3,3′,5,5′-Tetramethylbenzidme for the colorimetric determination of manganese in water

Manganese can be determined by colorimetry with previous oxidation of Mn(II) in a strong basic medium, using 3,3,5,5-tetramethylbenzidine as a chromogenic reagent. The molar absorptivity of the reaction product is 3.4 × 10⁴ mol^–1l cm^–1l, the detection limit 3 ng/ml, the RSD (0.5 mg/l,n = 8) 0.9% and the calibration range (1-cm cells) 0.02–0.8 mg/1 V(V), Cr(VI) and Co(II) are the most significant interferences. The new method was compared with an AAS procedure (air-acetylene flame) with previous solvent extraction and also with a colorimetric method for the determination of manganese in sea and drinking water.     

Energetics of the N-O bonds in 2-hydroxyphenazine-di-N-oxide

The standard enthalpy of formation and the enthalpy of sublimation of crystalline 2-hydroxyphenazine-di-N-oxide, at T = 298.15 K, were determined from isoperibol static bomb combustion calorimetry and from Knudsen effusion experiments, as -76.7 +/- 4.2 kJ.mol(-1) and 197 +/- 5 kJ.mol(-1), respectively. The sum of these two quantities gives the standard enthalpy of formation in the gas-phase for this compound, delta(f)H(m)degrees(g) = 120 +/- 6 kJ.mol(-1). This value was combined with the gas-phase standard enthalpy of formation for 2-hydroxyphenazine retrieved from a group estimative method yielding the mean (N-O) bond dissociation enthalpy, in the gas-phase, for 2-hydroxyphenazine-di-N-oxide. The result obtained with this strategy is (DH(m)degrees (N - O)) = 263 +/- 4 kJ.mol(-1), which is in excellent agreement with the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G(d) computed value, 265 kJ.mol(-1).     

Thermodynamic Stability of Fenclorim and Clopyralid

The present work reports an experimental thermodynamic study of two nitrogen heterocyclic organic compounds, fenclorim and clopyralid, that have been used as herbicides. The sublimation vapor pressures of fenclorim (4,6-dichloro-2-phenylpyrimidine) and of clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) were measured, at different temperatures, using a Knudsen mass-loss effusion technique. The vapor pressures of both crystalline and liquid (including supercooled liquid) phases of fenclorim were also determined using a static method based on capacitance diaphragm manometers. The experimental results enabled accurate determination of the standard molar enthalpies, entropies and Gibbs energies of sublimation for both compounds and of vaporization for fenclorim, allowing a phase diagram representation of the (p,T) results, in the neighborhood of the triple point of this compound. The temperatures and molar enthalpies of fusion of the two compounds studied were determined using differential scanning calorimetry. The standard isobaric molar heat capacities of the two crystalline compounds were determined at 298.15 K, using drop calorimetry. The gas phase thermodynamic properties of the two compounds were estimated through ab initio calculations, at the G3(MP2)//B3LYP level, and their thermodynamic stability was evaluated in the gaseous and crystalline phases, considering the calculated values of the standard Gibbs energies of formation, at 298.15 K. All these data, together with other physical and chemical properties, will be useful to predict the mobility and environmental distribution of these two compounds.     

Cytotoxicity of tantalum(V) and niobium(V) small carborane complexes and mode of action in P388 lymphocytic leukemia cells

