Aqueous Organic Media Research Articles

Speciation of organotin compounds by capillary electrophoresis: comparison of aqueous and mixed organic-aqueous systems.

A capillary electrophoresis method with direct ultraviolet detection was developed for the analysis of organotin species. Despite the fact that direct detection of organotin compounds by ultraviolet absorption is difficult because most organotins possess poor chromophoric properties, the application of low wavelength (lambda = 200 nm) and mixed organic-aqueous media enabled a significant enhancement in sensitivity. A mixed organic-aqueous system (10% methanol/40% acetonitrile/50% H2O) containing acetic acid and tetrabutylammonium perchlorate formed the basis for rapid, efficient and sensitive determinations of organotin cations such as tripropyltin, tributyltin, triphenyltin and diphenyltin. The concentration limits of detection (LOD) for the four organotin compounds were in the range of 0.4-14 microM, comparable to that obtained with the most sensitive indirect UV method reported until now, and took advantage of a stable baseline, a symmetric peak shape and an absence of disturbing system peaks. The relative standard deviations (n = 7) for the relative peak time and peak area were 0.44-0.77 and 4.8-5.8%, respectively. In addition to sensitivity enhancements, the use of organic-aqueous systems instead of pure aqueous media resulted in improved selectivity and efficiency of separations.

Substrate desolvation as a governing factor in enzymatic transformations of PAHs in aqueous-acetonitrile mixtures.

Conversion of hydrophobic substrates such as polyaromatic hydrocarbons (PAHs) was studied in aqueous-organic media using transformation of pyrene by cytochrome c. The experiments were conducted in pure solvents and aqueous-acetonitrile mixtures. The reaction rates dropped gradually as the solvent concentration was increased and were negligible in pure solvents. Thermodynamic calculations show that substrate desolvation was the governing factor in this reaction. The decrease in free energy of activation upon increasing the solvent concentration was found to be proportional to the substrate transfer free energy. This suggests that in order to make such reactions feasible in organic media, it will be necessary to improve the binding between the proteins and the substrates.

Determination of Hydroxamic Acids by Thermal-Lens Spectrometry

Conditions are determined for the determination of salicylhydroxamic and acetylhydroxamic acids by thermal-lens spectrometry using iron(III) in an aqueous medium. The detection limits are 6 × 10–8 and 3 × 10–8 M, respectively. The sensitivity can be enhanced 3–6 times with the use of aqueous–organic media.

Synthesis of new dinuclear and mononuclear peroxovanadium(V) complexes containing biogenic co-ligands: a comparative study of some of their properties

Dinuclear peroxo complexes of vanadium, [V 2O 2(O 2) 3(asn) 3] · H 2O ( 1) and [V 2O 2(O 2) 3(gln) 3] · H 2O ( 2) have been synthesized from the reaction of V 2O 5 with H 2O 2 and the respective amino acid ligand at pH ca. 2. Similar reactions conducted at pH ca. 5 afforded the monomeric complexes, Na[VO(O 2) 2(asn)] · H 2O ( 3) and Na[VO(O 2) 2(gln)] · H 2O ( 4). The compounds were characterized by elemental analysis and spectral studies. In complexes 1 and 2, the two V(V) centres are bridged by a peroxo group and an amino acid ligand occurring as a zwitterion. The monomeric complexes 3 and 4 contain peroxo groups bonded in a side-on fashion and an amino acid co-ligand binding the V(V) centre through O (carboxylate) atoms. The complexes 1 and 2 rapidly degraded in solution with release of O 2 and formation of diperoxovanadate and decavanadate as shown by 51V NMR spectra whereas complexes 3 and 4 remained stable in solution for over 24 h. Extent and rate of oxygen released from the two types of complexes under the effect of catalase action further evidenced the differences in their V:O 2 2− content and mode of peroxide binding in these species. The μ-peroxovanadate complexes 1 and 2 instantaneously oxidized bromide to a bromination competent intermediate in phosphate buffer at physiological pH, and also efficiently mediated bromination of organic substrates in aqueous-organic media. Complexes 3 and 4 were inactive for bromination under analogous conditions. These findings make the dinuclear complexes 1 and 2 possible candidates of mimic in the action of vanadium in bromoperoxidase.

Rapid automated assay of anti-oxidation/radical-scavenging activity of natural substances by sequential injection technique (SIA) using spectrophotometric detection.

