Kinetic and mechanistic investigation of the aminolysis of 3-methoxyphenyl 3-nitrophenyl thionocarbonate, 3-chlorophenyl 3-nitrophenyl thionocarbonate,and bis(3-nitrophenyl) thionocarbonate

The reactions of the title thionocarbonates (6, 7, and 8, respectively) with a series of secondary alicyclic amines are subjected to a kinetic investigation in 44 wt % ethanol-water, 25.0 degrees C, ionic strength 0.2 M (KCl). Under excess amine, pseudo-first-order rate coefficients (k(obsd)) are obtained for all reactions. Reactions of substrates 6 and 7 with piperidine and of thionocarbonate 8 with the same amine and piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine show linear k(obsd) vs [amine] plots, with slopes (k(1)) independent of pH. On the other hand, these plots are nonlinear upward for the reactions of substrates 6 and 7 with all the amines, except piperidine, and also for the reactions of compound 8 with 1-formylpiperazine and piperazinium ion. For all these reactions a mechanistic scheme is proposed with the formation of a zwitterionic tetrahedral intermediate (T(+/-)), which can transfer a proton to an amine to give an anionic intermediate (T(-)). Rate and equilibrium microcoefficients of this scheme, k(1), k(-)(1), K(1) (= k(1)/k(-)(1)), and k(2), are obtained by fitting the nonlinear plots through an equation derived from the scheme. The Br?nsted-type plots for k(1) are linear with slopes beta(1) = 0.19, 0.21, and 0.26 for the aminolysis of 6, 7, and 8, respectively. This is consistent with the hypothesis that the formation of T(+/-) (k(1) step) is the rate-determining step. The k(1) values for these reactions follow the sequence 8 > 7 > 6, consistent with the sequence of the electron-withdrawing effects from the substituents on the "nonleaving" group of the substrates. The k(1) values for the aminolysis of 6, 7, and 8 are smaller than those for the same aminolysis of 3-methoxyphenyl, 3-chlorophenyl, and 4-cyanophenyl 4-nitrophenyl thionocarbonates (2, 3, and 4, respectively). The k(2) values (expulsion of the nucleofuge from T(+/-)) increase as the electron withdrawal from the nonleaving group increases. These values are smaller for the aminolysis of 6, 7, and 8 compared to those for the same aminolysis of 2, 3, and 4, respectively.     

Ring-closing metathesis: development of a cyclisation-cleavage strategy for the solid-phase synthesis of cyclic sulfonamides

A series of novel 7-membered cyclic sulfonamides have been synthesised using a solid-phase cyclisation-cleavage RCM strategy. Model solution studies indicated the sulfonamides were suitable substrates for RCM using the Grubbs' catalyst 2. Starting from either 2-carboxyethyl polystyrene (21) or Merrifield resin, various seven-membered sulfonamides were prepared in good to excellent yields at low catalyst loadings (2.5-5 mol%) using a flexible spacer between the polymer and the substrate. In addition, a novel double-armed linker was shown to allow efficient RCM cleavage of sulfonamides with as little as 1 mol% of the ruthenium alkylidene complex 2.     

Simultaneous multiwavelength detection in flow injection analysis

The potential of the use of the diode-array detector in conjunction with flow injection analysis is outlined. Methods for multicomponent resolution can be based on the formation of complexes absorbing at different wavelengths. The example given is the determination of mixtures of copper(II) and iron(III) with a mixed 1:10-phenanthroline/neocuproine reagent. Amplification and dilution methods are based on the sum of the absorbances at several wavelengths and on monitoring absorbances at wavelengths away from the absorption maximum, respectively. The determination of nitrite via the Griess reaction is used as an example; the viable determination range is extended to 0.002/2-60.0 μg ml^?1 nitrite. Software for the implementation of the suggested methods is outlined.     

Ultrasound-assisted derivatization of phenolic compounds in spiked water samples before pervaporation, gas chromatographic separation, and flame lonization detection

Summary A fully automated method, based on continuous ultrasound-assisted derivatization coupled with pervaporation before gas chromatographic separation and flame ionization detection, has been developed for the determination of phenol and cresols in water. Spiked water samples were doped with acetic anhydride and dipotassium hydrogen phosphate, before introduction into the flow system, to achieve catalytic acetylation of the target compounds. A multivariate study was performed to optimize the main factors affecting the derivatization process. The correlation coefficients, r, of the calibration plots obtained were better than 0.999 for cresols and better than 0.99 for phenol. Detection limits were 0.02 μg mL¹ for phenol, o-cresol, andp-cresol, and 0.05 μg mL¹ form-cresol. Reproducibility and repeatability, expressed as relative standard deviation, ranged from 2.0–3.9% and from 1.0–3.5% respectively.     

Isotope effect in chemical shifts of μ+ and H+: MuBr versus HBr and MuHO versus H2O

The isotopic difference between the diamagnetic shielding of a positive muon (⁺) in ⁺ HBr and H₂O and the analogous shielding of protons is computed from first principles in free space and in Br₂ solution, using a self-consistent cellular cluster multiple scattering method (SC-CMS) for condensed matter and for the free molecules. The isotope shift of the chemical shift _is at the ⁺ in these molecules dissolved in liquid Br₂ is evaluated with the eigenfunctions and eigenvalues obtained using Ramsey's formalism. For HBr the computed chemical shifts _is are comparable with experiment and with the calculations of Breskman and Kanofsky and of Williams, and the solvent effect has the correct sign and order of magnitude. For H₂O, _is has also the correct sign and order of magnitude when compared with ⁺SR experiments.     

