Optimisation and comparison of microwave-assisted extraction and Soxhlet extraction for the determination of polychlorinated biphenyls in soil samples using an experimental design approach

Optimisation of microwave-assisted extraction (MAE) for the extraction of polychlorinated biphenyls (PCBs) from soil samples has been accomplished using an experimental design approach. Variables studied have been: percentage of acetone (v/v) in an acetone:n-hexane mixture, solvent volume, extraction time, microwave power and pressure inside the extraction vessel. Five samples of a certified soil (CRM 481) have been extracted under the optimum conditions of the developed method and the results have been compared to those obtained by Soxhlet extraction. Good recoveries (>95%) have been obtained for all the PCBs studied. All extracts have been analysed by gas chromatography/mass spectrometry (GC/MS) and an optimum determination method for the electron impact mass spectrometric (EIMS) has also been developed.     

Spectrofluorimetric determination of cisatracurium and mivacurium in spiked human serum and pharmaceuticals

Spectrofluorimetric methods to determine cisatracurium and mivacurium are proposed and applied to the determination of both substances in human serum and to the determination of mivacurium in pharmaceuticals. The fluorimetric methods allow the determination of 5-500 ng ml(-1) of mivacurium in aqueous solutions and 5-500 ng ml(-1) of cisatracurium in water-acetonitrile solutions, both containing acetic acid-sodium acetate buffer (pH 5.5) with lambda(exc)=230 nm and lambda(em)=324 nm.     

Theoretical modeling of enzyme catalytic power: analysis of "cratic" and electrostatic factors in catechol O-methyltransferase

A comparative theoretical study of a bimolecular reaction in aqueous solution and catalyzed by the enzyme catechol O-methyltransferase (COMT) has been carried out by a combination of two hybrid QM/MM techniques: statistical simulation methods and internal energy minimizations. In contrast to previous studies by other workers, we have located and characterized transition structures for the reaction in the enzyme active site, in water and in a vacuum, and our potential of mean force calculations are based upon reaction coordinates obtained from features of the potential energy surfaces in the condensed media, not from the gas phase. The AM1/CHARMM calculated free energy of activation for the reaction of S-adenosyl methionine (SAM) with catecholate catalyzed by COMT is 15 kcal mol(-1) lower the AM1/TIP3P free-energy barrier for the reaction of the trimethylsulfonium cation with the catecholate anion in water at 300 K, in agreement with previous estimates. The thermodynamically preferred form of the reactants in the uncatalyzed model reaction in water is a solvent-separated ion pair (SSIP). Conversion of the SSIP into a contact ion pair, with a structure resembling that of the Michaelis complex (MC) for the reaction in the COMT active site, is unfavorable by 7 kcal mol(-1), largely due to reorganization of the solvent. We have considered alternative ways to estimate the so-called "cratic" free energy for bringing the reactant species together in the correct orientation for reaction but conclude that direct evaluation of the free energy of association by means of molecular dynamics simulation with a simple standard-state correction is probably the best approach. The latter correction allows for the fact that the size of the unit cell employed with the periodic boundary simulations does not correspond to the standard state concentration of 1 M. Consideration of MC-like species allows a helpful decomposition of the catalytic effect into preorganization and reorganization phases. In the preorganization phase, the substrates are brought together into the MC-like species, either in water or in the enzyme active site. In the reorganization phase, the roles of the enzymic and aqueous environments may be compared directly because reorganization of the substrate is about the same in both cases. Analysis of the electric field along the reaction coordinate demonstrates that in water the TS is destabilized with respect to the MC-like species because the polarity of the solute diminishes and consequently the reaction field is also decreased. In the enzyme, the electric field is mainly a permanent field and consequently there is only a small reorganization of the environment. Therefore, destabilization of the TS is lower than in solution, and the activation barrier is smaller.     