A PC-controlled sequential injection analysis (SIA) system equipped with a spectrophotometric diode-array detector is used for rapid monitoring and evaluation of antioxidation/radical scavenging activity of biological samples. The automated method is based on the known reaction of stable 2,2'-diphenyl-1-picrylhydrazyl radical (DPPH) with antioxidants in organic or aqueous-organic media resulting in bleaching of DPPH due to its "quenching" by the interaction with the analytes. The decrease of the absorbance of DPPH (compared to blank experiment carried out with water-ethanol 1:1 instead of the test solution) measured at 525 nm is related to concentration of an antioxidant in the test solution. With the optimised SIA procedure it is possible to detect down to micromolar concentrations of model antioxidants such as ascorbic acid, caffeic acid, (+)-catechin, (-)-epicatechin and rutin and to evaluate the concentration of these antioxidants in the micromolar to millimolar range. The sample throughput is 45 h(-1). Thanks to its rapidity and sensitivity, the proposed SIA method is suitable for performing routine screening tests for the presence of various antioxidants in large series of lyophilised herbal or mushroom extracts (the amount of sample needed for the analysis is several milligrams).

Enzymatic Synthesis of N‐Acetyllactosamine in Aqueous‐Organic Reaction Media.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Reagent generation for chemical analysis assisted by ultrasonic irradiation

Few methods, such as coulometrics, have been developed to produce reagents in situ for analytical purposes. In this work the concept related to the generation of ultrasound-assisted reagents was exploited to yield oxidizing species in batch and flow systems of analysis. To evaluate the efficiency of ultrasound-assisted reagent generation, the conversion of Fe 2+ to Fe 3+ associated with the 1,10- o-phenantroline spectrophotometric method was tested to compare the oxidizing power of the produced species from pure water and aqueous solutions saturated with CCl 4 or CHCl 3 irradiated ultrasonic waves. Irradiation processes were conducted with an ultrasonic bath (40 kHz and 140 W). The borosilicate reactor was used in the batch studies, while the PTFE tube reactor was used for setting up the flow system, with the temperature during irradiation being controlled using a thermostatic bath. The sonochemical production of oxidizing agents was demonstrated to be efficient for chemical analysis in batch and flow systems. This technique was exemplified by oxidation of iodide and ferrous ions. It was observed that after 120 min of sonication approximately 40 μg of Fe 2+ was quantitatively oxidized to Fe 3+. Similar result was obtained by the irradiation of iodine in aqueous–organic medium.

A convenient route to 1,4-monoprotected dialdehydes, 1,4-ketoaldehydes, γ-lactols and γ-lactones through radical alkylation of α,β-unsaturated aldehydes in organic and organic-aqueous media

α,β-Unsaturated aldehydes were smoothly alkylated by radicals generated through photosensitised hydrogen abstraction of benzophenone. In this way, and by using 1,3-dioxolane as radical precursor, monoprotected 1,4-dialdehydes were obtained from crotonaldehyde, 2-hexenal, 4-methyl-2-pentenal and cyclohexencarboxyaldehyde in a moderate yield, and in a low yield from β-aryl-α,β-unsaturated aldehydes. With 2-alkyl-1,3-dioxolanes, monoprotected 1,4-ketoaldehydes were analogously prepared. By using methanol, ethanol and isopropanol as radical precursors γ-lactols were likewise obtained from the above aliphatic aldehydes. These single-step syntheses compared favorably with multi-step approaches previously proposed for some of these compounds. The lactols were conveniently oxidized to the corresponding γ-lactones. An alternative to the photosensitisation in organic medium was the use of mixed aqueous-organic solvent and a hydrosoluble photosensitiser (benzophenone disodium disulfonate was prepared for this purpose and successfully used), which allowed a more convenient work up.

Kinetics of Zinc and Mercury Oxidation by Iodine in Organic and Aqueous-Organic Media

The kinetics of the heterogeneous oxidation of zinc and mercury by iodine in organic and aqueous-organic media is experimentally studied. It is shown that the oxidation takes place in steps under the diffusion control.