Mutual annihilation of triplet excitons in a nearly one-dimensional system: 1,4-dibromonaphthalene

The analysis of the variation with incident flux of the time dependence of the delayed fluorescence in conjunction with the determination of the absolute ground state-first excited triplet absorption coefficients at room temperature, yields the value of γ_tot = (5.5 ± 2.0) × 10^?12 cm³ s^?1, for the total triplet-triplet annihilation rate constant in 1,4-dibromonaphthalene crystals. The one-dimensional mutual annihilation rate constant for the triplet exciton motion restricted to linear chains along the crystal c axis is γ¹_tot = (1.0 ± 0.4) × 10³ cm s^?1. The results are discussed in terms of recent theories of mutual annihilation of triplets in one-dimensional systems.     

Partition and location of nimesulide in EPC liposomes: a spectrophotometric and fluorescence study

Study of the mechanism of action of anti-inflammatory drugs (NSAIDs) and their side-effects may fall in the domain of membranology. In this work the extent of the interaction between an NSAID (nimesulide) and membrane phospholipids was quantified by the partition coefficient, K_p , using egg phosphatidylcholine (EPC) liposomes as cell membrane models. The liposome/aqueous phase partition coefficients were determined under physiological conditions, by derivative spectrophotometry and fluorescence quenching. Derivative spectrophotometry allows elimination of background signal effects (light scattering) due to the presence of liposomes. Theoretical models, accounting for simple partition of the NSAID between two different media, were used to fit the experimental data, allowing the determination of K_p in multilamellar vesicles (MLVs) and large unilamellar vesicles (LUVs). The location of nimesulide in MLVs and LUVs was evaluated by fluorescence quenching using spectroscopic probes located at different sites on the membrane. All n-AS probes were quenched and the relative quenching efficiencies were ordered as 2-AS<6-AS9-AS<12-AS; this suggests the drug is deeply buried in the membrane. Fluorescence quenching using the 12-AS probe was also used to determine the partition coefficient of the drug in MLVs and LUVs. The two techniques yield similar results. Finally, measurement of zeta-potential in the presence of different concentrations of nimesulide was performed to investigate possible changes in the zwitterionic phosphatidylcholine membranes. The membrane surface potential was not altered, which seems to be an indication that nimesulide binds to lipid bilayer mostly by hydrophobic interactions.     

Combining combinatorial chemistry and affinity chromatography: highly selective inhibitors of human betaine: homocysteine S-methyltransferase

A new method to find novel protein targets for ligands of interest is proposed. The principle of this approach is based on affinity chromatography and combinatorial chemistry. The proteins within a crude rat liver homogenate were allowed to interact with a combinatorial library of phosphinic pseudopeptides immobilized on affinity columns. Betaine: homocysteine S-methyltransferase (BHMT) was one of the proteins that was retained and subsequently eluted from these supports. The phosphinic pseudopeptides, which served as immobilized ligands for the isolation of rat BHMT, were then tested for their ability to inhibit human recombinant BHMT in solution. The most potent inhibitor also behaved as a selective ligand for the affinity purification of BHMT from a complex media. Further optimization uncovered Val-Phe-psi[PO(2-)-CH(2)]-Leu-His-NH(2) as a potent BHMT inhibitor that has an IC(50) of about 1 microM.     

Method development for the determination of manganese, cobalt and copper in green coffee comparing direct solid sampling electrothermal atomic absorption spectrometry and inductively coupled plasma optical emission spectrometry

A method has been developed for the determination of cobalt, copper and manganese in green coffee using direct solid sampling electrothermal atomic absorption spectrometry (SS-ET AAS). The motivation for the study was that only a few elements might be suitable to determine the origin of green coffee so that the multi-element techniques usually applied for this purpose might not be necessary. The three elements have been chosen as test elements as they were found to be significant in previous investigations. A number of botanical certified reference materials (CRM) and pre-analyzed samples of green coffee have been used for method validation, and inductively coupled plasma optical emission spectrometry (ICP OES) after microwave-assisted acid digestion of the samples as reference method. Calibration against aqueous standards could be used for the determination of Mn and Co by SS-ET AAS, but calibration against solid CRM was necessary for the determination of Cu. No significant difference was found between the results obtained with the proposed method and certified or independently determined values. The limits of detection for Mn, Cu and Co were 0.012, 0.006 and 0.004 μg g⁻¹ using SS-ET AAS and 0.015, 0.13 and 0.10 μg g⁻¹ using ICP OES. Seven samples of Brazilian green coffee have been analyzed, and there was no significant difference between the values obtained with SS-ET AAS and ICP OES for Mn and Cu. ICP OES could not be used as a reference method for Co, as essentially all values were below the limit of quantification of this technique.     

Phytochemical study,an evaluation of the antioxidant potential and the antimicrobial activity of Inga semialata (Vell.) C. Mart. hydroalcohol extract

Abstract

Inga semialata (Vell.) C. Mart. belongs to the family Fabaceae. It is known for its therapeutic properties, highlighting its antimicrobial and antioxidant potential. The objective of the present work was to obtain crude extract leaves of Inga semialata, to identify and quantify active compounds, to evaluate the antioxidant potential of the crude extract in vitro, as well as to determine its antimicrobial activity. The crude extract was obtained by the maceration process. The identified and quantified of compounds present in the crude extract of Inga semialata was performed by high performance liquid chromatography. The evaluation of the antioxidant potential of the extract was realized by in vitro tests (DPPH, diacetate dichlorofluorescein test and nitric oxide test) and the evaluation of the antimicrobial activity was carried out using the minimum inhibitory concentration methodology.