Comparison of ultra-performance liquid chromatography and high-performance liquid chromatography for the determination of priority pesticides in baby foods by tandem quadrupole mass spectrometry

Determination of 16 priority pesticides and transformation products specified in the EU Baby Food Directive 2003/13/EC has been compared using high-performance liquid chromatography (HPLC) and ultra-performance liquid chromatography (UPLC) coupled to tandem quadrupole mass spectrometry (MS/MS). Prior to analysis, co-extractives were removed from acetonitrile extracts using dispersive solid-phase extraction (SPE) with primary secondary amine (50 mg). Extracts spiked with pesticides at 1 microg kg(-1) yielded average recoveries in the range 85-119%, with relative standard deviations less than 17%. The HPLC-MS/MS and UPLC-MS/MS multi-residue methods developed are simple, rapid and suitable for the quantification and confirmation of the 16 priority pesticides in fruit-, potato- and cereal-based baby food at 1 microg kg(-1). The major advantages of UPLC, using 1.7 microm particles, over HPLC are the speed of analysis, the narrower peaks (giving increased signal-to-noise ratio) and improved confirmation for the targeted pesticides in the analyses of baby foods.     

Hydrolysis of 4-nitrophenyl acetate by a (N2S(thiolate))zinc hydroxide complex: a model of the catalytically active intermediate for the zinc form of peptide deformylase

A novel zinc(II) hydroxide complex with a rare alkylthiolate donor in the coordination sphere is formed in aqueous solution from the dissolution of the zinc alkyl precursor complex (PATH)ZnCH(3) (PATH = 2-methyl-1-[methyl(2-pyridin-2-ylethyl)amino]propane-2-thiolate) in H(2)O and protonolysis of the Zn-C bond to give (PATH)ZnOH (1). The (PATH)ZnOH complex has been shown to promote the hydrolysis of 4-nitrophenyl acetate (4-NA) by a detailed kinetic study and is the first functional model for the zinc form of the enzyme peptide deformylase. From a fit of the sigmoidal pH-rate profile a kinetic pK(a) of 8.05(5) and a pH-independent second-order rate constant (k" max)) of 0.089(3) M(-1) s(-1) have been obtained. The kinetic pK(a) is similar to the pK(a) of 7.7(1) determined by a potentiometric study (25 degrees C, I = 0.1 (NaNO3)). Observation of both rate enhancement and turnover shows that 1 acts as a catalyst for the hydrolysis of 4-NA, although the turnovers are modest. Activation parameters have been obtained from a temperature-dependence study of the rate constants (E(a) = 54.8 kJ mol(-1), DeltaH++ = 52.4 kJ mol(-1), and DeltaS++ = -90.0 J mol(-1) K(-1)), and support a reaction mechanism which depends on nucleophilic attack of 1 in the rate-determining step. This is the first kinetic and thermodynamic study of a 4-coordinate zinc hydroxide complex containing a thiolate donor. In addition it is only the second time that a complete set of activation parameters have been obtained for the zinc-promoted hydrolysis of a carboxylic ester. This study puts the basicity and nucleophilicity of a (N(2)S)ZnOH complex in context with those of other L(n)()ZnOH complexes and enzymes.     

Observation of the enhancement in enantioselectivity with conversion for the aziridination of styrene using copper bis(oxazoline) complexes

During the aziridination of styrene using copper bis(oxazoline) complexes the ee increases with conversion due to further reactions of the product.     

Effects of steric constraint on chromium(III) complexes of tetraazamacrocycles, 2. Comparison of the chemistry and photobehavior of the trans-dichloro- and trans-dicyano- complexes of cyclam, 1,4-C2-cyclam, and 1,11-C3-cyclam

The synthesis and characterization of several Cr(III) complexes of the constrained macrocyclic ligand 1,11-C3-cyclam (1,4,8,11-tetraazabicyclo[9.3.3]heptadecane) is reported. Only trans complexes are formed, and the structure of trans-[Cr(1,11-C3-cyclam)Cl2]PF6 is presented. The chemical and photophysical behavior of the 1,11-C3-cyclam complexes are compared with those of the corresponding cyclam (1,4,8,11 tetraazacyclotetradecane) and 1,4-C2-cyclam (1,4,8,11-tetraazabicyclo[10.2.2]hexadecane) complexes. The aquation rate of trans-[Cr(1,11-C3-cyclam)Cl2]+ is similar to that of the corresponding 1,4-C2-cyclam complex and is more than 5 orders of magnitude faster than the cyclam counterpart. A monotonic increase in the extinction coefficient is observed on going from the cyclam complexes to the 1,11-C3-cyclam complexes to the 1,4-C2-cyclam complexes, and this is related to the degree of centrosymmetry in each complex. The trans-[Cr(1,11-C3-cyclam)(CN)2]+ complex is a weak emitter in aqueous solution with a room-temperature emission maximum at 724 nm (tau=23 micros). Like the corresponding 1,4-C2-cyclam complex (tau=0.24 micros), the 1,11-C3-cyclam complex shows no deuterium-isotope effect in room-temperature solution. This is in marked contrast to the corresponding cyclam complex which has an emission lifetime of 335 micros and a significant deuterium isotope effect in room-temperature solution. Low temperature (77K) data are also presented in an attempt to understand the differences in photophysical behavior.     

