High performance liquid chromatography post-column chemiluminescence determination of sulfonamide residues in milk at low concentration levels using bis[4-nitro-2-(3,6,9-trioxadecyloxycarbonyl)phenyl] oxalate as chemiluminescent reagent

The determination of seven sulfonamides by means of HPLC with chemiluminescence detection is proposed for the first time. The analytes are derivatized with fluorescamine, separated and subsequently they participate in the post-column chemiluminescence (CL) peroxyoxalate system using imidazole as a catalyst. Among the different peroxyoxalates tested, bis[4-nitro-2-(3,6,9-trioxadecyloxycarbonyl)phenyl] oxalate provides higher sensitivities and stabilities, avoiding precipitation problems. A rigorous optimization of the significant variables by means of experimental designs has been developed in order to reconcile the chromatographic conditions with the CL reaction. The method provides detection limits in the low microgl(-1) range and has been satisfactorily applied to the analysis of spiked raw milk samples.     

The influence of the purine 2-amino group on DNA conformation and stability-II: Synthesis and physical characterization of d[CGT(2-NH2)ACG], d[CGU(2-NH2)ACG], and d[CGT(2-NH2)AT(2-NH2)ACG]

Synthesis of an N-protected derivative of 2-amino-2'-deoxyadenosine (dA') from 2'-deoxyguanosine is reported. The syntheses of several oligodeoxynucleotides containing this modified nucleoside are described, together with physical characterization via melting studies and CD conformational analysis. As expected, the 2-amino group is seen to add to the duplex stability. Although the sequence d(TA')₃ was found to undergo a salt-induced conformational transition, mixed sequences such as d(CGTA'CG) did not display this behavior. All guanine residues present in these sequences were O⁶ protected, either with the cyanoethyl group or the 4-nitrophenylethyl group, to eliminate guanine degradation during phosphorylation and condensation reactions. Procedures for the introduction and removal of these O⁶ protecting groups are described.     

Studies on the synthesis of pentacyclic strychnos indole alkaloids. photocyclization of n-chloroacetyl-1,2,3,4,5,6-hexahydro-1,5-methanoazocino[4,3-b]indole derivatives

Photocyclization of 2-chloroacetyl-1,2,3,4,5,6-hexahydro-1, 5-methanoazocino[4,3-b]indole ($\text{5}$) takes place at the indole 4-position to give a 1 ,2 ,3 ,4 , 5 ,6-hexahydro-2 ,11-ethano-1 ,5-methanoazocino [4 , 3-6] indole system. Consequently, the method appears to be unsuitable for constructing the pyrrolidine ring of pentacyclic Strychnos indole alkaloids.     

Linked crystallites in the conducting topmost layer of polymer bilayer films controlled by temperature: from micro- to nanocrystallites.

Temperature has great impact on the structure and size of the linked crystallites of the conducting topmost layer formed at the surface of a polycarbonate film via the reaction BEDT-TTF+IBr [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene]. We show that fine temperature control permits formation of a semiconducting topmost layer of alpha'-(BEDT-TTF)(2)(I(x)Br(1-x))(3) crystallites with either micro- or nanometre size, a result that opens a route to miniaturized conducting plastic materials.     

Microwave-assisted synthesis of bipyrazolyls and pyrazolyl-substituted pyrimidines

We describe the preparation of 1,4′-bipyrazolyls and 4-pyrazolylpyrimidines by the reaction of 2-pyrazolyl-3-dimethylamino acrylate and acrylonitrile with double nucleophilic reagents such as hydrazines, urea and guanidine. Reactions were performed under microwave irradiation in 5-60 min. This is a useful procedure for the preparation of valuable compounds with applications in medicinal and coordination chemistry.     

Revealing structural effects: electrochemical reactions of butanols on platinum

Spectroelectrochemical studies on the reactivity of butanol isomers on Pt electrodes in perchloric acid medium led to the observation of structural effects that result from the different arrangements of atoms in the organic molecules. The use of differential electrochemical mass spectrometry (DEMS) to detect volatile products showed that all four isomers react on the electrode, though different product yields were observed for each compound. In spite of the differences in the electrochemical behaviour of the butanol isomers, a series of general processes accounts for the results obtained. The formation of strongly adsorbed residues by a dehydration process leading to the formation of a C=C bond was proposed for all isomers. Electroreduction of the adsorbates produces C(4) and C(3) alkanes, and the latter reveal the existence of a fragmentation process. The C(4) hydrocarbons can be formed by hydrogenation of these residues and by hydrogenolysis of alcohol molecules in the bulk solution which react at the electrode with adsorbed hydrogen. On the other hand, CO(2) is formed during electrooxidation of the adsorbed species. Partial-oxidation products containing a carbonyl group were detected from 0.2 M solutions of 1-butanol, isobutyl alcohol and sec-butyl alcohol. The tertiary alcohol tert-butyl alcohol only reacts in its adsorbed state.     

