High-performance liquid chromatographic determination of ochratoxin A and its 4R-4-hydroxy metabolite in human urine

One of the metabolites of ochratoxin A (OA) is 4R-4-hydroxyochratoxin A (4-OH-OA), and the ratio of 4-OH-OA to OA excreted in urine can be linked to the carcinogenic potential of this compound. As further support to the hypothesis that OA can be involved in Balkan endemic nephropathy and the associated urinary system tumours, it was decided to investigate the presence of these two compounds in the urine of affected populations. A sensitive method is described for the determination of the compounds at the 10 ng l-1 level. It involves extraction, two purification steps by column chromatography and high-performance liquid chromatography (HPLC), an analysis by HPLC and a confirmatory test by HPLC after derivatisation.     

2-Nitroethanal conformations in solution: AnAb initio SCRF study

The structure of 2-nitroethanal has been studied at the MP2/6-31G^* level in the gas phase and in acetonitrile using a continuum model to represent the electrostatic solute-solvent interactions. The relative energies of the two stable conformations obtained are quite dependent on the media. Indeed, our computations predict a change of the most stable conformation from gas to polar solvent. These results are in agreement with experimental data for the axial/equatorial conformational equilibrium of 2-nitrocyclohexanone.     

Implementing the plasma-lasing potential for tabletop nano-imaging

Implementing the plasma-lasing potential for tabletop nano-imaging on across a hot plasma medium drives short-wavelength lasing, promising for "turnkey" nano-imaging setups. A systematic study of the illumination characteristics, combined with design-adapted objectives, is presented. It is shown how the ultimate nano-scale feature is dictated by either the diffraction-limited or the wavefront-limited resolution, which imposed a combined study of both the source and the optics. For nano-imaging, the spatial homogeneity of the illumination (spot noise) was shown as critical. Plasma-lasing from a triple grazing-incidence pumping scheme compensated for the missing spot homogeneity in classical schemes. We demonstrate that a collimating mirror pre-conditions both the pointing stability and the divergence below half a mrad.     

A tandem metal carbene cyclization-cycloaddition approach to the pseudolaric acids

An approach toward the synthesis of the antifungal and cytotoxic pseudolaric acids based on the tandem metal carbene cyclization-cycloaddition reaction is described. Using this strategy, the advanced intermediate 3a bearing three of the four stereocenters of the target molecules has been synthesized. The substrate-controlled diastereoselectivity of the tandem carbene cyclization-cycloaddition was preferential for the undesired diastereomer, but reagent control through the use of Hashimoto's chiral rhodium catalyst Rh(2)(S-BPTV)(4) reversed the selectivity in favor of 3a. Ring opening of the oxabicyclic nucleus to give a hydroxycycloheptene has been demonstrated in a model study.     

Self-assembled functional organic monolayers on oxide-free copper

The preparation and characterization of self-assembled monolayers on copper with n-alkyl and functional thiols was investigated. Well-ordered monolayers were obtained, while the copper remained oxide-free. Direct attachment of N-succinimidyl mercaptoundecanoate (NHS-MUA) onto the copper surface allowed for the successful attachment of biomolecules, such as β-d-glucosamine, the tripeptide glutathione, and biotin. Notably, the copper surfaces remained oxide-free even after two reaction steps. All monolayers were characterized by static water contact angle measurements, X-ray photoelectron spectroscopy, and infrared reflection absorption spectroscopy. In addition, the biotinylated copper surfaces were employed in the immobilization of biomolecules such as streptavidin.     

Protonation of water clusters induced by hydroperoxyl radical surface adsorption

We have investigated the HO(2) adsorption and acid dissociation process on the surface of (H(2)O)(20) and (H(2)O)(21) clusters by using quantum-chemistry calculations. Our results show that the radical forms a stable hydrogen-bond complex on the cluster. The HO(2) acid dissociation is more favorable in the case of the (H(2)O)(21) cluster, for which the inner water molecule plays a crucial role. In fact, acid dissociation of HO(2) is found to occur in two steps. The first step involves H(2) O autoionization in the cluster, and the second one involves the proton transfer from the HO(2) radical to the hydroxide anion. The presence of the HO(2) radicals on the surface of the cluster facilitates water autoionization in the cluster.     

Direct one-pot synthesis of primary 4-amino-2,3-diaryl-quinolines via Suzuki-Miyaura cross-coupling of 2-aryl-4-azido-3-iodoquinolines with arylboronic acids

Palladium-catalyzed Suzuki-Miyaura cross-coupling of 2-aryl-4-azido-3-iodo-quinolines with arylboronic acids afforded the corresponding primary 4-amino-2,3-diarylquinolines in a single-pot operation along with symmetrical biaryls and traces of the 2,3-diaryl-4-azidoquinolines. A plausible mechanism, which implicates palladium hydride species in the reduction of the incipient 2,3-diaryl-4-azidoquinolines to afford the 4-amino-2,3-diarylquinolines is proposed.     

Detailed kinetic study of the ring opening of cycloalkanes by CBS-QB3 calculations

This work reports a theoretical study of the gas-phase unimolecular decomposition of cyclobutane, cyclopentane and cyclohexane by means of quantum chemical calculations. A biradical mechanism has been envisaged for each cycloalkane, and the main routes for the decomposition of the biradicals formed have been investigated at the CBS-QB3 level of theory. Thermochemical data(DeltaHf(o), S(o), Cp(o)) for all the involved species have been obtained by means of isodesmic reactions. The contribution of hindered rotors has also been included. Activation barriers of each reaction have been analyzed to assess the energetically most favorable pathways for the decomposition of biradicals. Rate constants have been derived for all elementary reactions using transition-state theory at 1 atm and temperatures ranging from 600 to 2000 K. Global rate constant for the decomposition of the cyclic alkanes in molecular products have been calculated. Comparison between calculated and experimental results allowed us to validate the theoretical approach. An important result is that the rotational barriers between the conformers, which are usually neglected, are of importance in decomposition rate of the largest biradicals. Ring strain energies (RSE) in transition states for ring opening have been estimated and show that the main part of RSE contained in the cyclic reactants is removed upon the activation process.     

Lewis acid metal ion-exchanged MAPO-36 molecular sieve: Characterisation and catalytic activity

Magnesium aluminophosphate-36 (MAPO-36) molecular sieve was synthesised hydrothermally and subjected to wet ion-exchange with Fe³⁺, Zn²⁺, La³⁺ or Ce³⁺. They were characterised by using XRD, SEM, temperature programmed desorption (TPD) of ammonia and thermogravimetric analysis (TGA). The XRD patterns of ion-exchanged MAPO-36 exhibit similar features to that of MAPO-36, which revealed no structural degradation during ion-exchange. TPD (ammonia) showed selective ion-exchange of strong acid sites for Fe³⁺, La³⁺ and Ce³⁺ but not for Zn²⁺. Based on the results of TGA the actual species involved in the ion-exchange is suggested to be M(OH)₂⁺, which upon calcination converted to MO⁺ where M is Fe³⁺, La³⁺ or Ce³⁺. tert-Butylation of phenol was carried out in the vapour phase as a probe reaction to examine the catalytic activity of MAPO-36 and ion-exchanged MAPO-36 molecular sieves. The ion-exchanged catalysts were found to be more active than the parent MAPO-36 and also showed higher selectivity to 4-tert-butylphenol.