Electroenzymatic reactions. Investigation of a reductive dehalogenase by means of electrogenerated redox cosubstrates

As an illustration of how cyclic voltammetry can be used to unravel the mechanisms and kinetics of redox enzymes, the reductive dechlorination of trichloroethylene and tetrachloroethylene by a typical reductive dehalogenase, the tetrachloroethene reductive dehalogenase of Sulfurospirillum multivorans (formerly called Dehalospirillum multivorans), was investigated by means of several electrochemically generated cosubstrates. They comprised the monocation and the neutral form of methylviologen, the neutral form of benzylviologen, and cobaltocene. Cyclic voltammetry is used to produce the active form of the cosubstrate under controlled potential conditions. It shows large plateau-shaped catalytic responses, which are used to measure the kinetics of the enzymatic reaction as a function of the substrate and cosubstrate concentrations. The variation of the rate constant for the cosubstrate reaction with its standard potential shows the transition between two asymptotic behaviors, one in which the reaction is under diffusion control and the other in which it is under counter-diffusion control. Simple fitting of this plot allows an estimation of the standard potential of the electron acceptor center in the enzyme (E degrees = -0.57 V vs NHE).     

Ligand discrimination in the reaction of nitrones with [PtCl2(PhCN)2]. Selective formation of mono-oxadiazoline and mixed bis-oxadiazoline complexes under thermal and microwave conditions

[2+3] Cycloaddition of nitrones to the nitrile ligands in the complexes cis- or trans-[PtCl2(PhCN)2] occurs under ligand differentiation and allows for selective synthesis of complexes of the type cis- or trans-[PtCl2(oxadiazoline)(PhCN)]. Microwave irradiation enhances the reaction rates of the cycloaddition considerably and further favours the selectivity towards the mono-cycloadduct with respect to thermal conditions, because the first cycloaddition is accelerated to a higher extent than the second one. Reaction of the trans-substituted mono-oxadiazoline complexes with a nitrone different from the one used for the first cycloaddition step gives access to mixed bis-oxadiazoline compounds of the composition trans-[PtCl2(oxadiazoline-a)(oxadiazoline-b)]. The corresponding cis-configured complexes, however, do not undergo further cycloaddition. All reactions described occur without isomerisation of the stereochemistry around the platinum center, independently of whether thermal or microwave heating is applied.     

The chemistry of (1-azabuta-1,3-diene)tricarbonyliron(0) complexes

In this review, an overview of synthetic and structural aspects of 1-azabuta-1,3-diene complexes of iron is given and the reactivity of these complexes is discussed with regard to inorganic, organometallic, organic and stereochemical aspects of their chemistry. Their application in the synthesis of organic and organometallic target compounds, or as transfer reagents of the tricarbonlyiron(0) moiety is demonstrated.     

Charge separation versus recombination in dye-sensitized nanocrystalline solar cells: the minimization of kinetic redundancy

In this paper we focus upon the electron injection dynamics in complete dye-sensitized nanocrystalline metal oxide solar cells (DSSCs). Electron injection dynamics are studied by transient absorption and emission studies of DSSCs and correlated with device photovoltaic performance and charge recombination dynamics. We find that the electron injection dynamics are dependent upon the composition of the redox electrolyte employed in the device. In a device with an electrolyte composition yielding optimum photovoltaic device efficiency, electron injection kinetics exhibit a half time of 150 ps. This half time is 20 times slower than that for control dye-sensitized films covered in inert organic liquids. This retardation is shown to result from the influence of the electrolyte upon the conduction band energetics of the TiO2 electrode. We conclude that optimum DSSC device performance is obtained when the charge separation kinetics are just fast enough to compete successfully with the dye excited-state decay. These conditions allow a high injection yield while minimizing interfacial charge recombination losses, thereby minimizing "kinetic redundancy" in the device. We show furthermore that the nonexponential nature of the injection dynamics can be simulated by a simple inhomogeneous disorder model and discuss the relevance of our findings to the optimization of both dye-sensitized and polymer based photovoltaic devices.     

Electronic and Steric Influence of Axial and Equatorial Ligands in Vitamin B12 Model Complexes Derived from Cobaloxime: Electrochemical and 59Co-NMR Studies

In four series of strictly related organocobalt complexes, derived from cobaloximes by replacement of the O…H…O with O…-BF₂…O and/or (CH₂)₃ groups, the trends of ⁵⁹Co-NMR shielding and electrochemical data are discussed. A largely parallel behaviour of the plots of E_1/2(I) values for the first Co(III)/Co(II) electron transfer vs. the ₅₉Co chemical shifts reflects the similar sensitivity of the two parameters to a change in electron affinity of the central metal ion due to a variation of the organic group R. E_1/2(II) values for the second Co(II)/CO(I) electron transfer are less sensitive to the change of R, but the trend of the plot vs. δ(₅₉Co) is still parallel in the four series. Consistent deviations from a roughly linear dependence of E_1/2(I) on pK_a of the hydrocarbon acid corresponding to R, on Taft constant s?_* and on ⁵⁹Co shielding are noticed for the isopropyl derivatives and attributed to a steric effect. This was confirmed in a series of R? Co(DMG)pyridine complexes in which ⁵⁹Co shielding decreases steadily with increasing steric parameter E_s (Taft) of the alkyl group. There is experimental evidence from X-ray data that δ(⁵⁹Co) decreases with an increase of the Co? C bond length, illustrating steric hindrance in alkyl coordination to be responsible for the decreased shielding of the ⁵⁹Co nucleus. The relative displacements of the graphic displays for the different series reflect the effect of changes in electron affinity of the redox center, due to the equatorial ligand, which, in turn, is caused by variations in the electron-withdrawing power due to the introduction of the BF₂ group and by the change from ?2 to ?1 valence of the (CH₂)₃-capped ligands.     