The metallacarboranes (Et₂C₂B₄H₄)‐­TaCl₂(C₅H₅) ( 1 ), (Et₂C₂B₄H₄)NbCl₂(C₅H₅) ( 2 ), (Et₂C₂B₄H₄)TaCl₂(C₅Me₅) ( 3 ), [(Me₃Si)₂­C₂B₄H₄]TaCl₂(C₅H₅) ( 4 ) and (Me₂C₂B₄H₄)‐­TaCl₂(C₅H₅) ( 5 ) are potent cytotoxic agents against suspended tumors, producing cell death in several tissue culture lines; for example, all were effective against murine L1210 lymphoid leukemia, and all except 5 against murine P388 lymphocytic growth. Human leukemic growth is also retarded since 1–4 were effective against Tmolt₃ cell leukemia, all except 4 against Tmolt₄ leukemia, and 1, 2 , and 5 against HI‐60 leukemia. Cytotoxicity was found towards HuT‐8 lymphoma, THP‐1 acute monocytic leukemia and suspended HeLa‐S³ uterine carcinoma. Some but not all of the complexes were active against Sk‐2 melanoma and Mcf‐7 breast effusion growth. Mode‐of‐action studies in P388 lymphocytic leukemia cells showed that de novo purine synthesis was inhibited; this inhibition reduces DNA and RNA syntheses. Purine synthesis was reduced by compounds 3 and 4 at the regulatory enzymes, i.e. phosphoribosyl pyrophosphate (PRPP) amidotransferase and dihydrofolate reductase. The agents lowered d[ATP] and d[CTP] pools, further reducing DNA synthesis. The complexes afforded DNA fragmentation leading to apoptosis, but this was not by a mechanism of nucleoside alkylation, intercalation between base‐pairs or cross‐linking of the DNA strands. Copyright © 2000 John Wiley & Sons, Ltd.     

New perspectives about molecular arrangement of primary and permanent dentin

The dentin quality of primary and permanent teeth was inspected by Fourier transformed Raman spectroscopy (FT-Raman); scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) and hardness test. Middle dentin of crowns were reached by carbide bur abrading providing a uniform smear layer. Phosphoric acid was applied in order to simulate the etching of total etching adhesive systems. The groups were (n = 10): G1 (primary dentin smear layer); G2 (35% phosphoric acid etched primary dentin); G3 (permanent dentin smear layer); G4 (35% phosphoric acid etched permanent dentin). FT-Raman results were subjected to cluster analysis. SEM/EDS were made in order to add the data obtained by FT-Raman. The hardness data were subjected to ANOVA and Tukey test. FT-Raman showed differences among groups, either to organic or inorganic content. For the organic content, primary and permanent dentin became similar after the etching; conversely, the inorganic content showed differences for the two substrates. Hardness test showed no significant differences between primary and permanent dentin, before or after etching, but the etching decreased these values. The mineral content arrangement of primary dentin is different from permanent dentin, independently of the etching. The substrate type did no influence the hardness, but the etching decreased it.     

Production, purification, and characterization of a polygalacturonase from a new strain of kluyveromyces marxianus isolated from coffee wet-processing wastewater

A new high polygalacturonase (PG)-producing Kluyveromyces marxianus strain was isolated from coffee wet-processing wastewater. PG production in this strain is not repressed in the presence of 100g/L of glucose and, being growth-associated, reached its maximum accumulation in the culture medium at the beginning of the stationary phase. Oxygen and galacturonic acid negatively regulated enzyme synthesis, and glucose as the carbon source afforded better enzyme yields than lactose. The data reported here show that this strain exhibits the highest index of PG production among the wild-type strains reported so far (18.8U/mL). PG was readily purified by ion-exchange chromatography on SP-Sepharose FF. The activity corresponded to a single protein with an M _r of 41.7 kDa according to sodium dodecyl sulfate polyacrylamide gel electrophoresis. The enzyme was stable in the pH range of 3.0–5.0 and displayed an optimal temperature of 55°C; it showed a typical endo-splitting way of substrate hydrolysis and exhibited a fair degree of activity on pectin with a high degree of esterification.     

The first examples of the enantioselective Heck–Matsuda reaction: arylation of unactivated cyclic olefins using chiral bisoxazolines

Successful enantioselective Heck–Matsuda arylations were accomplished for the first time using chiral bisoxazolines as ligands. Enantioselective Heck arylations were performed with several cyclic nonactivated olefins to provide the Heck products in 63–96% isolated yields and in enantiomeric excesses of 54% up to 84%.