Monitoring multi-class pesticide residues in fresh fruits and vegetables by liquid chromatography with tandem mass spectrometry

A new analytical method was developed using liquid chromatography with tandem mass spectrometry for the routine analysis of 31 multi-class pesticide residues and applied to approximately 50 fresh fruit and vegetable samples (green bean, cucumber, pepper, tomato, eggplant, watermelon, melon and zucchini). Extraction of the pesticides with ethyl acetate was carried out. The optimal ionisation conditions were selected for each pesticide in the same run. The procedure was validated and the values of some merit figures, such as recovery, precision, linear range, detection limit and quantification limit for each pesticide were calculated together with its calculated expanded uncertainty (U). The average recoveries in cucumber obtained for each pesticide ranged between 74 and 105% at two different fortification levels (n = 10 each) that ranged between 9 and 250 ng g(-1) (depending on the pesticide). The uncertainty associated to the analytical method was lower than 23% for all compounds tested. The calculated limits of detection and quantitation were typically <1 ng g(-1) that were much lower than the maximum residue levels established by European legislation.     

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.     

Limited flexibility of lactose detected from residual dipolar couplings using molecular dynamics simulations and steric alignment methods

The conformational flexibility of lactose in solution has been investigated by residual dipolar couplings (RDCs). One-bond carbon-proton and proton-proton coupling constants have been measured in two oriented media and interpreted in combination with molecular dynamics simulations (MD). Two different approaches, known as PALES (Zweckstetter et al., J. Am. Chem. Soc. 2000, 122, 3791-3792) and TRAMITE (Azurmendi et al., J. Am. Chem. Soc. 2002, 124, 2426-2427), have been used to determine the alignment tensor from a shape-induced alignment model with the oriented medium. The steric alignment of the structures from several MD trajectories has provided ensemble averaged RDCs that have been compared with the experimental ones. The obtained results reveal the almost exclusive presence of a major low energy region defined as syn-phi/syn-psi (> 97%), for which sampling occurs in a dynamic manner. This result satisfactorily agrees with that determined by standard NOE-based methods.     

Iron(III)-catalyzed Prins-type cyclization using homopropargylic alcohol: a method for the synthesis of 2-alkyl-4-halo-5,6-dihydro-2H-pyrans

[reaction: see text] A new Prins-type cyclization between homopropargylic alcohol and aldehydes in the presence of FeX(3) to obtain 2-alkyl-4-halo-5,6-dihydro-2H-pyrans in good yield is described. Osmium-catalyzed cis dihydroxylation provided direct access to trans-2-alkyl-3-hydroxy-tetrahydro-pyran-4-ones. Anhydrous ferric halides are also shown to be excellent catalysts for the standard Prins cyclization using homoallylic alcohol. Isolation of an intermediate acetal provides substantiation of a proposed mechanism.     

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.     

Chemiluminescent determination of vanadium in steel

Summary A chemiluminescent method for the determination of vanadium in steel with cinchomeronic hydrazide as analytical reagent is proposed. The optimum conditions are pH 11.75 (phosphate buffer), 1.0×10^–3 mol/l cinchomeronic hydrazide, 6.6×10^–3 mol/l hydrogen peroxide and 2.8×10^–3 mol/l V(IV). The maximum chemiluminescent emission is obtained at 420 nm. A linear relationship exists in the range of 0.04–1.00 g/ml of V(IV) with a 3.6% variation coefficient at 0.50 g/ml of V(IV) level for ten replicates. Cobalt(II), copper(II) and chromium(VI) show strong interference and a chloroform extraction procedure with -benzo-inoxime is recommended to avoid these interferences. This method has been applied to determine vanadium in a certified steel with excellent results.Presented at Euroanalysis VII     

Darstellung,struktur und eigenschaften des dreikernigen titankomplexes (π-C2H5)2TiClOTi(π-C5H5)ClOTiCl(π-C5H5)2·CHCl3

The trinuclear titanium(IV) complex (π-C₅H₅)₂TiClOTi(π-C₅H₅)ClOTiCl(π-C₅H₅)₂ · CHCl₃ is formed by hydrolysis of (π-C₅H₅)₂TiCl₂ at pH > 3.5 and can be isolated in the crystalline state from the reaction of (π-C₅H₅)₂TiCl₂ with Ag₂O and water in chloroform. Its structure is determined by X-ray analysis.     

Combining thermal analysis with other techniques to monitor the decomposition of poly(m-phenylene isophthalamide)

The pyrolysis behaviour of Nomex, poly(m-phenylene isophthalamide) fibres under argon has been investigated up to a temperature of 1173 K with different methods to get direct information on the progressive changes taking place in the solid material and its carbon fibre residues. The main stages of the pyrolytic degradation of the fibres were determined by thermal analysis (TG and DTA) and their chemical and morphological evolution through the different steps was subsequently followed by Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) measurements, respectively, on samples treated to various temperatures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ab initio analytical potential energy surface and quasiclassical trajectory study of the O(+)((4)S)+H(2)(X (1)Sigma(g) (+))-->OH(+)(X (3)Sigma(-))+H((2)S) reaction and isotopic variants

An analytical potential energy surface (PES) representation of the O(+)((4)S)+H(2)(X (1)Sigma(g) (+)) system was developed by fitting around 600 CCSD(T)/cc-pVQZ ab initio points. Rate constant calculations for this reaction and its isotopic variants (D(2) and HD) were performed using the quasiclassical trajectory (QCT) method, obtaining a good agreement with experimental data. Calculations conducted to determine the cross section of the title reaction, considering collision energies (E(T)) below 0.3 eV, also led to good accord with experiments. This PES appears to be suitable for kinetics and dynamics studies. Moreover, the QCT results show that, although the hypotheses of a widely used capture model are not satisfied, the resulting expression for the cross section can be applied within a suitable E(T) interval, due to errors cancellation. This could be a general situation regarding the application of this simple model to ion-molecule processes.