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At least 19 recordsLinked to original sources
PubMed1997

Ultrasound and polar homogeneous reactions.

The effect of ultrasound on the rates of homogeneous heterolytic reactions not switched to a free radical pathway can be explained by the perturbation of the molecular organization of or the solvation in the reacting system. A quantitative analysis of the sonochemical acceleration on the basis of the microreactor concept was carried out. It was found that (1) the Diels-Alder reaction cannot be accelerated by ultrasound except when SET or free radical processes are promoted, (2) the rectified diffusion during cavitation cannot be responsible for the acceleration of reactions, and (3) the sonochemical acceleration of polar homogeneous reactions takes place in the bulk reaction medium. This implies the presence of a 'sound-field' sonochemistry besides the 'hot-spot' sonochemistry. The occurrence of a sonochemical deceleration effect can be predicted.

Journal Article↗
PubMed1975

Nonlinear phenomena in oscillatory systems of homogeneous reactions - experimental observations.

An oscillating system of homogeneous chemical reactions (oxidation of malonic acid by bromate with ceric/cerous ion system as the catalyst) was followed experimentally in several reactor configurations, operated both as close and open systems. Characteristic nonlinear phenomena observed include: damped oscillations of concentrations of reaction components in a well stirred batch reactor, undampted oscillations and sudden changes of oscillatory behaviour (jump to a new steady state) in CSTR and interaction of oscillations in the two connected CSTRs. In a tubular reactor stationary and travelling concentration waves were observed. In the system of CSTR connected with a tubular reactor spreading of waves from CSTR to the tubular reactor, and vice versa was followed. In the flow through tubular reactor the formation of non-monotomic concentration profiles was measured, which were basically stationary with respect to changing flow-rate. The course of oscillations in CSTR was simulated on the basis of a semiempirical reaction kinetics model. An attempt to simulate quantitatively the behaviour of the distributed system with the common type of Fickian diffusion balance equations failed. Difficulties in the proper description of the transport of ions through the concentration gradient and the gradient of the electrical potential found in this system are discussed.

Bromates↗
PubMed2005

Role of the direct reaction H2S + SO2 in the homogeneous Claus reaction.

Quantum chemical methods at the Gaussian-2 and -3 levels of theory have been used to investigate the reactions between H(2)S, SO(2), and S(2)O such as might occur in the front-end furnace of the Claus process. The direct reaction between H(2)S and SO(2) occurs via a 5-centered transition state with an initial barrier of approximately 135 kJ mol(-1) and an overall barrier of approximately 153 kJ mol(-1) to produce S(2)O and H(2)O. We indicate approximate values here because there are a number of isomers in the reaction pathway that have barriers slightly different from those quoted. The presence of a water molecule lowers this by approximately 60 kJ mol(-1), but the van der Waals complex required for catalysis by water is thermodynamically unfavorable under the conditions in the Claus reactor. The direct reaction between H(2)S and S(2)O can occur via two possible pathways; the analogous reaction to H(2)S + SO(2) has an initial barrier of approximately 117 kJ mol(-1) and an overall barrier of approximately 126 kJ mol(-1) producing S(3) and H(2)O, and a pathway with a 6-centred transition state has a barrier of approximately 111 kJ mol(-1), producing HSSSOH. Rate constants, including a QRRK analysis of intermediate stabilization, are reported for the kinetic scheme proposed here.

Journal Article↗
PubMed2002

Dramatic rate acceleration of the Baylis-Hillman reaction in homogeneous medium in the presence of water.

[reaction: see text] In homogeneous H(2)O/solvent medium, the reaction rate of aromatic aldehydes and acrylonitrile or acrylate was greatly accelerated, which led to shorter reaction time, lower reaction temperature, and higher yield. In this reaction, Me(3)N, DMAP, DABCO, and urotropine were good catalysts. Except for low-carbon alcohols, tetrahydrofuran, 1,4-dioxane, and acetonitrile could be chosen as the solvent. Under this condition, the diastereoselective reaction of nitrobenzaldehyde and L-menthyl acrylate was realized with 88-99% de.

Journal Article↗
PubMed1999

Automated kinetic exclusion assays to quantify protein binding interactions in homogeneous solution.

