Non-empirical LCAO-MO-SCF calculations with gaussian type functions on the electrocyclic transformation of cyclopropyl to allyl

Non-empirical molecular quantum chemical calculations have been performed on the electrocyclic transformation of planar and non planar cyclopropyl anions, to allyl anions in the LCAO-MO-SCF framework using gaussian type functions as atomic orbitals. Employing a total of 37 GTF 3 disrotatory and 2 conrotatory modes of transformation have been considered. A total of 34 calculations have been carried out. In the transformation of planar and non planar cyclopropyl anions to allyl anions, both the disrotatory and conrotatory modes possess activations barriers, those for the conrotatory modes being the lower by substantial margins. A detailed analysis of the energy terms involved show that is little theoretical justification for the postulate that the mode of ring opening depends only on the symmetries of the highest occupied orbitals.

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Zusammenfassung Nichtempirische SCF-MO-Rechnungen mit Gaußfunktionen (GF) für den Übergang des planaren und des unplanaren Cyclopropylanions zum Allyanion wurden vorgenommen. Mit einem Basissatz von 37 GF wurden 34 Rechnungen für 3 disrotatorische und 2 conrotatorische Übergänge durchgeführt. Sowohl die disrotatorischen als auch die conrotatorischen Übergänge besitzen Aktivierungsbarrieren, von denen diejenigen für die conrotatorischen Übergänge stets kleiner sind. Die Analyse der Energieterme gibt wenig Anlaß zu der Annahme, daß die Art der Ringöffnung nur von der Symmetrie des obersten besetzten Orbitals abhängt.

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Résumé Des calculs ab-initio en orbitales gaussiennes ont été effetués sur la transformation de cations cyclopropyliques en cations allyliques. Les barrières d'activation sont toujours plus basses pour la mode conrotatoire que pour la mode disrotatoire. Il est peu évident que l'ouverture du cycle dépende que de la Symmetrie de la plus haute orbitale occupée.

     

Aptamers as analytical reagents

Many important analytical methods are based on molecular recognition. Aptamers are oligonucleotides that exhibit molecular recognition; they are capable of specifically binding a target molecule, and have exhibited affinity for several classes of molecules. The use of aptamers as tools in analytical chemistry is on the rise due to the development of the "systematic evolution of ligands by exponential enrichment" (SELEX) procedure. This technique allows high-affinity aptamers to be isolated and amplified when starting from a large pool of oligonucleotide sequences. These molecules have been used in flow cytometry, biosensors, affinity probe electrophoresis, capillary electrochromatography, and affinity chromatography. In this paper, we will discuss applications of aptamers which have led to the development of aptamers as chromatographic stationary phases and applications of these stationary phases; and look towards future work which may benefit from the use of aptamers as stationary phases.     

Recombinant cell bioassay systems for the detection and relative quantitation of halogenated dioxins and related chemicals

Proper epidemiological, risk assessment and exposure analysis of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and related halogenated aromatic hydrocarbons (HAHs) requires accurate measurements of these chemicals both in the species of interest and in various exposure matrices (i.e. biological, environmental, food and feed). High-resolution instrumental analysis techniques are established for these chemicals, however, these procedures are very costly and time-consuming and as such, they are impractical for large scale sampling studies (i.e. for epidemiological studies and assessment of areas with widespread contamination). Accordingly, numerous bioanalytical methods have been developed for the detection of these chemicals in extracts from a variety of matrices, the majority of which take advantage of the ability of these chemicals to activate the aromatic hydrocarbon receptor (AhR) and the AhR signal transduction pathway. Here we review the currently available in vitro AhR-based cell bioassay systems with a focus on recent recombinant reporter gene cell lines that have been developed for detection and relative quantitation of TCDD and related HAHs. Comparison of the relative sensitivities of the various cell bioassays and examples of their use in screening and analysis of environmental, biological, and food and feed samples are presented. Currently available experimental results and validation studies demonstrate the utility of these cell bioassay systems to provide a relatively rapid, accurate, and cost effective screening approach for the detection of TCDD and related HAHs in a variety of environmental, biological, food and feed samples. The availability of these cell bioassay systems will not only facilitate the large scale sampling studies needed for accurate assessment of contamination and exposure to these environmental chemicals, but they provide avenues for the identification of novel classes of TCDD-like chemicals.     

Liquid chromatographic optimization of reaction conditions for the synthesis of 2,2-dimethyl-N-benzylmalonamide, a new anticonvulsant

Reversed-phase liquid chromatographic (RP/LC) procedures are used to optimize the reaction conditions for the large scale preparation of 2,2-dimethyl-N-benzylmalonamide, 1. In acute studies using mice and rats, compound 1 is a very effective anticonvulsant agent. Large quantities of 1 (ca. 300 g) are needed for detailed studies of its chronic effects. LC monitoring of the synthesis of intermediate products and 1 result in the optimum use of reagents, increased product yields, and decreased reaction times.     

High Performance Liquid Chromatographic Analysis of Pharmaceuticals Using Oil-In-Water Microemulsion Eluent and Monolithic Column

Novel and efficient separations of pharmaceutical substances were achieved using oil-in-water microemulsion eluent and a conventional C18 packing with a flow rate of 1 mL/min⁻¹. Attempts to decrease analysis time was limited due to the high viscosity of the microemulsion which generated relatively high back-pressures. Monolithic columns gave 3-fold lower back-pressures and allowed flow rates of 4 mL/min⁻¹. with the same microemulsion mobile phase which permitted rapid separations to be achieved. Separation of a test-mix of paraben preservatives was achieved in both isocratic and gradient mode in less than 1 min. The monolith-microemulsion combination was applied to rapidly quantitatively analyse two formulated products with excellent linearity, accuracy and repeatability. Quantitative analysis times were under 90 seconds. Successful quantitation of both nicotine lozenges and naprosyn tablets was performed using this approach.     

Liquid chromatographic and mass spectral analysis of 1-(3,4-methylenedioxyphenyl)-3-butanamines, homologues of 3,4-methylenedioxyamphetamines

The 1-(3,4-methylenedioxyphenyl)-3-butanamines (HMDAs) are prepared via reductive amination of the corresponding ketone with a series of low molecular weight alkylamines. These amines are homologues of the N-substituted 3,4-methylenedioxyamphetamines (MDAs). Compounds of the HMDA series have UV absorption properties similar to the MDAs because both series contain the same 3,4-methylenedioxyphenyl chromophore. The HMDAs are separated via reversed-phase liquid chromatographic methods using a C18 stationary phase and an acidic aqueous acetonitrile mobile phase. The mass spectra of these potential designer drugs are very similar to the spectra of the MDA homologues having the same N-substituent.     

Identification of novel macrocyclic peptidase substrates via on-bead enzymatic cyclization

Peptidase-catalyzed formation of macrocyclic lactams on solid phase identifies ring systems that are favorably bound in the enzyme active site. We evaluated several cyclic peptide motifs linked by ester bonds between the P2 and P1' or the P1 and P2' side chains. The depsipeptide represented by structure 5 was readily generated by a variety of peptidases from precursor omega-amino acids or omega-amino esters. This strategy for identifying ring systems for potential macrocyclic transition state analogues was demonstrated with the serine peptidases trypsin and chymotrypsin, with the aspartic peptidase pepsin, and with the zinc peptidase thermolysin.