Effect of phase composition on the whole-cell bioconversion of β-sitosterol in biphasic media

The multi-step bioconversion of β-sitosterol by Mycobacterium sp. NRRL B-3805 for the formation of androstenedione (AD) as major product, a key intermediate in the production of therapeutic steroids, is a well established industrial application of a biocatalytic process. In the present work, this sterol side-chain cleavage was selected as model system for a study of whole-cell biocatalyst operational stability in prolonged bioconversions in two-phase aqueous-organic media. The aim was to evaluate the effect of phase composition in the process yields by using four high logPoctanol phthalates as organic phases and phosphate buffer and nutritional media as aqueous phases. The effect of a non-ionic surfactant added to the aqueous phase was also studied. Results show no relation between bioconversion yields and solvent logPoctanol values, suggesting that, in the used range, solvent molecular structures and the resulting specific interactions with the cell envelope are determining for the biocatalyst behaviour. Different responses were obtained from the use of different aqueous media, the complex nutritional media leading to the highest conversion yields, while the lowest were observed with the mineral nutritional media. The effect of surfactant addition in the bioconversion yields was apparently not significant. In the systems with bis-(2-ethylhexyl) and bis-(3,5,5-trimethylhexyl) phthalates as the organic phases, the fed-batch operational mode allowed the maintenance of stable biocatalytic activity levels up to at least 6 days of operation, with high androstenedione yields.

Stabilization of horseradish peroxidase in aqueous-organic media by immobilization onto cellulose using a cellulose-binding-domain

A fused protein consisting of a cellulose-binding domain (CBD) and horseradish peroxidase (HRP) was bound to cellulose beads and evaluated in aqueous-organic solvent systems. The CBD–HRP fusion protein containing two different functionalities, a catalytic domain and a binding domain, preserved both capabilities in this non-conventional environment. A six-fold increase in the half-life of the enzyme in buffer resulted from immobilization onto cellulose via CBD. The immobilized enzyme was also more stable than the native enzyme in increasing concentrations of acetone (0–92%). There was a general decrease in activity as the solvent concentration in the mixture increased (in all solvent types: THF, acetone, acetonitrile and ethanol). However, the immobilized enzyme was at all times more active than the soluble enzyme forms. The thermostability of the enzyme in buffer, at 40–60 °C, was also improved by immobilization. The soluble CBD–HRP fusion protein exhibited greater stability (both to organic solvents and temperature), but lower activity, in comparison with the native HRP. This work demonstrates for the first time the use of a cellulose-bound CBD-enzyme as a catalyst in aqueous-organic solvent media.

Free Radical Formation and Characterization of Nitroanisole Isomer Reduction in Different Media

A comprehensive study of the electrochemical characteristics of 2-, 3-, and 4-nitroanisole isomers in three different electrolytic media has been carried out. Furthermore, kinetic characterization of the one-electron reduction product from these isomers in these media is also reported. Also, free radicals were characterized in aprotic media by electron paramagnetic resonance through their corresponding hyperfine splitting constants. In protic media (30% ethanol/0.1 M Britton-Robinson buffer pH 2-12) 2-, and 3-nitroanisole isomers gave an irreversible well-defined peak over the entire pH range on Hg in a reaction involving four electrons to give the hydroxylamine derivative. However, in the case of 4-nitroanisole the kinetic characterization of the corresponding nitro radical anion was successfully achieved in this medium at pH 10.5, exhibiting a value of 19,000 (M s)−1. In this medium the ease of reduction at pH 7 was 2-nitroanisole In mixed aqueous-organic media [0.015 M aqueous citrate dimethylformamide (DMF): 60:40, 0.3 M KCl, and 0.1 M tetrabutylammonium iodide (TBAI) at the isolation of the couple by cyclic voltammetry and the electrochemical characterization of the nitro radical anion corresponding to the three compounds were achieved. Calculated disproportionation second-order rate constants, had an average value of and for 2-NA, 3-NA, and 4-NA, respectively. Also, the reactivity of the radicals toward glutathione was quantitatively assessed through the calculation of the respective apparent interaction rate constants. In aprotic media (0.1 M TBAI in DMF) the nitro radical anions were more stable than in the mixed media, with the following second-order decay rate constant, values: and for 2-NA, 3-NA, and 4-NA, respectively. © 2002 The Electrochemical Society. All rights reserved.

Static and dynamic behavior of 2:1 inclusion complexes of cyclodextrins and charged porphyrins in aqueous organic media.