A method was developed for the quantification of protein-ligand interactions in which the free protein present in homogeneous reaction mixtures was separated and quantified using a KinExA immunoassay instrument. Separation was achieved by rapid percolation of the reaction mixture over a column of microbeads whose surfaces were coated with an immobilized form of the ligand. The protein thus captured was quantified using a fluorescently labeled anti-protein antibody. The features of this new method were illustrated using a model system in which each of the principal reagents was covalently labeled with a different fluorescent molecule: mouse monoclonal anti-biotin primary antibody (fluorescein), biotin (B-phycoerythrin), and goat anti-mouse polyclonal secondary antibody (indodicarbocyanin). Values for the equilibrium and kinetic rate constants for the binding between the anti-biotin antibody and biotin conjugated with B-phycoerythrin were determined and shown to be independent of whether the fluorescent label was located on the primary or secondary antibody. Equilibrium binding experiments conducted with (F(AB))(2) and corresponding F(AB) fragments showed that the valency of the binding protein had no influence on the value of the dissociation constant. The values of the equilibrium and rate constants obtained by this new method are those for the binding reaction in homogeneous solution; the immobilized ligand is only a tool exploited for the separation and quantification of the free protein.

Animals↗
PubMed2003

Interfacial versus homogeneous enzymatic cleavage of mandelonitrile by hydroxynitrile lyase in a biphasic system.

The question of an interfacial versus a homogeneous reaction is carefully addressed for the enzymatic biphasic cleavage of mandelonitrile to benzaldehyde by Prunus amygdalus hydroxynitrile lyase (pa-Hnl) (Hickel et al. [1999] Biotechnol Bioeng 36:425-436). Experimental evidence, including 1) the reaction ceases when the interface is populated by previously adsorbed denatured pa-Hnl, 2) the reaction continues even after washout of the bulk enzyme from the aqueous phase, 3) highly nonpolar organic solvents initially promote fast reaction kinetics that relatively quickly decay to zero product production, and 4) the reaction rate is nonlinear in the bulk enzyme concentration, provide robust grounds for an interfacial reaction. We also model enzymatic mandelonitrile cleavage assuming a homogeneous aqueous-phase reaction. The homogeneous reaction scheme does not simultaneously account for the experimental observations of a linear dependence of the reaction rate on organic/water interfacial area, no dependence on the aqueous-phase volume, and a nonlinear dependence on pa-Hnl aqueous concentration. Further, simple calculations demonstrate that the homogeneous reaction rate is at least three orders of magnitude slower than those observed by Hickel et al. (1999). We again conclude that enzyme adsorbed at the organic solvent/water interface primarily catalyzes the biphasic mandelonitrile cleavage reaction.

Acetonitriles↗
PubMed2003

A recyclable catalyst that precipitates at the end of the reaction.

Homogeneous catalysts--which exist in the same (usually liquid) phase as reactants and products--are usually more selective than heterogeneous catalysts and far less affected by limitations due to slow transport of reactants and products, but their separation from reaction products can be costly and inefficient. This has stimulated the development of strategies that facilitate the recycling of homogeneous catalysts. Some of these methods exploit the preference of a catalyst for one of two solvents with thermoregulated miscibility; others exploit a dramatic decrease in catalyst solubility as one reagent is consumed or temperature changed after completion of the reaction. Here we describe a tungsten catalyst for the solvent-free hydrosilylation of ketones that retains its activity until essentially all of the liquid substrate is converted to liquid products, which we can then simply decant to separate the catalyst that precipitates from the products of the reaction. We attribute the ability of the catalyst to retain its solubility and hence activity until completion of the reaction to the transient formation of liquid clathrate that contains a few molecules of the substrate per molecule of the otherwise solid catalyst. Insights into the fundamental processes controlling the formation of this liquid clathrate might help to tailor other catalysts and substrates, so as to develop efficient and solvent-free schemes for reactions of practical interest.

Journal Article↗
PubMed1990

Aromatase inhibition by R 76 713: a kinetic analysis in rat ovarian homogenates.

Reaction kinetics of the aromatase enzyme and of a new nonsteroidal aromatase inhibitor, R 76 713 (6-[(4-chlorophenyl)(1H-1,2,4-triazol-1-yl)-methyl]-1-methyl-1H- benzotriazole), were studied in ovarian homogenates obtained from pregnant mare's serum gonadotropin (PMSG)-injected female Wistar rats. The Km (Michaelis constant) of the aromatase enzyme with androstenedione as the substrate was 47 +/- 13 nM; for testosterone as the substrate, a value of 159 +/- 10 nM was found. In the presence of increasing concentrations of R 76 713, the Km increased while the Vmax (maximal velocity of enzyme-catalyzed reaction) remained unchanged. Using androstenedione and testosterone as the substrate, Lineweaver-Burk analysis of the data showed a Ki (dissociation constant of the enzyme-inhibitor complex) for R 76 713 of 0.7 +/- 0.3 nM and 1.6 +/- 0.4 nM, respectively. R 76 713 appeared to competitively inhibit the rat ovarian aromatase.

