Recherches sur la structure électronique de composés soufrés (méthode L.C.A.O. améliorée)

Improved L.C.A.O. method primitively built for conjugated molecules holding only atoms of second row in the periodic classification is extended to molecules holding atoms of the third row. The application is done for thiophen, thioketones, SO₂ and SO₃. d orbitals are not introduced in this study. Discussion about results shows that d orbitals must be introduced when the sulphur atom is strongly positively charged like in sulphur oxides.     

High-performance liquid chromatographic determination of tocopherols in infant formulas

A method for the simultaneous determination of alpha-tocopherol acetate and alpha-, delta-, and gamma-tocopherols by normal-phase high-performance liquid chromatography (HPLC) with a fluorescent detector in infant formula is proposed. The values obtained in the determination of the analytical parameters: linearity, precision, limit of detection and accuracy (analysis of a standard reference material, SRM 1846), confirm the quality of the method. The proposed method is useful for the determination of alpha-, delta-, and gamma-tocopherols and alpha-tocopherol acetate in infant formulas at a low cost and in a total time of 2 h.     

Tungsten(VI)-gluconic acid complexes: Polarimetric and13C-N.m.r. Studies in an excess of tungsten(VI)

Summary A polarimetric study of the tungsten (VI)-gluconic acid system in an excess of metal reveals the formation of four stable complexes: two monomers with 1 : 2 and 2: 1 stoichiometries and two dimers of 2:2 composition. The pH ranges of these species, the amount of acid equivalents their formation requires, and their conditional stability constants have also been calculated.The probable coordination of the organic ligand to the metallic centre in solutions containing different metal : ligand ratios has been investigated by¹³C-n.m.r. spectroscopy. The results confirm the formation of different complexes depending on the reagent which is in excess. The behaviour of the system when the metal is in excess is related to that of other polyhydroxylic ligands such as mannitol and sorbitol.     

Reactivity of 2-halo-2H-azirines. 1. Reactions with nucleophiles.

Nucleophilic substitution reactions of 2-halo-2H-azirines 1a, 1b, 1d, and 1e with potassium phthalimide and aniline allowed the preparation of new substituted 2H-azirines 2-5. The reactions of 2-bromo-2H-azirine 1a with methylamine led to the synthesis of alpha-diimines 7 and 8. 2-Halo-2H-azirines were also established as building blocks for the synthesis of a range of heterocyclic compounds, namely, quinoxalines 10a-10d, 3-oxazoline 14, and 2H-[1,4]oxazines 18 and 20. X-ray crystal structures of alpha-diimine 7, 3-oxazoline 14, and 2H-[1,4]oxazine 18 are reported.     

Liquid chromatographic determination of diminazene diaceturate (Berenil) in raw bovine milk

A simple liquid chromatographic (LC) method is presented for the determination of diminazene (DZ) in raw bovine milk. DZ is extracted from raw milk by chilled aqueous centrifugation and is isolated from milk components on a cyano solid-phase extraction column. DZ is eluted by using a methanol-ion pairing reagent. A Phenomenex LUNA CN column and an acetonitrile-buffered mobile phase with a counter ion are used for gradient LC. The LC effluent is monitored at a detection wavelength of 372 nm by using a deuterium lamp. Under the parameters described, the retention time of DZ is 8-10 min with a peak area response of 6.5 mAU/ng. The method demonstrated excellent precision over all levels tested (25-400 ppb) with an overall average recovery of 90.4 +/- 14.5%. The method is applicable to the monitoring of milk for DZ residues at the 25 ppb level with a limit of quantitation of 10 ppb.     

Effect of absolute laser phase on reaction paths in laser-induced chemical reactions

Potential surfaces, dipole moments, and polarizabilities are calculated by ab initio methods [unrestricted MP2(full)/6-311++G(2d,2p)] along the reaction paths of the F+CH4 and Cl+CH4 reaction systems. It is found that in general dipole moments and polarizabilities exhibit peaks near the transition state. In the case of X=F these peaks are on the products side and in the case of X=Cl they are on the reactants side indicating an early transition state in the case of fluorine and a late transition state in the case of chlorine. An analysis of the geometric changes along the reaction paths reveals a one-to-one correspondence between the peaks in the electric properties and peaks in the rate of change of certain internal geometric coordinates along the reaction path. Interaction with short infrared intense laser fields pulses leads to the possibility of interferences between the dipole and polarizability laser-molecule interactions as a function of laser phase. The larger dipole moment in the Cl+CH4 reaction can lead to the creation of deep wells (instead of energy barriers) and new strongly bound states in the transition state region. This suggests possible coherent control of the reaction path as a function of the absolute phase of the incident field, by significant modification of the potential surfaces along the reaction path and, in particular, in the transition state region.     

Unique tautomeric properties of isoguanine

The tautomeric properties of isoguanine (also named 2-oxoadenine or 2-hydroxyadenine) have been studied in the gas phase, in different pure solvents, and in the DNA environment using state of the art theoretical methods. Our results show that isoguanine constitutes an unique example of how tautomerism can be modulated by the environment. Compared to the tautomeric preference in the gas phase, both polar solvents and the DNA microenvironment dramatically change the intrinsic tautomeric properties of isoguanine. Tautomers which are important in physiological conditions are less than 1/10(5) of the total population of isoguanine in the gas phase. The impact of the present findings in the understanding of spontaneous mutations and in the design of new nucleobases with multiple recognition properties is discussed.     

