Flow injection determination of oxalate in urine based on an inhibitory effect

Summary Two methods based on the use of the normal and stopped-flow injection modes were developed for the determination of oxalate based on its inhibitory effect on the catalytic action of Fe(III) on the 2,4-diaminophenol/hydrogen peroxide system. The linear determination ranges achieved were between 0.2 and 12.0 g ml^–1 and between 0.2 and 40.0 g ml^–1, the precision was ±5.4%, and ±3.5%, and the sampling rate was 30 and 20 samples h^–1 for the normal and stopped-flow method, respectively. Both methods have been applied to the determination of oxalate in urine with excellent results.     

Complete 1H and 13C NMR assignments for two new monodesmoside saponins from Pentaclethra macroloba (Willd.) Kuntze

A detailed NMR study and full assignments of the 1H and 13C spectral data for two novel triterpenoid saponins isolated from the stem bark of Pentaclethra macroloba (Willd.) Kuntze are described. Their structures were established using a combination of 1D and 2D NMR techniques including 1H,1H-COSY, TOCSY, NOESY, gs-HMQC and gs-HMBC, and also electrospray ionization mass spectrometry and chemical methods. The structures were established as 3beta-O-([O-beta-D-glucopyranosyl-(1-->2)-O-beta-D-glucopyranosyl-(1-->4)-O-beta-D-glucopyranosyl-(1-->3)-O-alpha-L-rhamnopyranosyl-(1-->2)]-[O-beta-D-glucopyranosyl-(1-->3)-O-beta-D-glucopyranosyl-(1-->4)])-alpha-L-arabinopyranosylhederagenin (1) and 3beta-O-)[O-beta-D-glucopyranosyl-(1-->2)-O-beta-D-glucopyranosyl-(1-->4)-O-beta-D-glucopyranosyl-(1-->3)-O-alpha-L-rhamnopyranosyl-(1-->2)]-[O-beta-D-glucopyranosyl-(1-->3)-O-beta-D-glucopyranosyl-(1-->4)])-alpha-L-arabinopyranosyloleanolic acid (2).     

Effect of ethylene glycol on the thermodynamic and micellar properties of Tween 20

The aggregation behaviour of Tween 20 in ethylene glycol-water mixed solvents has been investigated using surface tension, density, static and dynamic light scattering, and fluorescence measurements. Micellar and surface thermodynamics data were obtained from the temperature dependence of critical micelle concentrations in various aqueous mixtures of ethylene glycol. In order to evaluate the influence of the cosolvent, the differences in the Gibbs energies of micellization of Tween 20 between water and binary solvents were determined. This study allowed us to conclude that the ability of ethylene glycol to act as a structure breaker and its interaction with the surfactant hydrophilic group are the controlling factors of the micellization process. From the evaluation of the thermodynamics of adsorption at the solution-air interface, it was determined that the surface activity of the surfactant decreases slightly with increasing concentration of ethylene glycol at a given temperature. Partial specific volume data, obtained by density measurements, indicate that the fraction of solvent molecules interacting with the micelle, via hydrogen bonds, remained roughly constant. The effect of cosolvent on the size and solvation of the aggregates was analysed by means of static and dynamic light scattering measurements. It was found that the aggregation number decreased, whereas the whole micellar solvation increased with the ethylene glycol content. Micellar micropolarity was examined using two different probes, pyrene and 8-anilinonaphthelene-1-sulfonic acid, and was found to increase with ethylene glycol addition, accompanied by an enhanced solvation. Fluorescence polarization measurements found by using coumarin 6 as a hydrophobic probe revealed an increase in the micellar microviscosity. The observed trends in these microenvironmental properties were ascribed to a participation by ethylene glycol in the micellar solvation layer.     

