated at all the temperatures studied, enabling calculation of activation parameters. A reaction mechanism is suggested to involve two pre-equilibria, leading to formation of an intermediate Ag(III) complex, namely [Ag(HIO₆)(OH)(MeNHEt)]²⁻. In the subsequent rate-determining steps, this intermediate undergoes inner-sphere electron transfer from the coordinated amine to the metal center via two distinct routes, one of which is spontaneous while the other is mediated by a hydroxide ion.     

Kinetics and mechanism of the reaction of hydroxopentaaquarhodium(III) ion with <Emphasis Type="SmallCaps">l</Emphasis>-Arginine in aqueous solution

The kinetics of the aqua ligand substitution from hydroxopentaaquarhodium(III) ion, [Rh(H₂O)₅(OH)]²⁺, by l-Arginine has been studied spectrophotometrically as a function of Arginine concentration, and temperature, at pH 4.3. The reaction proceeds via a rapid outer sphere association complex formation step followed by two consecutive steps. The first of these involves ligand-assisted anation, while the second involves chelation as the second aqua ligand is displaced. The association equilibrium constant for the outer sphere complex formation has been evaluated together with the rate constants for the two subsequent steps. The activation parameters for both steps have been evaluated using Eyring’s equation. Thermodynamic parameters calculated from the temperature dependence of the outer sphere association equilibrium constants are also consistent with an associative mode of activation. The product of the reaction has been characterized by conductivity measurement and IR spectroscopic analysis.     

Complexing properties of 1-[5-(hydrazidomethylsulfinyl)pentyl]-3,5-dimethylisocyanurate with cobalt(<Emphasis Type="SmallCaps">ii</Emphasis>), nickel(<Emphasis Type="SmallCaps">ii</Emphasis>), and iron(<Emphasis Type="SmallCaps">iii</Emphasis>)

Composition and thermodynamic stability of complexes of 1-[5-(hydrazidomethylsulfinyl)pentyl]-3,5-dimethylisocyanurate with cobalt(ii), nickel(ii), and iron(iii) in H₂O—DMSO medium (60 vol.% DMSO) was studied by spectrophotometry, potentiometric titration, and mathematical modeling (CPESSP program). The geometries of all studied structures were optimized by the MM2 molecular mechanics method to obtain the primary target data (estimates) on coordination pattern of 1-[5-(hydrazidomethylsulfinyl)pentyl]-3,5-dimethylisocyanurate in complexes with cobalt(ii), nickel(ii), and iron(iii). Cobalt(ii) and nickel(ii) formed the 1 : 1 complexes; while iron(iii) under experimental conditions excluding redox reactions gave the 1 : 1 and 1 : 2 complexes. All complexes contain ligands in neutral form.     

High Soluble Expression of <Emphasis Type="SmallCaps">d</Emphasis>-Amino Acid Oxidase in <Emphasis Type="Italic">Escherichia coli</Emphasis> Regulated by a Native Promoter

To express high-active soluble d-amino acid oxidase (DAAO), a constitutive plasmid that is regulated by a native hydantoinase promoter (P_Hase), was constructed. A d-amino acid oxidase gene (dao) was ligated with the P_Hase and cloned into pGEMKT to constitutively express protein of DAAO without the use of any inducer such as isopropyl β-d-1-thiogalactopyranoside which is poisonous to the cells and environment. The ribosome binding site region, host strain, and fermentation conditions were optimized to increase the expression level. When cultivated in a 5-m³ fermenter, the enzyme activity of JM105/pGEMKT-R-DAAO grown at 37 °C was found to be 32 U/mL and increase 16-fold over cells of BL21(DE3)/pET-DAAO grown at 28 °C. These results indicate the success of our approaches to overproducing DAAO in soluble form in Escherichia coli.     

A Novel 2-Keto-<Emphasis Type="SmallCaps">d</Emphasis>-Gluconic Acid High-Producing Strain <Emphasis Type="Italic">Arthrobacter globiformis</Emphasis> JUIM02

2-Keto-d-gluconic acid (2KGA) is mainly used for industrial production of erythorbic acid, a food antioxidant. In this study, a 2KGA producing strain JUIM02 was firstly identified as Arthrobacter globiformis by morphological observation and 16S rDNA sequencing. The 2KGA synthetic capacity of A. globiformis JUIM02 was evaluated by both fermentation and bioconversion, with 180 g/L dextrose monohydrate as substrates, in shake flasks and 5 L fermenters. For fermentation, 2KGA titer, yield, molar yield, and productivity of JUIM02 reached 159.05 g/L, 0.97 g/g, 90.18%, and 6.63 g/L/h in 24 h. For non-sterile and buffer-free bioconversion by free resting cells (~?3.2 g/L dry cell weight) of JUIM02, these data were 172.96 g/L, 1.06 g/g, 98.07%, and 5.41 g/L/h in 32 h. Moreover, JUIM02 resting cells could be repeatedly used. Resting cells stored at 4 °C within 30 days showed stable bioconversion capacity, with 2KGA titers ≥?171.50 g/L, yields ≥?1.04 g/g, and molar yields ≥?97.24%. The 2KGA synthetic pathway in A. globiformis, which was rarely reported, was also speculated similar to Pseudomonas and verified preliminarily. In conclusion, A. globiformis JUIM02 is a promising 2KGA industrial-producing strain suitable for various production methods and a suitable object for 2KGA metabolism research of A. globiformis.