Two heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (TMe-beta-CD) molecules strongly include the peripheral substituents at the 5- and 15-positions of the charged meso-tetrasubstituted porphyrins, PorSub(4) [TPPS(4) (Sub = p-C(6)H(4)-SO(3)(-)), TPPOC3PS (p-C(6)H(4)-O-(CH(2))(3)-p-C(6)H(4)-SO(3)(-)), TCPP (Sub = p-C(6)H(4)-CO(2)(-)), and TPPOC3Py (p-C(6)H(4)-O-(CH(2))(3)-Py(+)Br(-)), where Py(+) = N-alkylpyridinium] in aqueous ethylene glycol. The binding constants (K(1) and K(2)) and the rate constants (k(1) and k(2)) for formation of the 1:1 and 2:1 complexes of TMe-beta-CD and PorSub(4) were determined. Both the binding constants and the rate constants for anionic TPPS(4), TCPP, and TPPOC3PS were much larger than those for cationic TPPOC3Py. The smaller k(1) and k(2) values for TPPOC3Py indicate a higher barrier for penetration of a cationic guest into the TMe-beta-CD cavity. The methyl groups at the rims and the cavity wall of the host are positively polarized due to the inductive effect of the ethereal oxygens. The positively polarized rims and interior of the host cavity should prevent the penetration of the cationic substituent of TPPOC3Py into the TMe-beta-CD cavity. The 2:1 TMe-beta-CD-PorSub(4) complexes are extraordinary stable in aqueous solutions, even in the case of cationic TPPOC3Py. Formation of both 1:1 and 2:1 complexes is promoted by negative and large enthalpy changes, suggesting a strong van der Waals interaction as the main binding force.

Capillary electrophoresis in aqueous–organic media: Ionic strength effects and limitations of the Hubbard–Onsager dielectric friction model

The mobilities of three aromatic sulfonates, ranging in charge from −1 to −3, were investigated by capillary electrophoresis using buffers containing 0 to 75% ethanol or 2-propanol. Absolute mobilities were determined by extrapolation of the effective mobilities to zero ionic strength according to the Pitts’ equation. For all buffers studied, ions of higher charge experienced larger ionic strength effects. The resulting ionic strength-induced selectivity alterations were more dramatic when organic solvents were present in the media. Furthermore, for different organic modifier types and contents, the magnitude of the ionic strength effect was governed to a large extent by the 1/( ηε 1/2) dependence in the electrophoretic effect of the Pitts’ equation. Addition of ethanol or 2-propanol to the electrophoretic media resulted in changes in the absolute mobilities of the ions. These solvent-induced mobility changes are attributed to dielectric friction. As predicted by the Hubbard–Onsager model, dielectric friction increased with increasing organic content and with increasing analyte charge. As a result, dramatic changes in the relative absolute mobilities were observed, such as a reversal in migration order between sulfonates of −1 and −3 charge in 75% 2-propanol. Within the alcohols, the Hubbard–Onsager model was successful at predicting the relative mobility trends upon changing solvent. However, the relative trends observed between acetonitrile–water and alcohol–water media were not consistent with the model. This may be explained by the continuum nature of the model, whereby the different ion–solvent interactions characteristic to each solvent class are not taken into account.

Enzymatic hydrolysis of nitrides by an engineered nitrile hydratase (Papain Gln19Glu) in aqueous‐organic media

A mutant of the cysteine protease papain, displaying nitrile hydratase and amidase activities, was expressed in Pichia pastoris and used for the hydrolysis of peptide nitriles in aqueous-organic media. The rate of hydrolysis of these nitriles is lowered by increasing acetone concentration. This is caused by an increase of the Michaelis constant, and a variation of Vmax proportional to the amount of water in the mixture. The hydrolysis of the amide is less affected by the increase in co-solvent, which results in lower accumulation of this intermediate product. With the peptide nitrile tested, high nitrile concentrations could be used to promote the production of the amide and prevent its hydrolysis to the acid by diminishing the relative rate of amide hydrolysis. A number of non-peptidyl nitriles were also tested as potential substrates but activity was detected for only one compound with structural resemblance to peptide nitriles.

Isokinetic relationship for nucleophilic substitution reactions with a participating thiosulfate ion in aqueous-organic media: a special reaction mechanism

For 72 nucleophilic substitution reactions at a saturated carbon atom with the participation of the S 2 O 3 2– anion in aqueous-organic media, an isokinetic relationship withT iso = 1130 K and lg k iso = 5.949 was found.

