Enantioselective Synthesis of α-Thiocarboxylic Acids by Nitrilase Biocatalysed Dynamic Kinetic Resolution of α-Thionitriles

The enantioselective synthesis of α-thiocarboxylic acids by biocatalytic dynamic kinetic resolution (DKR) of nitrile precursors exploiting nitrilase enzymes is described. A panel of 35 nitrilase biocatalysts were screened and enzymes Nit27 and Nit34 were found to catalyse the DKR of racemic α-thionitriles under mild conditions, affording the corresponding carboxylic acids with high conversions and good-to-excellent ee. The ammonia produced in situ during the biocatalytic transformation favours the racemization of the nitrile enantiomers and, in turn, the DKR without the need of any external additive base.     

Spectroscopic investigation of Rose Bengal/cyclodextrin interactions in aqueous solution: the case of the hydroxypropyl-cyclodextrins

The interaction of Rose Bengal (RB) with hydroxypropyl-α-cyclodextrin (HP-α-CD), hydroxypropyl-β-cyclodextrin (HP-β-CD) and hydroxypropyl-γ-cyclodextrin (HP-γ-CD) has been studied in water and in acetate buffer at pH 4.5 by UV–Vis absorption, fluorescence spectroscopy and Induced Circular Dichroism at 298 K. Evidence of the complex formation between the RB and all HP-CDs have been obtained both in water and in buffer. Binding constants and stoichiometry of RB/HP-CD complexes in water have been determined by applying the modified Benesi-Hildebrand equation to the fluorescence measurements.     

The effects of increasing NaCl concentration on the stability of inclusion complexes in aqueous solution

Equilibrium constants and standard molar enthalpies of reaction were determined by titration calorimetry for the reaction of 1-butanol with 2-hydroxypropyl-b-cyclodextrin (HP-b-CD) in aqueous solution at different concentrations of NaCl (0-1.9 M). The standard molar free energy and entropy changes associated to the complexation were calculated from the corresponding equilibrium constants, K, and standard enthalpies determined experimentally. In NaCl solutions the inclusion complexes ButOH/HP-b-CD are more stable than in water and their stability increases at increasing NaCl concentration; otherwise, the standard molar enthalpy associated to the formation of the complexes does not change with the increasing of salt concentration. The dependence of K on NaCl concentration were used to evaluate the number of water molecules displaced from the hydration shells of HP-b-CD and ButOH in forming complexes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Direct kinetic formation of nonanomeric [6.5]-spiroketals in aqueous media

The direct kinetic formation of spiroketals from mixed ketal-alcohol precursors under acid catalysis was studied using four differently substituted systems. In all cases, the exclusive formation of the anomeric isomer was observed under equilibrating conditions. However, the formation of the nonanomeric spiroketal isomer was observed if the reaction was performed under kinetic conditions using an appropriately tuned acid. Water had a dramatic accelerating effect on the spiroketalization reactions that were performed in THF, and the highest yields of the nonanomeric products were obtained in aqueous THF. The nonanomeric/anomeric product ratio was also strongly affected by the substituents and the stereochemistry of the starting alcohol.     

Transfer of coenzyme Q0 from water to aqueous surfactant solutions: the case of Triton X-100

The effect of UQ0 on the micellization equilibrium of Triton X-100 has been studied by the analysis of the UV absorption spectra of Triton X-100. In the range of the UQ0 concentration investigated, the critical micelle concentration (CMC) increases at increasing of the solute concentration. The dependence of the CMC on UQ0 concentration has been used to calculate the generalized Setchenov constant. Mixing and dilution enthalpies of aqueous solutions of UQ0 and Triton X-100 were measured and used to calculate the enthalpies of transfer of UQ0 from water to Triton X-100 aqueous solutions. From the dependence of the enthalpy of transfer on surfactant concentration, the distribution constant between aqueous and micellar phase and the standard enthalpy of transfer from water to Triton X-100 micelles were evaluated along with the standard transfer free energy and entropy. All measurements were carried out at 298 K.     

Interactions between surfactant capped CdS nanocrystals and organic solvent

The dispersibility of organic capped nanocrystals (NCs) in diverse solvents is one of the key factor of the success of such a class of nanostructured materials. In this work the α-cyclodextrins, mediated phase transfer has been considered as an effective procedure to direct the NC transfer from the organic solvent to water. The effect of the original organic solvent, namely hexane and chloroform, and of the ligand molecule coordinating CdS nanocrystal surface has been investigated by optical (UV-Vis, Photoluminescence, FTIR, Dynamic Light Scattering) and calorimetric techniques. The calorimetric investigation has been carried out by performing dilution experiments and the correlation between thermal effects and dilution ratio has been evaluated using the McMillan approach. The obtained results have provided relevant insight on the parameters driving the phase transfer process and on the NC mutual interaction, thus resulting valuable on the effectiveness of the phase transfer procedure.     

