Kinetics of electrode reaction coupled to ion transfer across the liquid/liquid interface

In the theoretical model it is assumed that a graphite disk electrode is covered by a thin film of solution of decamethylferrocene (dmfc) and some electrolyte CX in nitrobenzene and immersed in an aqueous solution of the electrolyte MX. Oxidation of dmfc is accompanied by the transfer of anion X ⁻ from water into nitrobenzene since it is also assumed that cations dmfc ⁺ and C ⁺ are insoluble in water and cation M ⁺ is insoluble in nitrobenzene. Kinetic parameters of the electrode reaction can be determined if the total potential difference across the nitrobenzene/water interface is maintained constant by adding the electrolytes CX and MX in concentrations which are much higher than the initial concentration of dmfc in nitrobenzene.     

On the substituent effects of halogens in aromatic substitutions

The general kinetic features of the bromination of 4-bromophenol by molecular bromine in acetic acid medium has been studied andArrhenius activation energies for the bromination of fourpara substituted phenols of the general formulaRC₆H₄OH (R=CH₃, F, Cl, and Br) have been accurately determined. These values, which indicate an anomalous order in substituent effect of halogens, are discussed.

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Der Substituenten-Effekt von Halogenen bei der aromatischen Substitution

Zusammenfassung Die Bromierung von 4-Bromphenol mit Br₂ in Essigsäure wurde untersucht. Außerdem wurden dieArrhenius-Aktivierungs-Energien für die Bromierung von vierp-substituierten PhenolenRC₆H₄OH (R=CH₃, F, Cl und Br) genau bestimmt. Die beobachteten Substituenten-Effekte werden diskutiert und interpretiert.

     

Proficiency testing in the light of a new rationale in metrology

The novel proposed definition of measurement result in the international metrology vocabulary requires a revision of standards and guidelines for proficiency testing (PT), and a new approach to processing proficiency data is needed to test the ability of laboratories to present not only unbiased quantity values, but reliable estimates of their uncertainty. Hence, an accepted reference value with the smallest possible uncertainty is needed to ascertain the proficiency of laboratories reporting results with lower than average uncertainty. A strategy based on the T-statistic is proposed leading to an accepted reference value that fully reflects the uncertainties reported by participants in a PT scheme and permits calculation of E _n numbers to distinguish whether or not measurement results are consistent with the accepted definition of the measurand. The strategy is applied to PT data from a recent international laboratory intercomparison of uranium isotopic ratios.     

Effect of the amount and type of the crosslinker on the swelling behavior of temperature-sensitive poly(N-tert-butylacrylamide-co-acrylamide) hydrogels

Temperature-sensitive poly(N-tert-butylacrylamide-co-acrylamide) [P(NTBA-co-AAm)] hydrogels were synthesized by free-radical copolymerization in a water–methanol mixture using three types of crosslinkers: 1,2-ethyleneglycol dimethacrylate, N,N-methylenebisacrylamide, and 1,3-butandiol dimethacrylate. These thermosensitive hydrogels were swollen to equilibrium in water at 20°C and examined by gravimetric measurements. The influence of type and content of crosslinkers on the swelling ratio, the polymer–solvent interaction parameter (χ), the average molecular mass between crosslinks and the effective crosslinking density (ν _E) of the hydrogels were reported and discussed. The swelling process in water was found to be non-Fickian diffusion. The enthalpy (ΔH) and entropy (ΔS) changes appearing in the χ parameter for the hydrogels were determined by using the Flory–Rehner theory based on the phantom network model of swelling equilibrium. Negative values for ΔH and ΔS indicated that the hydrogels had a negative temperature-sensitive property in water; that is, swelling at a lower temperature and shrinking at a higher temperature. The temperature-reversibility and on–off switching properties of the P(NTBA-co-AAm) hydrogels may be considered as good candidates for designing novel drug-delivery systems.     

Probing the role of N-linked glycans in the stability and activity of fungal cellobiohydrolases by mutational analysis