Comprehensive study of the methyl effect on the solvolysis rates of bridgehead derivatives

The effect of a bridgehead methyl group on the hydride ion affinity in the gas phase of bicyclo[1.1.1]pent-1-yl (1+), 1-norbornyl (3+), cubyl (5+), 1-adamantyl (7+), bicyclo[2.2.2]oct-1-yl (9+),and bicyclo[3.1.1]hept-1-yl (11+) cations has been studied using density functional theory and ab initio methods. It is concluded that the methyl group always increases the stability of the substituted cations. The effect of the solvent on the stability of methyl-substituted cations in relation to the unsubstituted cations has been studied using the polarizable continuum model of the self-consistent reaction field theory. In the case of rearranging cations, the nucleophilic assistance of the solvent is determined by means of the interaction energy of the corresponding water complexes. It is concluded that the solvent causes the relative stabilization of the parent cations. As a consequence, most of the methyl-substituted bridgehead derivatives show a lower solvolysis rate than the corresponding unsubstituted compounds. A nonqualitative explanation of the methyl effect on the relative stability of bridgehead cations in both gas phase and solution is given for the first time. The ratios of solvolysis products in the case of rearranging bridgehead cations have also been computed from the relative stability of the intermediate water complexes.     

Kinetics and mechanism of the phenolysis of asymmetric diaryl carbonates

The reactions 4-methylphenyl 4-nitrophenyl carbonate (MPNPC), 4-chlorophenyl 4-nitrophenyl carbonate (CIPNPC), 4-methylphenyl 2,4-dinitrophenyl carbonate (MPDNPC), and 4-chlorophenyl 2,4-dinitrophenyl carbonate (CIPDNPC) with a homogeneous series of phenoxide anions are subjected to a kinetic investigation in aqueous solution (25.0 degrees C, ionic strength 0.2 M (KCI)). Under an excess of phenoxide with respect to the substrate, all of these reactions obey pseudo-first-order kinetics and are first order in phenoxide. The Br?nsted-type plots for the nucleophilic rate constants (k(N)) are linear, with slopes beta = 0.48 (MPNPC), 0.67 (ClPNPC), 0.41 (MPDNPC), and 0.32 (ClPDNPC). The magnitude of these slopes and the absence of a curvature in the Br?nsted plot at pK(a) = 7.1 for the CIPNPC reactions are consistent with concerted mechanisms (one step). The carbonates MPDNPC and ClPDNPC are more reactive than MPNPC and CIPNPC, respectively, toward phenoxide nucleophiles. This can be explained by the presence of a second nitro group in the nucleofuge of the dinitro derivatives, which (i) leaves their carbonyl carbon more positively charged, making them better electrophiles, and (ii) makes 2,4-dinitrophenoxide a better leaving group than 4-nitrophenoxide. The 4-chloro derivatives are more reactive than the corresponding 4-methyl derivatives. This should be due to the greater electron withdrawal of 4-chloro than 4-methyl, which makes the former carbonyl more electrophilic. Comparison of the concerted phenolysis of MPNPC with the stepwise reactions of secondary alicyclic amines with the same substrate indicates that substitution of a secondary alicyclic amine group in a zwitterionic tetrahedral intermediate by a phenoxy group greatly destabilizes the intermediate. An equation is deduced for log k(N) in terms of the basicity of the nucleophile, the nonleaving moiety, and the leaving group. This equation shows that for these reactions, the sensitivity of log k(N) to the basicity of the nonleaving moiety (beta(nlg) = -0.27) is very similar to that of the nucleofuge (beta(lg) = -0.25).     

Consequence of boar edible tissue consumption on urinary profiles of nandrolone metabolites. II. Identification and quantification of 19-norsteroids responsible for 19-norandrosterone and 19-noretiocholanolone excretion in human urine

In previous work (Le Bizec et al., Rapid Commun. Mass Spectrom. 2000; 14: 1058), it was demonstrated that a boar meal intake could lead to possible false accusations of abuse of 17beta-nortestosterone in antidoping control. The aim of the present study was to identify and quantify endogenous 19-norsteroids in boar edible tissue at concentrations that can alter the steroid urinary profile in humans, and lead to excretion of 19-norandrosterone (19-NA) and 19-noretiocholanolone (19-NE). The samples were analysed in two laboratories. The methodologies used for extraction and detection (GC/MS(EI) and LC/MS/MS(APCI+)) are compared and discussed. 19-Norandrostenedione (NAED), 17beta- and 17alpha-nortestosterone (bNT, aNT), and 17beta- and 17alpha-testosterone (bT, aT) were quantified. The largest concentrations of NAED and bNT were observed in testicles (83 and 172 microg/kg), liver (17 and 63 microg/kg) and kidney (45 and 38 microg/kg). A correlation between the bNT and NAED content of a typical meal prepared with boar parts and the excreted concentrations of 19-NA and 19-NE in human urine was demonstrated.