Solute-Solvent interactions in water-rich mixtures. II. Ionic conductances in water-dimethylsulfoxide mixtures at 25°C

Conductances of sodium bromide, iodide, and perchlorate, potassium chloride, and tetraphenylboride (BPh ₄ ^– ) as well as triisoamyl-n-butylammonium iodide (i-Am₃BuNI) have been measured in aqueous mixtures containing up to 20 mole percent dimethylsulfoxide (DMSO) at 25°C. Experimental data were analyzed by the 1965 Fuoss-Onsager-Skinner (FOS) equation. Single-ion limiting equivalent conductances were calculated by assuming that ₀ (i–Am ₃ BuN ⁺=₀ (BPh ₄ ^– ). The variations of the limiting ionic Walden products are discussed on the basis of acid-base type interactions for cations, and on the basis of structural effects for anions.     

Enthalpies of solution and dilution of butanol and pentanol in dodecyltrimethylammonium bromide micellar solutions

The enthalpies of solution and of dilution of 1-butanol and 1-pentanol were measured in micellar solutions of dodecyltrimethylammonium bromide by systematically changing the concentration of alcohols and surfactant. The enthalpies of solution at infinite dilution of alcohols at each surfactant concentration were evaluated from a linear plot. This quantity increases with surfactant concentration (up to 0.8m) with a curvature which depends on the alcohol alkyl chain length. The difficulties arising for a quantitative treatment of both the enthalpies of dilution and of solution at finite alcohol concentrations are discussed. The dependence on the surfactant concentration of the standard enthalpies of solution and the enthalpies of dilution for m0 are rationalized. From the resulting equations the distribution constant, standard enthalpy of transfer, standard enthalpy of solution, and the alcohol-alcohol interaction parameter in the micellar phase are evaluated. The enthalpies of transfer obtained using this technique agree well with those previously reported from enthalpies of mixing. The distribution constants also agree with those reported in the literature from several approaches: mixing enthalpies, partial molar volumes, and the dependence of the cmc on added alcohol.     

Mass action model for solute distribution between water and micelles. Partial molar volumes of butanol and pentanol in dodecyl surfactant solutions

The densities of 1-butanol and 1-pentanol were measured in aqueous solutions of dodecyltrimethylammonium bromide and dodecyldimethylamine oxide and the partial molar volumes at infinite dilution of the alcohols in aqueous surfactants solutions were obtained. The observed trends of this quantity as a function of the surfactant concentration were rationalized using a mass-action model for the alcohol distribution between the aqueous and the micellar phase. At the same time, the model was revised to account for the alcohol effect on the surfactant micellization equilibrium. The partial molar volume of alcohols in the aqueous and in the micellar phases and the ratios between the binding constant and the aggregation number were calculated. These thermodynamic quantities are nearly the same in the two surfactants analyzed in this paper but differ appreciably from those in sodium dodecylsulfate. The apparent molar volume of surfactants in some hydroalcoholic solutions at fixed alcohol concentration were also calculated. In the micellization region the trend of this quantity as a function of the surfactant concentration shows a hump, which depends on the alcohol concentration and on the alcohol alkyl chain length. The alcohol extraction from the aqueous to the micellar phase due to the addition of the surfactant can account for the observed trends.     

Standard partial molar volumes of alcohols in aqueous dodecyltrimethylammonium bromide solutions

Density measurements of water-dodecyltrimethylammonium bromide (DTAB)-alcohol ternary systems as a function of alcohol and surfactant concentrations were carried out at 25°C. The alcohols were propanol (PrOH), 2-propanol (2-PrOH) and hexanol (HexOH). The apparent molar volume V_,R of alcohols have been calculated and the standard (infinite dilution) partial molar volumes of alcohols V _R at each surfactant concentration were obtained by means of a least squares fit of V_,R vs. the alcohol concentration. The V _R vs. surfactant concentration curves have been rationalized in terms of the partial molar volume of alcohol in the aqueous V _f and the micellar V _b phases and the distribution constant of alcohol between the aqueous and the micellar phases K. The V _b values for PrOH and HexOH together with those of butanol and pentanol previously reported satisfy the additivity rule giving a methylene group contribution of 16.7 cm³-mol^–1 which is identical to that reported in the literature from the study of pure liquid alcohols. No difference between V _b for PrOH and 2-PrOH has been found. From density data of water-alcohol and water-surfactant binary systems and of water-surfactant-alcohol ternary system, the apparent molar volume of the surfactant in the water-alcohol mixed solvent V_,S have been calculated as a function of the surfactant concentration and of the mixed solvent composition. The effect of the alkyl chain length of the alcohols and the effect of isomerization of the alcohols on the V_,S vs. surfactant concentration trends have been analyzed.