π-Interaction in metal Β-ketoenolates

The available data (supplemented where necessary) on the electronic, nuclear magnetic resonance and vibrational spectra of metal -ketoenolates has been analysed to assess the relative contributions of donor and acceptor metal-ligand -bonding in the first transition period metal (III) -ketoenolates. The effects of the metal ion and the ligand substituent on the -interaction in the -ketoenolate ring are discussed in terms of established theoretical concepts.

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Zusammenfassung Die bekannten Daten (ergÄnzt, falls notwendig) der elektronischen und kernmagnetischen Resonanz sowie der Schwingungsspektren von Metall--ketoenolaten wurden analysiert, um die relativen BeitrÄge von Donor- und Akzeptor (Metall-Ligand) -Bindung in Metall(III)ketoenoIaten der ersten übergangsperiode zu ermitteln. Die Einwirkung des Metallions und des Ligandsubstituenten auf die -Bindung im -Ketoenolatring werden diskutiert auf der Grundlage von bekannten theoretischen Prinzipien.

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Résumé L'examen des résultats antérieurement publiés (supplementés ou nécessaire) des spectres électroniques, résonance nucléaire magnétiques et vibrationnel nous a amenés à préciser les contributions relatifs de liaison- donneur et accepteur entre métal et ligand de la première période transitionelle métal(III) -kétoénolates. Les effects de l'ion métallique et du ligand substituent exercés sur l'interaction- dans le noyau -kétoénolate sont discutés selon les idées théoriques déjà établies.

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This work was supported by University of Cape Town and C.S.I.R. Research Grants. We thank the National Institute for Metallurgy for a bursary (to R.D.H.).     

An efficient multigram synthesis of the potent histamine H3 antagonist GT-2331 and the reassessment of the absolute configuration

GT-2331 is a potent histamine H(3) antagonist which has entered clinical trials. Efficient multigram syntheses of this compound and its enantiomer are described. The literature reports that GT-2331 is the dextrorotatory (+), more potent, enantiomer of 4-[2-(5,5-dimethylhex-1-ynyl)cyclopropyl]-1H-imidazole with the absolute configuration of (1R,2R)-1. However, we found that the dextrorotatory, more potent, enantiomer of 4-[2-(5,5-dimethylhex-1-ynyl)cyclopropyl]-1H-imidazole has the (1S,2S) absolute configuration. We suggest a reconsideration of the absolute configuration of GT-2331.     

Correlation-Driven Magnetic Frustration and Insulating Behavior of TiF3

The halide perovskite TiF₃, renowned for its intricate interplay between structure, electronic correlations, magnetism, and thermal expansion, is investigated. Despite its simple structure, understanding its low-temperature magnetic behavior has been a challenge. Previous theories propose antiferromagnetic ordering. In contrast, experimental signatures for an ordered magnetic state are absent down to 10 K. The current study has successfully reevaluated the theoretical modeling of TiF₃, unveiling the significance of strong electronic correlations as the key driver for its insulating behavior and magnetic frustration. In addition, frequency-dependent optical reflectivity measurements exhibit clear signs of an insulating state. The analysis of the calculated magnetic data gives an antiferromagnetic exchange coupling with a net Weiss temperature of order 25 K as well as a magnetic response consistent with a S = 1/2 local moment per Ti³⁺. Yet, the system shows no susceptibility peak at this temperature scale and appears free of long-range antiferromagnetic order down to 1 K. Extending ab initio modeling of the material to larger unit cells shows a tendency for relaxing into a noncollinear magnetic ordering, with a shallow energy landscape between several magnetic ground states, promoting the status of this simple, nearly cubic perovskite structured material as a candidate spin liquid.     

Computer simulation as a tool for the rapid optimization of the high-performance liquid chromatographic separation of a tryptic digest of human growth hormone.

Computer simulation was used to optimize the separation of a tryptic digest of recombinant human growth hormone using reversed-phase high-performance liquid chromatography in a gradient mode. DryLab G/plus software modelled the retention behavior of the complex tryptic digest mixture as a function of gradient conditions, based on data from two experimental gradient runs. The theoretical optimum separation conditions were rapidly obtained and reproduced experimentally. Resolution did not simply increase as gradient steepness was decreased, rather, an intermediate gradient time provided maximum sample resolution. The simulation results also indicate that the method is reasonably rugged, with little change in the separation expected for different high-performance liquid chromatography systems, and changes in the separation can be compensated by a change in the gradient steepness. Computer simulation can also be useful to quickly reoptimize conditions for a new column, if it fails to provide the same separation.