Kinetic investigation of the reactions of S-4-nitrophenyl 4-substituted thiobenzoates with secondary alicyclic amines in aqueous ethanol

The reactions of S-4-nitrophenyl 4-X-substituted thiobenzoates (X = H, Cl, and NO(2): 1, 2, and 3, respectively) with a series of secondary alicyclic amines (SAA) were subjected to a kinetic investigation in 44 wt % ethanol-water, at 25.0 degrees C and an ionic strength of 0.2 M (KCl). The reactions were followed spectrophotometrically by monitoring the release of 4-nitrobenzenethiolate anion at 420-425 nm. Under excess amine, pseudo-first-order rate constants (k(obsd)) are obtained for all reactions. The plots of k(obsd) vs [SAA] at constant pH are linear with the slope (k(N)) independent of pH. The statistically corrected Br?nsted-type plots (log k(N)/q vs pK(a) + log p/q) for the reactions of 1 and 2 are nonlinear with slopes at high pK(a), beta(1) = 0.27 and 0.10, respectively, and slopes at low pK(a), beta(2) = 0.86 and 0.84, respectively. The Br?nsted curvature is centered at pK(a) (pK(a)(0)) 10.0 and 10.4, respectively. The reactions of SAA with 3 exhibit a linear Br?nsted-type plot of slope 0.81. These results are consistent with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T(+/-)). For the reactions of 1 and 2, there is a change in rate-determining step with amine basicity, from T(+/-) breakdown to products at low pK(a), to T(+/-) formation at high pK(a). For the reactions of 3, breakdown to products of T(+/-) is rate limiting for all the SAA series (pK(a)(0) > 11). The increasing pK(a)(0) value as the substituent in the acyl group becomes more electron withdrawing is attributed to an increasing nucleofugality of SAA from T(+/-). The greater pK(a)(0) value for the reactions of SAA with 1, relative to that found in the pyridinolysis of 2,4-dinitrophenyl benzoate (pK(a)(0) = 9.5), is explained by the greater nucleofugality from T(+/-) of the former amines, compared to isobasic pyridines, and the greater leaving ability from T(+/-) of 2,4-dinitrophenoxide relative to 4-nitrobenzenethiolate.     

Kinetics and mechanism of the aminolysis of O-aryl S-methyl thiocarbonates

[reaction: see text] The reactions of secondary alicyclic (SA) amines and quinuclidines (QUI) with 4-nitrophenyl and 2,4-dinitrophenyl S-methyl thiocarbonates (1 and 2, respectively) and those of SA amines with 2,3,4,5,6-pentafluorophenyl S-methyl thiocarbonate (3) are subjected to a kinetic study in aqueous solution, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). The reactions of thiocarbonates 1, 2, and 3 were followed spectrophotometrically at 400, 360, and 220 nm, respectively. Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs amine concentration at constant pH are linear, with the slope (kN) independent of pH. The Br?nsted-type plots (log kN vs pKa of aminium ions) are linear for all the reactions, with slopes beta = 0.9 for those of 1 with SA amines and QUI, beta = 0.36 and 0.57 for the reactions of 2 with SA amines and QUI, respectively, and beta = 0.39 for the reactions of SA amines with 3. The magnitude of the slopes indicates that both aminolyses of 1 are governed by stepwise mechanisms, through a zwitterionic tetrahedral intermediate (T+/-), where expulsion of the nucleofuge from T+/- is the rate-determining step. The values of the Br?nsted slopes found for the aminolyses of thiocarbonates 2 and 3 suggest that these reactions are concerted. By comparison of the reactions under investigation between them and with similar aminolyses, the following conclusions arise: (i) Thiocarbonate 2 is more reactive than 1 toward the two amine series. (ii) The change of the nonleaving group from MeO in 4-nitrophenyl methyl carbonate to MeS in thiocarbonate 1 results in lower kN values. (iii) The greater reactivity of this carbonate than thiocarbonate 1 is attributed to steric hindrance of the MeS group, compared to MeO toward amine attack. (iv) The change of a pyridine to an isobasic SA amine or QUI destabilizes the T+/- intermediate formed in the aminolyses of 2. (v) The change of 4-nitrophenoxy to 2,3,4,5,6-pentafluorphenoxy or 2,4-dinitrophenoxy as the leaving group destabilizes the tetrahedral intermediate formed in the reactions with SA amines, changing the mechanism from a stepwise process to a concerted reaction.     

