Dioxouranium(VI)--carboxylate complexes. Interaction with dicarboxylic acids in aqueous solution: speciation and structure

In this paper we report the results of an investigation performed by potentiometric (H+-glass electrode) and visible spectrophotometric measurements on the interaction of UO2(2+) ion towards some carboxylic ligands (acetate, malonate, succinate, azelate). The measurements were carried out at T= 25 degrees C in different ionic media (KNO3 and NaCl) at different ionic strengths (0.1 < or = I/mol L(-1) < or = 1.0, NaCl; I/mol L(-1) = 0.1, KNO3). The dependence on ionic strength of formation constants was taken into account by using both a simple Debye-Hückel type equation and the SIT (Specific ion Interaction Theory) approach. Different speciation models (depending on concentration of reagents, ionic strength, pH-range) both for different carboxylates and different ionic media have been obtained. Linear combinations between formation constants, stoichiometric coefficients and length of alkyl chain of dicarboxylates have been observed and predicted formation constants at I= 0 mol L(-1) are reported for the interaction of UO2(2+) with HOOC-(CH2)n-COOH with 1 < or = n < or = 7. Finally, a visible absorption spectrum for each complex reaching a significant percentage of formation in solution (KNO3 medium) has been calculated to characterise the compounds found by pH-metric refinement.     

Synthesis of 2,3-dihydro-1,4-dithiinyl nucleosides via Pummerer-type glycosidation

A straightforward procedure for the preparation of nucleoside analogue 1 and its regioisomer 2 containing a dihydro-1,4-dithiin as sugar moiety has been accomplished in four steps by our readily available heterocyclic system 5. Nucleobase insertion was carried out by direct addition of N⁴-acetylcytosine to sulfoxide derivatives via Pummerer-type glycosidation reaction.     

Determination of epichlorohydrin by sulfite derivatization and ion chromatography: characterization of the sulfite derivatives by ion chromatography-mass spectrometry

This work is an upgrade of a previously developed method (J. Chromatogr. A 884 (2000) 251] for epichlorohydrin determination by ion chromatography (IC) and conductivity detection. Here, an ion chromatography-mass spectrometry (IC-MS) coupling has been employed for the separation and the identification of products of epichlorohydrin when reacted with the nucleophilic agent SO3(2-). The high capacity column (IonPac AS11-HC) used for separation provided good resolution. This allowed evaluation of the IC behavior and mass spectrometric identification of epichlorohydrin sulfite derivatives. By using atmospheric pressure interfaces (ESI and APCI) the following species were tentatively identified: 2,3-dihydroxy-1-propanesulfonic, 2,3-epoxy-1-propanesulfonic,1,3-dihydroxy-2-propanesulfonic and 3-oxetanesulfonic acids and 2-hydroxy-1,3-propanedisulfonic acid (or its isomer 3-hydroxy-1,2-propanedisulfonic acid). The study showed that chlorine atoms are displaced from epichlorohydrin during the reaction, while mass spectrometry confirmed that none of the products formed contains chlorine atoms.     

A Convenient Route to 5,6-Diphenyl-5,6-dihydro-uracils

Diphenylcyclopropenone(1) reacts with a variety of thiopseudoureas to eventually afford dihydrouracils     

Thermodynamics of Proton Association of Polyacrylates and Polymethacrylates in NaCl(aq)

Enthalpies of protonation of polyacrylates and polymethacrylates with different molecular weights in aqueous NaCl solutions, 0 I 2 mol-L^–1 were determined by titration calorimetry at 25°C. H values are dependent on both the neutralization degree, , and the molecular weight of polyacids. T S of protonation was obtained using pK values already reported and the present H results. Empirical equations for the dependence on I, , and molecular weight are reported for both H and T S.     

Barbituric and thiobarbituric acids: a conformational and spectroscopic study

A conformational study on Barbituric (BA) and Thiobarbituric (TBA) acids was performed at ab initio MP2/6-31G** level on the neutral, protonated, mono- and di-anionic forms. Acid-base equilibria were studied by comparing the electronic transitions evaluated for the most stable conformations and the experimental spectra at different pH values. The electronic transitions were obtained through the ZINDO approach.     

