Dearomatising rearrangements of lithiated thiophenecarboxamides

Thiophene-3-carboxamides bearing allyl or benzyl substituents at nitrogen undergo dearomatising cyclisation on treatment with LDA. Rearrangements transform the dearomatised products into pyrrolinones, azepinones or partially saturated azepinothiophenes.     

Surface layering in ionic liquids: an X-ray reflectivity study

The surface structure and thermodynamics of two ionic liquids, based on the 1-alkyl-3-methylimidazolium cations, were studied by X-ray reflectivity and surface tensiometry. A molecular layer of a density approximately 18% higher than that of the bulk is found to form at the free surface of these liquids. In common with surface layering in liquid metals and surface freezing in melts of organic chain molecules, this effect is induced by the lower dimensionality of the surface. The concentrations of the oppositely charged ions within the surface layer are determined by chemical substitution of the anion. The temperature-dependent surface tension measurements reveal a normal, negative-slope temperature dependence. The different possible molecular arrangements within the enhanced-density surface layer are discussed.     

Chiral induction in quinoline-derived oligoamide foldamers: assignment of helical handedness and role of steric effects

Chiral groups attached to the end of quinoline-derived oligoamide foldamers give rise to chiral helical induction in solution. Using various chiral groups, diastereomeric excesses ranging from 9% to 83% could be measured by NMR and circular dichroism. Despite these relatively weak values and the fact that diastereomeric helices coexist and interconvert in solution, the right-handed or left-handed helical sense favored by the terminal chiral group could be determined unambiguously using X-ray crystallography. Assignment of chiral induction was performed in an original way using the strong tendency of racemates to cocrystallize, and taking advantage of slow helix inversion rates, which allowed one to establish that the stereomers observed in the crystals do correspond to the major stereomers in solution. The sense of chiral helical induction was rationalized on the basis of sterics. Upon assigning an Rs or Ss chirality to the stereogenic center using a nomenclature where the four substituents are ranked according to decreasing sizes, it is observed that Rs chirality always favors left-handed helicity and Ss chirality favors right-handed helicity (P). X-ray structures shed some light on the role of sterics in the mechanism of chiral induction. The preferred conformation at the stereocenter is apparently one where the bulkiest group should preferentially point away from the helix, the second largest group should be aligned with the helix backbone, and the smallest should point to the helix.     

Fourth-order diagrammatic MB-RSPT calculations of the interaction energy between two helium atoms

The efficiency of the MB-RSPT in the calculations of the correlation contributions to the interaction energies was investigated, using He₂ as a model Van der Waals system. The attention has been focused on the convergency of the perturbation expansion in the calculations of the interaction energy and on the analysis of the fourth-order terms of MB-RSPT. The rôle of the renormalization term in the correct long-range behaviour of the interaction potential has been emphasized.     

Determination of diamine oxidase by a kinetic method with 2,2''-azino-di(3-ethylbenzthiazoline-6-sulfonate)

Hydrogen peroxide, the product of diamine oxidase-catalyzed putrescine or cadaverine oxidation, formed in proportion to the enzyme activity, is measured spectrophotometrically by using the above sulfonate (ABTS) and peroxidase. Only one reagent solution containing 3 mmol of putrescine or 10 mmol of cadaverine, 4 mmol of ABTS and 3000 U of peroxidase per litre of 0.2 mol l^-1 Tris—0.1 mol l^-1 HCl buffer pH 7.5 is needed. Absorbance changes are measured at 410 nm over the first 3 min of the reaction. This initial oxidation rate of the chromogen enables diamine oxidase activity up to 230 U l^-1 to be determined.     

Antichain cutsets

A subset A of an ordered set P is a cutset if each maximal chain of P meets A; if, in addition, A is an antichain call it an antichain cutset. Our principal result is a characterization, by means of a forbidden configuration, of those finite ordered sets, which can be expressed as the union of antichain cutsets.     

Use of high-molecular-mass polyacrylamides as matrices for microchip electrophoresis of DNA fragments

DNA fragment analysis requires the use of polymer solutions as sieving matrices. Generally, such matrices are constituted of high-molar-weight polymers employed at a concentration higher than their entanglement threshold concentration. These polymer solutions are highly viscous and difficult to use in the narrow channels of a microchip. Ultralarge polyacrylamides synthesized via a nonconventional method, being the low-temperature plasma-induced polymerization (PIP), were used as DNA sieving matrices for microchip electrophoresis. The distinctive features of these polymers (ultralarge molecular mass and linearity) allow their use at a dilute concentration. Dilute PIP polyacrylamides revealed a constant value of resolution in a broad range of DNA fragment sizes (123 bp-1353 bp), thus proving to be effective in common genotyping applications. Moreover, the low viscosity of the dilute solutions enable it to be easier and faster in filling the channel between runs, thus enhancing the throughput of the microchip devices.     

Algebraic Kekulé structures of benzenoid hydrocarbons

An algebraic Kekulé structure of a benzenoid hydrocarbon is obtained from an ordinary Kekulé structure by inscribing into each hexagon the number of pi-electrons which (according to this Kekulé structure) belong to this hexagon. We show that in the case of catafusenes, there is a one-to-one correspondence between ordinary and algebraic Kekulé structures. On the other hand, in the case of perifusenes, one algebraic Kekulé structure may correspond to several ordinary Kekulé structures.     

Selenium speciation in foods: Preliminary results on potatoes

The present paper deals with the speciation of selenium in potatoes (enriched or not in selenium). The study was carried out by using differential pulse cathodic stripping voltammetry (DPCSV) for quantifying selenium. Results obtained provide evidence that the selenium content in the protein fraction is rather independent from the selenium added to the plants during their growth. On the contrary, the amount of Se in the non-protein fraction (water and starch) in Se-enriched sample is significantly higher than in non-enriched one, suggesting that it is the main selenium-storing site. In this fraction the Se(VI)/Se(IV) ratio seems independent from selenium application but it may be related to the redox conditions. The accumulation of selenium in the non-protein fraction is tentatively ascribed to the “Se–starch interaction” that should be able to modulate both the Se absorption into proteins and, possibly, its toxic effect for the plant itself.     

Dynamic NMR Studies of Some Halo and Mercaptide Bridged Palladium (II) Dimers

Some halo or mercaptide bridged palladium (II) dimers, [Pd(S₂CNR₂)X]₃ (R=ibutyl, X=Cl, Br, I, S-ethyl and S-t-butyl) were studied by variable temperature ¹H nmr spectroscopy. Line shape changes of the chloro and bromo bridged dimers were interpretated by the solvolytic breaking of the Pd-X bond, while mercaptide bridged complexes were explained in terms of slow N-C single bond rotation. The results consist with the strengthness of the class b metal ion with various soft donor ligands.