P Ka values of peptides in aqueous and aqueous–organic media : Prediction of chromatographic and electrophoretic behaviour

In the present work, models describing the effect of the pH on the chromatographic and electrophoretic behaviour for polyprotic peptides were compared. The proposed models can be simultaneously used for determination of dissociation constants and selection of the optimum pH for the separation of peptides, in water and acetonitrile–water mixtures widely used in liquid chromatography and in capillary electrophoresis. The models use the pH value measured in the acetonitrile–water mixture instead of the pH value in water and take into account the effect of the activity coefficients. They permit the determination of the acidity constants in the aqueous and hydro–organic mobile phase from chromatographic retention and electrophoretic migration measurements, respectively. The values obtained by both proposed techniques agree with the potentiometric values previously determined. The suitability of the proposed models for predicting chromatographic and electrophoretic behaviour of compounds studied from a limited number of experimental data was also compared. The separation between solutes by both techniques in a complex mixture can be easily predicted, making simple and rapid pH selection to achieve optimum separation.

Synthesis of novel deoxy λ 5 phospha sugar nucleosides: 1,3-dipolar cycloaddition of an azidophospholane with alkynes

Several novel phospha sugar nucleosides, analogs of normal sugar nucleosides, were synthesized from a phospholene 1-oxide derivative. Bromination of a phospholene precursor in aqueous organic medium gave regio diastereomers, the threo and erythro bromohydrins 3 (1-bromo-1,3,4-trideoxy-1,4- C-[( R, S)-phenylphosphinylidene]- glycero-tetrofuranose). Further substitution of the threo isomer 3a with sodium azide led to its corresponding azidophospholane 4 (1-azido-1,3,4-trideoxy-2-methyl-1,4- C-[( R)-phenylphosphinylidene]-β- d- glycero-tetrofuranose). 1,3-Dipolar cycloaddition of 4 with various electron-deficient and electron-rich alkynes afforded triazole derivatives that are nucleoside analogues. The strong electron-withdrawing phosphoryl group in the hemiacetal ring exerted no effect over reaction regioselectivity of the 1,3-dipolar cycloaddition, but steric effects of the alkynes played a vital role on the selectivity, since the regioisomer ratios and the rates and yields of cycloadducts changed as the bulkiness of the substituents on the acetylene changes. Structures of all compounds were unequivocally confirmed by 1H, 13C, and 31P NMR and mass spectral studies. Single crystal X-ray crystallographic analysis of some derivatives allowed determination of configuration of the phospha sugar nucleosides.

Stabilization of various α-chymotrypsin forms in aqueous-organic media by additives

The effect of different polyols (glucose, sorbitol or polyethylene glycol) on the stability of soluble and different covalently immobilized α-chymotrypsin forms in aqueous-organic solvent (ethanol, dioxane or acetonitrile) systems were investigated. Generally, immobilization resulted in remarkable improvement of the stability of the enzyme in aqueous-organic media: while immobilized forms retained 80–85, 70–90 or 30–60% of their initial activity in 50% (v/v) ethanol, dioxane or acetonitrile, respectively, the soluble enzyme showed only about 13, 35 or 2% remaining activity in the above aqueous-organic mixtures, respectively (after 60 min preincubation). Further improvement was found in the stability by the addition of polyhydroxy compounds. This effect depended on the feature of the support and the organic solvent. The most explicit increase in the stability was observed when the enzyme was immobilized on a silica-based support (Silochrome activated with p-benzoquinone) in aqueous-acetonitrile mixtures, where the activity of α-chymotrypsin was enhanced about 1.5–2 times by the addition of sorbitol or glucose as compared with its control.

Key parameters for carbamate stability in dilute aqueous–organic solution

Within the framework of the determination of pesticides of the carbamate family, the isocyanates prove to be excellent derivatizing agents of the hydrolysis products of pesticide residues. The optimum conditions for derivatization require the use of an aqueous organic solution (water–acetonitrile) with the presence of triethylamine. Under these conditions, the derivatives formed (ureas or carbamates) must be stable to enable a reliable and representative determination. Our objective, therefore, was to study the parameters, such as solvent, structure of the derivatizing agent, pH, nature of the catalyst, and the optimum conditions needed for stability. The kinetic and statistical analysis of 24 compounds (an analysis of variance based on 142 measurements) demonstrate that neither the effects of pH, nor of the group introduced by the derivatizing agent make it possible to simultaneously stabilize all of the formed derivatives. To ensure reliable and repeatable determination of the carbamate (obtained after derivatization), it is necessary to expulse the carbamate, in the organic phase, by adding NaCl to the aqueous–organic medium.