Substituent effects in eight-electron electrocyclic reactions

The main features of transition structures associated with eight-electron electrocyclic reactions have been studied with Density Functional Theory. It is found that conrotatory electrocyclization reaction of (3Z,5Z)-octa-1,3,5,7-tetraenes takes place via M?bius aromatic transition structures of helical conformation. The reaction is completely periselective. In general, transition structures having outward substituents are preferred with respect to the inward transition structures, irrespective of the pi-donor or pi-acceptor character of the substituent. In contrast with four-electron thermal conrotatory electrocyclic reactions, there is no satisfactory correlation between the difference in energy of activation between inward and outward substituents and the Taft resonance sigma(R) parameter.     

Selenium and mercury bioaccessibility in fish samples: an in vitro digestion method

The assessment of hazard from selenium (Se) and mercury (Hg) contaminants in the food chain based on their potential bioaccessibility and on estimate of their actual content in fish is reported under an in vitro model. Atomic fluorescence spectroscopy (AFS) was applied for total selenium and mercury quantification. Selenium and mercury bioaccessibility varied depending on the type of fish analyzed. Se solubility in the gastrointestinal supernatants was higher in swordfish and sardine (76 and 83%, respectively) than in tuna (50%). A low Hg bioaccessibility (9-17%) was found for all the samples.Simulated human gastric and intestinal digestion led to the identification of selenomethionine (SeMet) and organic mercury in the three digested fish. Furthermore, these species were not modified during the digestion. Speciation of selenium compounds was done by liquid chromatography in conjunction with inductively coupled plasma mass spectrometry (LC-ICP-MS).Sardine had the most favorable Se:Hg, [Se:Hg]_{bioaccessible} and [SeMet:Hg]_{bioaccessible} molar ratios, making it preferable to tuna and swordfish.The effect of cooking was also evaluated.     

Approximate transferability in alkanenitriles

The atomic and bond properties of a series of alkanenitriles were calculated in order to analyze the transferability of the CN, methyl, and methylene groups. The calculations were carried out using the atoms in molecules (AIM) theory on RHF/6‐31++G**// RHF/6‐31G** wave functions obtained for compounds CN–R (R ranging from H to C₁₁H₂₃). Linear correlations between L(Ω) and N(Ω) were used to establish N(CH₂) and N(CH₃) nearly transferable values. Average values and maximum differences to the mean value of several properties were used for classifying the CN group. It shows a transferable behavior along the CN–R series for R>Et. The methyl group presents specific properties when R<Pr. The methylene groups can be classified considering both their position with respect to the end of the chain and the position with respect to the CN group. The atomic energy displays a dependence on the molecular size. Although this behavior does not allow to consider this property as transferable, both the ab initio total electronic molecular energies and the experimental heats of formation can be fitted, by linear regression analysis, as a function of the number of methylene groups. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Thermodynamic properties of quinoxaline-1,4-dioxide derivatives: a combined experimental and computational study

The mean (N-O) bond dissociation enthalpies were derived for three 2-methyl-3-(R)-quinoxaline 1,4-dioxide (1) derivatives, with R = methyl (1a), ethoxycarbonyl (1b), and benzyl (1c). The standard molar enthalpies of formation in the gaseous state at T = 298.15 K for the three 1 derivatives were determined from the enthalpies of combustion of the crystalline solids and their enthalpies of sublimation. In parallel, accurate density functional theory-based calculations were carried out in order to estimate the gas-phase enthalpies of formation for the corresponding quinoxaline derivatives. Also, theoretical calculations were used to obtain the first and second N-O dissociation enthalpies. These dissociation enthalpies are in excellent agreement with the experimental results herewith reported.     

Identifying acetylated lignin units in non-wood fibers using pyrolysis-gas chromatography/mass spectrometry

A series of non-wood plant fibers, namely kenaf, jute, sisal and abaca, have been analyzed upon pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) of the whole material. The pyrolysis products mainly arise from the carbohydrate and lignin moieties of the fibers. The lignin-derived phenols belonged to the p-hydroxyphenylpropanoid (H), guaiacylpropanoid (G) and syringylpropanoid (S) structures, and showed a high S/G ratio of between 2.0 and 5.4, the highest corresponding to kenaf. Among the lignin-derived phenols released, small amounts of sinapyl and coniferyl acetates (in both cis- and trans-forms) were identified for the first time upon Py-GC/MS of lignocellulosic materials. Acetylation of the sinapyl and coniferyl alcohols was at the gamma-position of the side chain. The release of these alcohols derived from intact acetylated lignin units upon pyrolysis seems to indicate that the native lignin in the fibers selected for this study is at least partially acetylated. Sinapyl (and coniferyl) acetates have recently been suggested to be authentic lignin precursors involved in the polymerization of lignin along with the normal sinapyl and coniferyl alcohols. Py-GC/MS will offer a convenient and rapid tool for analyzing naturally acetylated lignins, as well as to screen plant materials for the presence of acetylated units in lignin.     