The filamentous fungi Trichoderma reesei and Penicillium funiculosum produce highly effective enzyme mixtures that degrade the cellulose and hemicellulose components of plant cell walls. Many fungal species produce a glycoside hydrolase family 7 (Cel7A) cellobiohydrolase, a class of enzymes that catalytically process from the reducing end of cellulose. A direct amino acid comparison of these two enzymes shows that they not only have high amino acid homology, but also contain analogous N-linked glycosylation sites on the catalytic domain. We have previously shown (Jeoh et al. in Biotechnol Biofuels, 1:10, 2008) that expression of T. reesei cellobiohydrolase I in a commonly used industrial expression host, Aspergillus niger var. awamori, results in an increase in the amount of N-linked glycosylation of the enzyme, which negatively affects crystalline cellulose degradation activity as well as thermal stability. This complementary study examines the significance of individual N-linked glycans on the surface of the catalytic domain of Cel7A cellobiohydrolases from T. reesei and P. funiculosum by genetically adding or removing N-linked glycosylation motifs using site directed mutagenesis. Modified enzymes, expressed in A. niger var. awamori, were tested for activity and thermal stability. It was concluded that N-linked glycans in peptide loops that form part of the active site tunnel have the greatest impact on both thermal stability and enzymatic activity on crystalline cellulose for both the T. reesei and P. funiculosum Cel7A enzymes. Specifically, for the Cel7A T. reesei enzyme expressed in A. niger var. awamori, removal of the N384 glycosylation site yields a mutant with 70% greater activity after 120 h compared to the heterologously expressed wild type T. reesei enzyme. In addition, similar activity improvements were found to be associated with the addition of a new glycosylation motif at N194 in P. funiculosum. This mutant also exhibits 70% greater activity after 120 h compared to the wild type P. funiculosum enzyme expressed in A. niger var. awamori. Overall, this study demonstrates that “tuning” enzyme glycosylation for expression from heterologous expression hosts is essential for generating engineered enzymes with optimal stability and activity.     

On the thermodynamics of partially miscible liquid binary systems

Isobaric coexistence curves of two liquid phases are predicted from the temperature and composition at the consolute point of binary solutions consisting of a hydrocarbon and the corresponding perfluorocarbon. Excess enthalpies obtained from the free volume theory ofFlory as well as from calorimetric measurements are used to account for the temperature dependence of theGibbs energy of mixing. The treatment is based on the extension of theFlory-Huggins model to nonathermal polymer solutions. Formal applicability to mixtures of molecules not largely differing in size is achieved by introducing generalized variables characterizing composition in place of volume fractions. Phase diagrams of following systems are predicted and compared with experimental results: methane+carbontetrafluoride,n-hexane+perfluoro-n-hexane,n-heptane+perfluoro-n-heptane, methylcyclohexane+perfluoromethyl-cyclohexane.

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Zur thermodynamik beschränkt mischbarer flüssiger zweistoffsysteme

Zusammenfassung Isobare Koexistenzkurven zweier flüssiger Phasen werden aus Temperatur und Zusammensetzung am kritischen Entmischungspunkt binärer Lösungen bestehend aus einem Kohlenwasserstoff und dem entsprechenden Perfluor-kohlenstoff vorausgesagt. Die Temperaturabhängigkeit derGibbs-Energie der Mischung wird mit Hilfe von Mischungsenthalpien, die sowohl der Freien Volumentheorie vonFlory als auch kalorimetrischen Messungen entstammen, berücksichtigt. Als Berechnungsgrundlage dient das auf nichtathermische Polymerlösungen erweiterteFlory-Huggins-Modell. Die Einführung verallgemeinerter Variablen zur Charakterisierung der Zusammensetzung an Stelle von Volumenbrüchen ermöglicht die formale Anwendbarkeit auf Mischungen von Molekülen mit wenig verschiedener Größe. Zustandsdiagramme folgender Mischsysteme werden vorausberechnet und mit experimentellen Ergebnissen verglichen: Methan+Tetrafluorkohlenstoff,n-Hexan+Perfluor-n-Hexan,n-Heptan+Perfluor-n-Heptan, Methylcyclohexan+Perfluormethylcyclohexan.

     

Synthesis and studies of organic nitrates of the heterofunctional series

N-Nicotinoyltris(hydroxymethy)aminomethane trinitrate hydronitrate, an analog ofN-nicotinoylethanolamine nitrate (the active principle of the antianginal drug nicorandil), was prepared byO-nitration of the corresponding triol with concentrated HNO₃. The structure of the reaction product was established by X-ray structural analysis. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2224–2227, December, 1997.     

High-Yield Syntheses of Tetra-O-benzyl-α-D-glucopyranosyl bromide and Tetra-O-pivaloyl-α-D-glucopyranosyl bromide and their Advantage in the Koenigs-Knorr Reaction

Summary. Several improved approaches for the preparation of tetra-O-benzyl-α-D-glucopyranosyl bromide and tetra-O-pivaloyl-α-D-glucopyranosyl bromide are discussed. The importance of these compounds, which are useful glycosyl donors, was demonstrated by successful preparation of cholesteryl glucopyranosides in an almost neutral medium without the formation of orthoesters. In addition, accurate ¹H and ¹³C NMR resonance assignments of the synthesized cholesteryl glycosides were performed by 2D NMR spectroscopy.     