Säurekatalysierte Umlagerungen von 1,5-Dimethyl-6-methyliden-tricyclo[3.2.1.02,7]oct-3-en-8endo-olen

Acid Catalysed Rearrangement of 1,5-Dimethyl-6-methyliden-tricyclo[3.2.1.0^2,7]oct-3-en-8endo-ols The tricyclic alcohols 2,3,4 and 6 (Scheme 1) are synthesized by the reaction of the tricyclic ketone 1 with sodiumborohydrid or metalloorganic reagents. Their configuration at C(8) is determined by NMR. in the presence of Eu(fod)₃. The exo-attack of 1 by the nucleophil forming the endo-alcohol is favored, the π-electrons of C(3) = C(4) hindering the endo-attack. On treatment with sulfuric acid in dioxane/water at 25° the tertiary alcohols yield aryl-substituted ketones. 3 gives in 78.5% yield a mixture of the 3-(dimethylphenyl)-2-butanones 12 and 13 , in addition to 16.5% of (2,3,4-trimethylphenyl)-2-propanon ( 14 ) (Scheme 2). The alcohols 4 and 6 yield mixtures of the 2-(dimethylphenyl)-3-pentanones 19 and 20 (72%), and 2-(dimethylphenyl)-propiophenones 21 and 22 (68%), respectively (Scheme 2). In the case of the secondary alcohol 2 mainly products derived from hydration at the C(6), C(9) double bond are formed, namely the mixture of diols 23 and 24 (21%), and the mixture of the isomeric 2-(dimethylphenyl)propanals 25, 26 and 27 (3%) (Scheme 3). - The structures of 12–14, 19/20, 21/22, 23/24 and 25/26/27 were established by spectroscopic data. In the case of 12 and 13 the degradation of their mixture to the known 1-(dimethylphenyl)ethanols 17/18 confirmed the assignment. - The most probable mechanism for the rearrangement of 3 is shown in Schemes 4 and 5. The reaction proceeds from 3 through a, b and g to 12 and 13; 14 is formed via e, f and i . In the case of 4 and 6 only the reaction analogue to 3 → a → b → g ?12/13 takes place. The isomeric aldehyds 25–27 formed from 2 could have the structures s, t , and v . The former two could be generated in a similar way as 12/13 from 3 , the latter one as shown in Scheme 8.     

Selenium speciation in foods: Preliminary results on potatoes

The present paper deals with the speciation of selenium in potatoes (enriched or not in selenium). The study was carried out by using differential pulse cathodic stripping voltammetry (DPCSV) for quantifying selenium. Results obtained provide evidence that the selenium content in the protein fraction is rather independent from the selenium added to the plants during their growth. On the contrary, the amount of Se in the non-protein fraction (water and starch) in Se-enriched sample is significantly higher than in non-enriched one, suggesting that it is the main selenium-storing site. In this fraction the Se(VI)/Se(IV) ratio seems independent from selenium application but it may be related to the redox conditions. The accumulation of selenium in the non-protein fraction is tentatively ascribed to the “Se–starch interaction” that should be able to modulate both the Se absorption into proteins and, possibly, its toxic effect for the plant itself.     

Genes encoding enzymes responsible for biosynthesis of L-lyxose and attachment of eurekanate during avilamycin biosynthesis

The oligosaccharide antibiotic avilamycin A is composed of a polyketide-derived dichloroisoeverninic acid moiety attached to a heptasaccharide chain consisting of six hexoses and one unusual pentose moiety. We describe the generation of mutant strains of the avilamycin producer defective in different sugar biosynthetic genes. Inactivation of two genes (aviD and aviE2) resulted in the breakdown of the avilamycin biosynthesis. In contrast, avilamycin production was not influenced in an aviP mutant. Inactivation of aviGT4 resulted in a mutant that accumulated a novel avilamycin derivative lacking the terminal eurekanate residue. Finally, AviE2 was expressed in Escherichia coli and the gene product was characterized biochemically. AviE2 was shown to convert UDP-D-glucuronic acid to UDP-D-xylose, indicating that the pentose residue of avilamycin A is derived from D-glucose and not from D-ribose. Here we report a UDP-D-glucuronic acid decarboxylase in actinomycetes.     

Kontinuierliche Konzentrationsbestimmung von Elektrolyten mit Hilfe der Oscillometrie

Zusammenfassung Die Messung der Elektrolytkonzentration in fließenden Lösungen, auch bei solchen, die Niederschläge oder Suspensionen enthalten, wird auf Grund der Veränderung des Dielektrikums einer Zelle, welche von einem elektrischen Hochfrequenzstrom durchflossen wird, durchgeführt. Auf diesem Wege ist die quantitative Bestimmung der Konzentration der gelösten Substanz oder die der durchlaufenden Menge beträchtlich vereinfacht und automatisiert. Dabei werden subjektive und Manipulationsfehler weitgehend ausgeschlossen. Die untersuchte Probe erleidet keinerlei Veränderungen.

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Summary The electrolyte concentration of flowing solutions and of suspensions or solutions containing precipitates can be measured by the variation of the dielectrics in a cell, which is coupled with a high-frequency circuit. By means of this method the quantitative determination of the concentration of solutions or the determination of the quantity of ions passed through the tube can be simplified. The method makes possible an automatization and it precludes the possibility of subjective and manipulation errors. During the measurement the sample remains unchanged.