Synthesis of imidazo[5,1-b]thiazoles or spiro-β-lactams by reaction of imines with mesoionic compounds or ketenes generated from N-acyl-thiazolidine-2-carboxylic acids

New mesoionic compounds (2H, 3H-thiazolo[3,2-c]oxazol-7-ones) (β) or ketenes ((3-acyl-1,3-thiazolidin-2-ylidene)methanone) (β′) were generated from N-acetyl and N-benzoyl-thiazolidine-2-carboxylic acids (7a,b) using different methods, and their reactivity towards N-(phenylmethylene)benzenesulfonamide (2) and N-(phenylmethylene)aniline (3) was tested. When (7a,b) were treated with (2) and acetic anhydride in refluxing toluene solution, only imidazo[5,1-b]thiazoles (8a,b) were obtained from the mesoionic compound intermediates (β). When the ketene intermediates (β′) were generated from (7a,b) by means of Mukaiyama's reagent, only spiro-β-lactams (9a,b) were isolated.     

Salt effects on the protonation of imidazole in aqueous solution at different ionic strengths: A tentative explanation by a complex formation model

Imidazole protonation constants were determined potentiometrically, using a (H⁺)-glass electrode, in LiCl, NaCl, KCl, CaCl₂, MgCl₂, tetramethylammonium (Me₄N-) iodide and chloride, Et₄N-, Pr₄N- and Bu₄N-iodides. Salt effects were tentatively explained by assuming that complexes [H(Im)X]^o (Im = imidazole, X^–=Cl^– or I^–), [M(Im)]²⁺ (M²⁺=Mg²⁺ or Ca²⁺) and [A(Im)]⁺ (A⁺ = tetraalkylammonium cation) were formed in solution. Calcium(II)-selective electrode measurements confirmed our hypothesis about the formation of the Ca²⁺-imidazole complex. The reliability of the complex formation model is discussed on the basis of its self consistency and in light of previous results.     

A New Alkaliphilic Cold-Active Esterase from the Psychrophilic Marine Bacterium Rhodococcus sp.: Functional and Structural Studies and Biotechnological Potential

The special features of cold-adapted lipolytic biocatalysts have made their use possible in several industrial applications. In fact, cold-active enzymes are known to be able to catalyze reactions at low temperatures, avoiding side reactions taking place at higher temperatures and preserving the integrity of products. A lipolytic gene was isolated from the Arctic marine bacterium Rhodococcus sp. AW25M09 and expressed in Escherichia coli as inclusion bodies. The recombinant enzyme (hereafter called RhLip) showed interesting cold-active esterase activity. The refolded purified enzyme displayed optimal activity at 30 °C and was cold-active with retention of 50 % activity at 10 °C. It is worth noting that the optimal pH was 11, and the low relative activity below pH 10 revealed that RhLip was an alkaliphilic esterase. The enzyme was active toward short-chain p-nitrophenyl esters (C2–C6), displaying optimal activity with the butyrate (C4) ester. In addition, the enzyme revealed a good organic solvent and salt tolerance. These features make this an interesting enzyme for exploitation in some industrial applications.     

Interaction of Alkyltin(IV) Compounds with Ligands of Interest in the Speciation of Natural fluids: Complexes of (CH3)2Sn2+ with Carboxylates

Complex formation by (CH₃)₂Sn²⁺ with acetate (ac), malonate (mal), 1,2,3-propane-tricarboxylate (tricarballylate, tca) and 1,2,3,4-butanetetracarboxylate (btc) ligands in aqueous solution is reported. The study has been performed by potentiometry ([H⁺]–glass electrode) at T=25 °C, and in the 0⩽I⩽1 mol dm⁻³ ionic strength range. In order to evaluate the salt effects on the formation constants of the complex species, and to contribute to the definition of the chemical speciation of diorganotin(IV) compounds in natural waters where carboxylic ligands are naturally present, interactions of NaCl (which is the major component of all natural fluids), with the components of the systems under investigation have also been considered. A model for the ionic strength dependence of the formation constant was used in order to calculate the values of thermodynamic constants, at infinite dilution, for all the proposed species, by considering hydrolytic species too. A relationship between the charges on the different ligands considered and the stabilities of the complex species formed is also discussed. © 1997 John Wiley & Sons, Ltd.