Androstenedione↗
PubMed1986

Role of molecular oxygen in lignin peroxidase reactions.

Homogeneous lignin peroxidase (diarylpropane oxygenase) oxidized veratryl alcohol to veratryl aldehyde under anaerobic conditions in the presence of either H2O2, m-chloroperoxybenzoic acid (mCPBA), or p-nitroperoxybenzoic acid (pNPBA). Lignin peroxidase also oxidized the 1-(3',4'-diethoxyphenyl)-1,2-dihydroxy-(4"-methoxyphenyl)-propane I under anaerobic conditions in the presence of mCPBA to yield 3,4-diethoxybenzaldehyde III and 1-(4'-methoxyphenyl)-1,2-dihydroxyethane IV. In contrast to what occurs under aerobic conditions, under anaerobic conditions no 2-hydroxy-1-(4'-methoxyphenyl)-1-oxoethane V was obtained. During the diarylpropane I cleavage under anaerobic conditions, 18O from H2(18)O was incorporated into the alpha-position of the phenylglycol IV. Lignin peroxidase also hydroxylated 1-(4'-ethoxy-3'-methoxyphenyl)propane II at the alpha-position to yield 1-(4'-ethoxy-3'-methoxyphenyl)-1-hydroxypropane VI under anaerobic conditions in the presence of mCPBA. During the phenylpropane II hydroxylation under anaerobic conditions, 18O from H2(18)O was incorporated into the alpha-position of VI. These results are rationalized according to a mechanism involving an initial one-electron oxidation of the diarylpropane I by the lignin peroxidase compound I to form a benzene pi cation radical which undergoes alpha, beta cleavage to produce a benzaldehyde and a C6C2 benzylic radical. The latter is then attacked by O2 to form a hydroperoxy radical which may decompose through a tetroxide to form the phenylglycol IV and phenylketol V. Under anaerobic conditions the C6C2 benzylic radical is probably oxidized to a carbonium ion which would be subsequently attacked by H2O to yield the phenylglycol V.

Anaerobiosis↗
PubMed1995

A transient model of mass transfer and kinetics in a passive vapor sampler.

A transient model of vapor contaminant diffusion and homogeneous reaction in a two-compartment passive sampler was formulated. The mathematical analysis considered finite reaction kinetics, bulk air boundary-layer effects, and the solubility of the vapor contaminant in the liquid absorbing/reacting medium. The model was evaluated using experimentally measured Cl2 uptake rates in a series of samplers of different dimensions containing a 0.1% aqueous sulfamic acid solution in the absorbing medium chamber. The theory predicts accurately the effects of sampler diameter, stagnant air chamber path length, and sampler orientation with respect to the flowing air on Cl2 uptake, with an average error of less than 10%. Chlorine uptake in a 33-mm diameter sampler with a 10.8-mm path length was found to be essentially independent of wind direction and varied by approximately 23% for wind speeds ranging from 5.1 to 203 cm s-1 (10-400 ft min-1). Calculations show that the sampler is sensitive to wind speed partly because of the Cl2 hydrolysis reaction, which produces HOCl and drops the vapor phase concentration of chlorine to near zero at the stagnant air/absorbing medium chamber interface, regardless of the rate of homogeneous reaction between HOCl and sulfamate anions. If the solubility of the vapor contaminant in the liquid absorbing medium were to obey Henry's Law (no hydrolysis reaction) and the homogeneous trapping reaction were sufficiently slow, then model calculations show that contaminant uptake could be controlled solely by reaction kinetics, although the sampling rates would be lower than those observed and predicted for the Cl2 case.

Air Pollutants, Occupational↗
PubMed2006

Homogeneous enzymatic reactions in ionic liquids with poly(ethylene glycol)-modified subtilisin.