Immobilization of the restriction endonucleasesPvuII andHindIII

The effects of several chemical reagents on the activity of the restriction endonucleasesPvuII andHindIII were investigated. Carbodiimide, which reacts preferentially with carboxyl groups, was found to inactivate these enzymes. This specific effect could be prevented by Mg²⁺ cation. pBR322 DNA, which containsPvuII andPvuII* sites andHindIII andHindIII* sites, did not protect the enzymes from the carbodiimide. On the other hand, glutaraldehyde, which reacts primarily with lysine residues, inactivatesPvuII andHindIII enzymes. This specific effect could not be prevented by pBR322 DNA. Preincubation with high concentrations ofN-ethylmaleimide, which reacts with sulfhydryl groups, caused slight inhibition ofPvuII activity, but had no effect on the activity ofHindIII enzyme. The effects of glutaraldehyde, carbodiimide, andN-ethylmaleimide on other restriction endonucleases were also investigated. Restriction endonucleasesPvuII andHindIII were immobilized by covalent coupling to various insoluble carriers. Both immobilized enzymes retained partial enzyme activities, when immobilized through phenolic groups and were stable for at least two months.     

Influence of a new axial impeller on K L a and xylanase production by Penicillium canescens 10-10c

The effects of a new axial impeller (HTPG4) on oxygen volumetric transfer coefficient, K _L a, and xylanase production by Penicillium canescens 10-10c were studied and compared for dual-impeller systems, one with one DT4 impeller below and one HTPG4 above (DT4-HTPG4) and one with two DT4 (DT4-DT4) impellers, in a 5-L bioreactor. The volumetric coefficient of oxygen transfer was measured in culture medium using a gassing-out method at different gassing rates and agitation speeds. We observed that the DT4-HTPG4 combination provided better K _L a performance than the DT4-DT4 combination. The two combinations were also tested for their influence on xylanase production by a filamentous microorganism; P. canescens 10-10c. These experiments demonstrated that the DT4-HTPG4 combination impeller enhanced enzyme production up to 23% compared with the DT4-DT4 combination at an aeration rate of 1 vvm and an agitation speed of 600 rpm. The main cause for this difference is thought to be a higher shear stress generated by the DT4-DT4 combination, which damages the mycelium of P. canescens and decreases xylanase production.     

o-benzoquinone photoreduction products in the presence of N,N-dimethylanilines

Photoreduction of o-benzoquinones in the presence of para-substituted N,N-dimethyl-anilines under irradiation at λ ≥ 500 nm affords pyrocatechol monoethers of the 2-(amino-methoxy)phenol type. In the subsequent dark reaction, these monoethers undergo quantitative decomposition by a heterolytic mechanism to give the corresponding pyrocatechols and nitrogen-containing compounds. The rate of this decomposition decreases with decreasing size of the substituent at the position adjacent to the ether bond that is formed upon photoreduction. The redox characteristics of such pyrocatechol monoethers can serve as the criterion of their stability. A weakening of the electron-withdrawing properties of quinones and the electron-donating properties of amines leads to an increase in stability of their reaction products. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1528–1535, September, 2006.     

Quinonimines and aminoquinones, the reaction products of 3,6-di(tert-butyl)-o-benzoquinone with primary and secondary amines

Reactions of 3,6-di(tert-butyl)-o-benzoquinone with primary amines occur by the nucleophilic 1,4-addition mechanism and lead to the corresponding 2-hydroxy-p-quinonimines, which exist in solutions in equilibrium with tautomeric 4-amino-o-quinones. The thermodynamic parameters of this prototropic isomerism were determined by NMR spectroscopy. In the case of a secondary amine (piperidine), a derivative of 4-amino-o-quinone was obtained; the corresponding o-semiquinone complexes were studied in solution by ESR spectroscopy. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1151–1155, July, 2006.     

Uncontrolled pseudoliving free-radical polymerization of methyl methacrylate in the presence of butyl-<Emphasis Type="Italic">p</Emphasis>-benzoquinones

Polymerization of methyl methacrylate initiated by dicyclohexyl peroxydicarbonate in the presence of tri-n-butylboron and butyl-p-benzoquinone or 2,5-di-tert-butyl-p-benzoquinone occurred with no induction period. The yields and molecular masses of the polymers linearly increased with an increase in the conversion degree, which suggests the free-radical mechanism of “living” chain polymerization. The poly(methyl methacrylate) macrochains of the prepolymers contained sterically hindered aromatic structures with labile C-O bonds. The latter underwent reversible homolytic dissociation to give a growth-inducing radical and sterically hindered aryloxyls. Pseudoliving free-radical polymerization in the presence of the prepolymer (macroinitiator) was studied at 45, 60, and 80 °C. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1119–1122, June, 2007.     