Kinetics and mechanism of the reactions of S-2,4-dinitrophenyl 4-substituted thiobenzoates with secondary alicyclic amines

[reaction: see text] The title reactions, in 44 wt % ethanol-water at 25.0 degrees C, exhibit slightly curved Br?nsted-type plots (log kN versus pKa of amines) with slopes beta1 = 0.1-0.44 (at high pKa) and beta2 ca. 0.7 (at low pKa). The magnitude of some of these slopes, together with the fact that the curvature center (pKa(0) = 9.5-10.8) does not change with the electronic effects of the benzoyl substituent, suggests that these reactions are not stepwise, but concerted.     

Analysis of endosulfan isomers and endosulfan sulfate in air and tomato leaves by gas chromatography with electron-capture detection and confirmation by gas chromatography-mass spectrometry

Rapid analytical methods for the determination of endosulfan isomers and endosulfan-sulfate in air and plant samples were developed. The insecticides were trapped from air using a column containing Florisil and extracted with a low volume of ethyl acetate, assisted by sonication. Pesticide residues were determined by gas chromatography with electron-capture detection using a nonpolar capillary column. Residue identities were confirmed by gas chromatography coupled with mass spectrometry. Recoveries of these compounds from air samples were always higher than 78% with an RSD lower than 11% and the detection limits obtained were at least 0.3 ng/l air. Leaf samples were homogenised with ethyl acetate and extracts cleaned-up on an aluminium oxide column. Pesticides were eluted with a hexane-ethyl acetate (80:20, v/v) mixture. Recoveries obtained from plant samples were higher than 78% with an RSD lower than 14% and detection limits in leaves were 0.02 microg/g for each pesticide. These methods were applied to study the volatilisation of endosulfan from tomato leaves under laboratory conditions. A volatilisation rate near 1% of the initial amount of endosulfan per hour was obtained during the first 24 h at room temperature.     

(Z)-3-p-tolylsulfinylacrylonitrile as a chiral dienophile: diels-alder reactions with furan and acyclic dienes

The behavior of (Z)-3-p-tolylsulfinylacrylonitrile (1) as a chiral dienophile has been evaluated from its reactions with furan and acyclic dienes. Electrostatic interactions of the cyano group with the sulfinyl one restrict the conformational mobility around the C-S bond, thus controlling the pi-facial selectivity, which is almost complete in all cases, the approach of the diene from the less-hindered face of the dienophile (that bearing the lone electron pair) in the predominant rotamer being the favored one. The regioselectivity is also completely controlled by the cyano group. Additionally, the reactivity of compound 1 as well as its endo-selectivity are both higher than those observed for the corresponding (Z)-3-sulfinylacrylates, thus proving the potential of sulfinylnitriles as chiral dienophiles.     

Individual and simultaneous enzymatic determination of ethanol and acetaldehyde in wines by flow injection analysis

The individual and simultaneous enzymatic determination of ethanol and acetaldehyde in wine by flow injection analysis is described. Individual determinations of 0.002–0.016% (v/v) ethanol or 1.0–8.0 μg ml^?1 acetaldehyde with r.s.d. 0.7% and 0.5%, respectively, are done with a single-beam spectrophotometer, based on the use of alcohol dehydrogenase and aldehyde dehydrogenase. A diode-array detector and dual reagent injections are used for the simultaneous determination of the two compounds. The errors are 〈 3.5% and 〈 2.0% for ethanol and acetaldehyde, respectively, when the method is applied to wine samples.     

A pyrazine bis‐adduct of a binuclear rhodium(II) carboxylate containing 3,4,5‐triethoxybenzoate as the ­equatorial ligand

The title compound, tetrakis(μ‐3,4,5‐triethoxy­benzoato‐κ²O:O′)­bis­[(pyrazine‐κN)­rhodium(II)](Rh—Rh), [Rh₂(C₁₃H₁₇O₅)₄(C₄H₄N₂)₂], crystallizes on an inversion centre in the triclinic space group . The equatorial carboxyl­ate ligands bridge the two Rh^II atoms, giving a binuclear lantern‐like structure. The pyrazine mol­ecules occupy the two axial coordination sites. The phenyl rings are tilted by ca 10° with respect to the attached carboxyl­ate groups. The pyrazine planes have a torsion angle of ca 19° around the Rh—N bond with respect to the plane of the nearer carboxyl­ate group and are not coplanar with the Rh—Rh bond.