Bestimmung von C-Methylgruppen in organischen Sauerstoffverbindungen durch Oxydation mit Chromsäure unter optimalen Bedingungen. I

Zusammenfassung Eine Reihe sauerstoffhaltiger Modellsubstanzen, die Methylgruppen am primären, sekundären oder doppelt gebundenen Kohlenstoffatom enthalten, wurden mit einem Gemisch von 2 T. 5-n Chromsäure und 1 T. konz. Schwefelsäure 45 Minuten bei 130° C oxydiert. Die entstandene Essigsäure wurde mit Wasserdampf abdestilliert und alkalimetrisch titriert. Die angeführten Reaktionsbedingungen können als optimal angesehen werden, da sie die größte oxydative Wirkung haben, bei der jedoch Essigsäure nicht weiter oxydiert wird. Der Abbau des Kohlenstoffskeletts verläuft schneller und vollständiger als bei der Methode vonKuhn undRoth. Die Essigsäureausbeute lag zwischen 0,70 und 0,95 Mol Essigsäure je C-Methylgruppe und wurde in Abhängigkeit von der Struktur der untersuchten Stoffe diskutiert.

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Determination of C-methyl groups in organic oxygen compounds through oxidation with chromic acid under optimal conditions

Summary A number of oxygen-containing model substances, which contained methyl groups on primary, secondary or doubly bound carbon atoms, were oxidized with a mixture of 2 parts of 5N chromic acid and 1 part of conc. sulfuric acid for 45 minutes ar 130° C. The resulting acetic acid was distilled off with steam and titrated alkametrically. The cited reaction conditions may be regarded as optimal, because they exert the greatest oxidative action, which however the acetic acid is not further oxidized. The degradation of the carbon skeleton proceeds faster and more completely than in the method ofKuhn andRoth. The yield of acetic acid lay between 0.70 and 0.95 mol acetic acid per C-methyl group and was discussed with respect to the structure of the investigated material.

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In dieser Mitteilung ist der erste Teil der Dissertationsarbeit von Frau Dipl.-Ing.B. Voláková inbegriffen.     

Highly diastereoselective cyanation of methyl ketimines obtained from (R)-glyceraldehyde

[reaction: see text] Addition of trimethylsilyl cyanide to ketimines derived from (R)-2,2-dimethyl-1,3-dioxolan-4-yl methyl ketone to generate a quaternary stereocenter has been achieved with high yields and excellent diastereoselectivity. The stereoselectivity was found to be temperature and solvent dependent. The beta-hydroxy-alpha-amino nitrile of syn configuration was the major compound in kinetically controlled reactions, whereas the anti stereoisomer was obtained in excess in thermodynamically controlled reactions. Double stereodifferentiation under kinetic control conditions was successful, and the cyanation reaction occurred with complete syn diastereoselectivity using the matched pair. The versatility of the resulting amino nitrile as a synthetic intermediate was tested by performing the synthesis of orthogonally protected (R)-(2-aminomethyl)alanine.     

Optimisation of high performance liquid chromatography separation of neuroprotective peptides. Fractional experimental designs combined with artificial neural networks

The study of experimental design conjunction with artificial neural networks for optimisation of isocratic ion-pair reverse phase HPLC separation of neuroprotective peptides is reported. Different types of experimental designs (full-factorial, fractional) were studied as suitable input and output data sources for ANN training and examined on mixtures of humanin derivatives. The independent input variables were: composition of mobile phase, including its pH, and column temperature. In case of a simple mixture of two peptides, the retention time of the most retentive component and resolution were used as the dependent variables (outputs). In case of a complex mixture with unknown number of components, number of peaks, sum of resolutions and retention time of ultimate peak were considered as output variables. Fractional factorial experimental design has been proved to produce sufficient input data for ANN approximation and thus further allowed decreasing the number of experiments necessary for optimisation. After the optimal separation conditions were found, fractions with peptides were collected and their analysis using off-line matrix assisted laser desorption/ionisation time of flight mass spectrometry (MALDI-TOF-MS) was performed.     