Diastereoselective bromination of(R)-N-cinnamoyl-4-phenyloxazolidin-2-one in the presence of Lewis acids

The stereochemistry of addition of Br₂ toE-andZ-(R)-N-cinnamoyl-4-phenyloxazolidin-2-ones was studied. It was established that both theE-andZ-isomers give only two out of four possible diastereoisomers in the presence of Lewis acids (BPr₃ or AlBr₃). The absolute configurations of the diastereoisomers [(2S', 3R') and (2R', 3S') of the side chain] were established by X-ray structural analysis. The stereochemistry observed is a consequence of the stepwise bromination and the absence of bridging bromine atoms along the reaction coordinate. In the case of theZ-isomer, the diastereoselectivity of the reaction was high, whereas it is low in the case of theE-isomer. It was suggested that at the first stage of addition of Br₂ at the C=C bond, the attack of the Br⁺ cation occurs at the α position, and the second stage of transfer of Br⁻ occurs with the participation of boron or aluminum complexes in the intermediate state of the bromination reaction. This hypothesis as well as the results of calculations of the initial conformations of the substrates provide an explanation of the regularities observed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1022–1028, May, 1997.     

Determination of <Emphasis Type="Italic">p</Emphasis>-nitrobenzene-azo-naphthol by an oscillating chemical reaction using the analyte pulse perturbation technique

A simple analytical method for the determination of p-nitrobenzene-azo-naphthol was proposed by a sequential perturbation with different amounts of p-nitrobenzene-azo-naphthol on an oscillating chemical system. The method involves a Cu(II)-catalysed oscillating reaction between hydrogen peroxide and sodium thiocyanate in alkaline medium with the aid of continuous-flow stirred tank reactor (CSTR). A good linear relationship between the changes, oscillation amplitude or/and period, and the concentration of p-nitrobenzene-azo-naphthol was obtained. The use of analyte pulse perturbation technique provides a possibility of sequential determination in a same oscillating system, due to a new steady state that reappeared rapidly after each perturbation. The calibration curve fits a linear equation very well when the concentration of p-nitrobenzene-azo-naphthol ranging from 5.2×10⁻⁷ to 3.3×10⁻³M. Influence of temperature, injection point, flow rate and reactants variables on this system were investigated in detail.     

The tert-butyl-NNO-azoxy group in the synthesis of 1,2,4-benzotriazin-3(4H)-one 1-oxides

Treatment of 2-(tert-butyl-NNO-azoxy)anilines with phosgene at 20 °C was proposed as a novel route to 1,2,4-benzotriazin-3(4H )-one 1-oxides. This method involves a new reaction, viz., an intramolecular interaction of the tert-butyl-NNO-azoxy group with a C-electrophile (leading to the formation of the N(2)—C(3) bond of the triazine ring) followed by elimination of the tert-butyl group. Complete assignment of the signals in the ¹H and ¹³C NMR spectra of the compounds obtained was performed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1507–1509, August, 2007.     

Utilization of cascade chemo-bio catalysis for the synthesis of R -1-phenylethyl acetate

Liquid phase tert-butylation of m-cresol was carried out using 12-tungstosilicic acid supported on neutral alumina by varying different parameters. The reaction has also been carried out by using calcined samples under optimized conditions. The present catalyst shows an excellent activity of 93.2%, conversion of 100% selectivity for the o-isomer, an important industrial product.     

MRINDO/S-CI calculation on the electronic spectra of higher azines

Summary MRINDO/S calculation completed by singly excited configuration interaction was performed on the higher aziness-triazine,s-tetrazine and pentazine. The results enable the main characteristics of the observed electronic spectra to be interpreted. The importance of outer (Rydberg) atomic orbitals is stressed and it is found that a few singlet-singlet transitions of the higher azines lead to an excited state with considerable Rydberg character.     

Quantum Mechanical Study of the Syn and Anti Conformations of Solvated Cyclic GMP

Self-consistent Reaction Field (SCRF) computational methods have been applied to guanosine 3:5-cyclic monophosphate (cGMP) to determine the geometries and energetics of the syn and anti conformations of this cyclic nucleotide in aqueous solution. The syn conformation of cGMP has been predicted to be more stable in the gas phase due to an internal hydrogen bond. The syn conformation is observed in the crystal structure of the sodium tetrahydrate salt, although a bridging water molecule is present in lieu of the internal hydrogen bond. In the gas phase, we find from Hartree–Fock/6-31+G(d) optimizations that the syn conformation is more stable than the anti by about 4 kcal/mol. However, we report here that the anti conformation is more stable in aqueous solution, according to estimates based upon results from both the Onsager model and the Isodensity Polarized Continuum Method (IPCM). Our best estimate from single-point IPCM B3LYP/6-31+G(d) calculations has the anti conformation 19 kcal/mol lower in energy. For comparison purposes, we also present SCRF results for syn and anti adenosine 3:5-cyclic monophosphate (cAMP). For cAMP, we estimate the anti conformation to be more stable than the syn by about 6 kcal/mol. We suggest that the relative stability of the anti conformation of cGMP be considered in studies, such as, enzyme docking.