Screening and analytical confirmation of sulfonamide residues in milk by capillary electrophoresis-mass spectrometry

A new methodology is proposed to automate the monitoring of sulfonamide residues in milk samples. It combines a screening unit for the total amount of sulfonamide with capillary electrophoresis-mass spectrometry (CE-MS) equipment for processing the samples containing a detectable level of sulfonamide. The screening unit consists of continuous-flow system (CFS) to precipitate the proteins connected on-line to the CE-MS equipment, in which a common characteristic ion of all sulfonamides was monitored with the MS detector by flushing the sample through the capillary. The confirmatory method is based on the purification and preconcentration of sulfonamides in a CFS unit and posterior analysis by CE-MS. The sample treatment unit was also on-line connected to the CE-MS equipment. In order to increase sensitivity, the flow rate of the sheath liquid was diminished from 0.5 to 0.2 microL.min(-1) by increasing the content in water from 0 to 50% and the formic acid from 0.5 to 1.5% in this liquid and by applying an overimposed pressure of 5 mbar during the electrophoretic separation. The method allowed the analysis of 30 samples per hour.     

Density functional theory study of the mechanism of the proline-catalyzed intermolecular aldol reaction

Transition structures associated with the C-C bond-formation step of the proline-catalyzed intermolecular aldol reaction between acetone and isobutyraldehyde have been studies using density functional theory methods at the B3LYP/6-31G** computational level. A continuum model has been selected to represent solvent effects. For this step, which is the stereocontrolling and rate-determining step, four reactive channels corresponding to the syn and anti arrangement of the active methylene of the enamine relative to the carboxylic acid group of l-proline and the re and si attack modes to both faces of the aldehyde carbonyl group have been analyzed. The B3LYP/6-31G** energies are in good agreement with experiment, allowing us to explain the origin of the catalysis and stereoselectivity for these proline-catalyzed aldol reactions. Received: 2 April 2002 / Accepted: 18 July 2002 / Published online: 11 October 2002 Acknowledgements. This work was supported by research funds provided by the Ministerio de Educación y Cultura of the Spanish Government by DGICYT (project PB98–1429). All the calculations were performed on a Cray–Silicon Graphics Origin 2000 of the Servicio de Informática de la Universidad de Valencia. We are most indebted to this center for providing us with computer capabilities. Correspondence to: L. R. Domingo e-mail: domingo@utopia.uv.es     

Carbon dendron nano-chains with π-extended conjugation based on end-capped N,N-dimethylamino in linear 1,4-phenylethynyl or in 1,5-naphthylethynyl subunits: fluorescence analysis

The synthesis of dendrons with the end-capped 5-(N,N-dimethylamino)naphthyl-1-ethynyl unit connected to conjugated naphthylethynyl or p-phenylethynyl chains, as the branches of the 1,3,5-substituted benzene core, have been undertaken by heterocoupling reaction between 1,3,5-triiodobenzene and the convenient end-capped 5-(N,N-dimethylamino)naphthylacetylene or 5-(N,N-dimethylamino)naphthylethynylphenylacetylene in the presence of the palladium-copper catalyst system, in excellent yields. The influence of the alternating naphthylethynyl-phenylethynyl chains on the fluorescence emission radiation, in the dendron structures, has been analyzed.     

Gas chromatography/mass spectrometric study of non-commercial C-4-substituted 1,4-dihydropyridines and their oxidized derivatives

A gas chromatography/mass spectrometry (GC/MS) method for the qualitative and quantitative determination of the calcium-channel antagonists C-4-substituted 1,4-dihydropyridines, and their corresponding N-ethyl derivatives, is presented. Also, the electrochemical oxidation and the reactivity of the compounds with alkyl radicals derived from 2,2'-azobis-(2-amidinopropane) were monitored by GC/MS. Mass spectral fragmentation patterns for the C-4-substituted 1,4-dihydropy-ridine parent drugs were significantly different from those of their oxidation products, generated either by electrochemical oxidation or by reaction with alkyl radicals. However, for N-ethyl-1,4-dihydropyridine compounds it was not possible to detect the final products (pyridinium salts) using these experimental conditions.