Reduction of sulfoxides with boranes catalyzed by MoO2Cl2

The first example of an organic reduction with boranes catalyzed by a high valent oxo-complex is reported. The systems catecholborane/MoO₂Cl₂(H₂O)₂ (5 mol %) and BH₃·THF/MoO₂Cl₂ (5 mol %) proved to be very efficient for the reduction of sulfoxides to the corresponding sulfides in excellent yields.     

A Comparative Study of Naproxen – Beta Cyclodextrin Complexes Prepared by Conventional Methods and Using Supercritical Carbon Dioxide

Naproxen is a poorly soluble anti-inflammatorydrug, the solubility of which canbe enhanced by complexation withbeta-cyclodextrin. Besides that, the inclusioncomplex reduces the incidence of gastrointestinal side effects of the drug. The aim of this work was to compare the physicochemical characteristics of the solid complexes prepared by traditional methods (kneading, freeze-drying and spray-drying) and using a supercritical fluid technology. The unusual solvent properties of carbon dioxide above their critical temperature and pressure were exploited in order to prepare inclusion compounds. Complexes prepared using supercritical fluid technology showed similar properties to those of freeze-drying andspray-drying complexes as proved by DSC, FT-IRand UV.     

Flow injection system for hydrolysable tannin determination

A flow injection system was proposed to evaluate the transient product of a colorimetric reaction between hydrolysable tannin and potassium iodate (KIO₃) solution. The system optimization was accomplished by using statistical methods based on experimental design. Flow rate of KIO₃ solution, sample volume, carrier flow rate, and reaction coil were the selected factors for evaluation. On screening step, complete factorial 2⁴ was used and two levels for each selected factor were studied. For the optimization phase, a centered face composite design 2² + star was employed to evaluate sample volume and flow rate of KIO₃ solution, which were the factors identified in the screening phase as having more influence on the absorbance signal. After optimization, the proposed system was compared with batch determination. Some characteristics, such as analytical frequency, reagent consumption and chemical residues generation presented better results by the use of the proposed system if compared with batch method. The system presented good repeatability with standard deviation lower than 3%, for n = 10, linearity (R² = 0.9974) for tannic acid standard, analytical frequency of 15 injections h^− 1 and limit of quantification of 24 mg L^− 1 of tannic acid. Good results were obtained when the proposed system was applied to hydrolysable tannin determination in Stryphnodendron barbatimão, Eucalyptus citriodora and Phyllanthus niruri, samples of plants commonly used in popular medicine.     

Characterization of a Thermotolerant and Alkalotolerant Xylanase from a Bacillus sp.

In a recent screening for thermophilic bacteria from Azores hot springs, a Bacillus sp strain 3M, exhibiting cellulase-free extracellular xylanolitic activity, was isolated. Further enzyme characterization from liquid cultures grown on birchwood xylan revealed that the endo-l,4-βxylanase retains 100% of activity for at least 3 d at 55°C. At 80°C, it retains 47% of its maximal activity, and the enzyme is still active at 90°C. The optimum pH of the enzyme has a broad pH range, between 6.0 and 7.5, and it is remarkably active for the alkaline region, exhibiting 89% of relative activity at pH 9.O. The enzyme was partially inactivated by different divalent metal ions. Because of its tolerance for high temperature and pH conditions, and the absence of contaminating cellulase activity, the xylanase produced byBacillus sp 3M appears to be attractive for use in the pulp and paper industry. Indeed, the efficiency of the enzyme application to the kraftEucalyptus pulp was studied for bleaching pretreatment, resulting in a moderate increase of pulp bleachability.     

Synthesis and characterization of carbohydrate-based phospholipids

Novel carbohydrate-based phospholipids containing two saturated C(12) (dilauroyl ribo-phosphocholine) (DLRPC), C(14) (dimyristoyl ribo-phosphocholine) (DMRPC), and C(20) (diarachadonyl ribo-phosphocholine) (DARPC) carboxylic acid chains were synthesized. The physical properties of the supramolecular structures formed by these compounds were compared to those formed by their direct glycerol analogues dilauroyl phosphocholine (DLPC), dimyristoyl phosphocholine (DMPC), and diarachadonyl phosphocholine (DAPC). Modulated differential scanning calorimetry (MDSC) and X-ray diffraction data indicated that with chain lengths < or =14 carbons, the carbohydrate backbone increased the thermal stability of the bilayer below the phase-transition temperature (T(m)) as compared to the glycerol-based lipids. With longer chains (C(20)), the bilayer structure was destabilized as compared to glycerol-based lipids. NMR studies of a DMRPC vesicle dispersion reveal split choline headgroup signals and distinct magnetization transfer effects arising from the "inner" and "outer" surfaces of the bilayer vesicle. Modulated differential scanning calorimetry also demonstrated that glycerol- and carbohydrate-based lipids mix, as evidenced by a single intermediate T(m). In addition, carbohydrate-based lipid/cholesterol mixtures exhibited a decrease in enthalpy with an increase in cholesterol concentration. Unlike glycerol phospholipids, carbohydrate lipids were resistant to enzymatic degradation by phospholipase A(2) (PLA(2)).