Quantitative approach for the screening of cyclodextrins by nuclear magnetic resonance spectroscopy in support of chiral separations in liquid chromatography and capillary electrophoresis enantioseparation of norgestrel with alpha-, beta- and gamma-cyclodextrins

A quantitative NMR approach is proposed for the screening of cyclodextrins with regard to their enantioselectivity as chiral mobile phase additives in column reversed-phase chromatography and capillary electrophoresis. Similarities and differences between the mechanism of enantiomeric peak-separation in NMR and HPLC and CE are interpreted. The affinity of d-norgestrel to bind to (alpha-, beta-, gammay-) cyclodextrins in aqueous solution was quantified and compared by determining the association constants from chemical shift data. The association constant of l-norgestrel was estimated from titration of the racemate. Differences between the apparent association constants of the enantiomerically pure drug and the racemate are discussed from the point of view of enantiomeric competition for the cyclodextrin. The apparent association constants and chiral selectivities determined by 'H NMR for dl-norgestrell/gamma-CD system at various water-methanol ratios are correlated with the corresponding chromatographic results found in the literature. The pitfalls of previously proposed screening methods based on comparison of chemical shift differences with separation parameters are discussed.     

Zusammenhang zwischen Struktur und Komplexbildungseigenschaften der C1–C6-Xanthogenate mit Hg(II)-, Cd(II)- und Zn(II)-Ionen

The thermodynamic parameters D[`(H)], D[`(G)], D[`(S)]₂₉₈\Delta \bar H, \Delta \bar G, \Delta \bar S_{298} and lg _n resp. of the reactions indicated in the title have been computed from polarographic data. The numerical values obtained are nearly independent from the xanthate used. The overall formation constants increase as follows: Zn(II)<>相似文献   

Stereoselective conjugate addition of benzyl phenylsulfonyl carbanions to enoates derived from d-mannitol

The conjugate addition of benzylic phenylsulfonyl carbanions (2a'-d') to enoates derived from d-(+)-mannitol (E- or Z-1a-c) was studied using THF and THF/HMPA as solvent. Under kinetic conditions (-78 degrees C), enoate E-1a,b led to a mixture of syn-(R,S) and anti-(S,S) adducts (55/45), and syn-(R,S) adducts were the main product obtained ( approximately 90/10) from enoate Z-1a. Under thermodynamic conditions (-78 degrees C to room temperature) syn-(R,S) adducts were also preferentially formed ( approximately 90/10), despite the geometry at the double bond in the acceptor. Enoate 1c (E/Z = 57/43), bearing an additional benzyl group at the alpha-position, also reacted with carbanions 2'a,b, under thermodynamic conditions, leading to syn-adducts in excellent de (control at the three newly generated stereogenic centers). The adducts were quantitatively transformed into the corresponding beta-gamma-disubstituted gamma-butyrolactones and alpha,beta,gamma-trisubstituted gamma-butyrolactones. (1)H NMR studies (NOE and J-coupling) of these lactones allowed us to determine their configuration at the newly generated chiral centers. The reduction of the C-S bond in adducts syn-(R,S) with Na/Hg, followed by treatment of the resulting products in aqueous acid media, led to enantioenriched beta-benzyl-gamma-hydroxymethyl-gamma-butyrolactones. The conformational equilibrium of enoates E- and Z-1b was evaluated by theoretical calculations (ab initio, MP2/6-31G), and a mechanistic rationale was proposed to explain the observed stereoselectivities.     

The use of Tillandsia usneoides L. as bioindicator of air pollution in São Paulo, Brazil

Epiphytic bromeliadTillandsia usneoides was used as a biomonitor of metal atmospheric pollution in the city of São Paulo, Brazil. The samples were collected from an unpolluted area and were exposed for 8 weeks in 10 sites of the city and in a control site. The data obtained showed that the control site presented lower concentration for most elements analyzed and the highest concentrations of Na, Cl, Br, K. The highest concentrations of Zn, Cu and V were observed in stations submitted to industrial and vehicular sources. The results obtained for rare earth elements, Al, Fe, Mg, Mn, Rb and Sc indicate mineral dust origin. The elements Ba, As and Sb presented higher values in stations near streets with heavy traffic.