Subtilisin Carlsberg was covalently modified with comb-shaped poly(ethylene glycol) (PM13). PM13-modified subtilisin (PM13-Sub) was readily solubilized in three different ionic liquids (ILs), i.e., [Emim][Tf2N], [C2OC1mim][Tf2N] and [C2OHmim][Tf2N]. Analysis of homogeneous enzymatic reactions in the ILs revealed that PM13-Sub exhibited excellent catalytic performance while the native enzyme suspended in ILs showed no activity. Hydrophobicity of ILs slightly affected enzyme activity, and the relatively hydrophobic IL [Emim][Tf2N] was the preferred medium for enzymatic reactions, similar to enzymatic reactions in conventional organic solvents. Enzyme activity was much higher in [Emim][Tf2N] than in conventional organic solvents, and excellent activity was associated with unique properties of ILs such as hydrophobicity and high polarity. Furthermore, PM13-Sub showed good stability in [Emim][Tf2N], and maintained 80% of its initial activity after 60 h.

Bacillus subtilis↗
PubMed1983

Capture of spatially homogeneous chemical reactions in tissue by freezing.

A useful technique in studying the saturation of hemoglobin in erythrocytes or myoglobin in tissue is cryophotometry, in which tissue is frozen for later spectrophotometric analysis. A general question associated with this technique is whether the freezing process alters the chemical state. This paper presents a theoretical analysis of the simplest model relevant to that question. We study the effect of rapid cooling on a spatially homogeneous chemical reaction. The analysis shows that changes during freezing are negligible near the boundary to which the heat sink is applied, but can be significant deeper in the sample. The distance from the boundary at which the changes during freezing become appreciable can be expressed simply in terms of the chemical reaction rates and the thermal diffusivity of the tissue. Detailed results are given for the case of oxygen and myoglobin in skeletal muscle.

Animals↗
PubMed1999

The incidence of spontaneous acrosome reaction in homogeneous populations of hyperactivated human spermatozoa.

The incidence of spontaneous acrosome reaction occurring in 1314 individually selected hyperactivated (HA) human spermatozoa was compared to that occurring in 8226 individually selected non-hyperactivated spermatozoa (non-HA) sampled over an incubation time course to allow for capacitation. Two-way analysis of variance showed a significant difference between HA and non-HA spermatozoa for the mean percent acrosome reacted (R), partially acrosome reacted (PR) and combined total (R+PR) (P < 0.001). One-way analysis showed that among the HA spermatozoa there were marked differences among the proportion showing R+PR at the various time points (P = 0.005). Using the same end point, there was no significant evidence of change with time for the non-HA spermatozoa. The overall data indicated that HA human spermatozoa have a greater propensity for spontaneous acrosomal loss than non-HA spermatozoa during incubation in synthetic culture media.

Acrosome Reaction↗
PubMed2001

Selenium-catalyzed oxidations with aqueous hydrogen peroxide. 2. Baeyer-Villiger reactions in homogeneous solution.

Several diselenides were tested for catalytic activity in Baeyer-Villiger reactions with 60% aqueous hydrogen peroxide. Bis[3,5-bis(trifluoromethyl)phenyl] diselenide forms the corresponding 3,5-bis(trifluoromethyl)benzene seleninic acid in situ, which is a highly reactive and selective catalyst for the oxidation of carbonyl compounds in 1,1,1,3,3,3-hexafluoro-2-propanol, 2,2,2-trifluoroethanol, or dichloromethane.

Journal Article↗
PubMed1989

Characterization of a monoclonal antibody, OVB1, which binds to a unique determinant in human ovarian carcinomas and myeloid cells.

A monoclonal antibody, OVB1, was generated against a human ovarian carcinoma cell line, OVCAR-3. The antigen reacting with this antibody was strongly expressed on the external surface of the plasma membrane of OVCAR-3 cells and cells of 4/4 other ovarian carcinoma lines. Variable density and homogeneity of expression was found on cells from 5/5 breast carcinoma lines. Various ovarian tumor specimens and normal human tissues were frozen, cryostat-sectioned, and examined for OVB1 reactivity using immunoperoxidase methods. A strong, uniform, homogeneous reaction on 10/10 ovarian carcinoma specimens and variable, non-homogeneous reactions on breast tumors were seen. Normal tissues reacting with the antibody include thyroid, pituitary pars intermedia, breast ductal epithelium, Auerbach's plexus and neuronal processes in the GI tract, colonic mucosal epithelium, and salivary gland ductal epithelium. Polymorphonuclear leukocytes, eosinophils, and approximately 13% of peripheral lymphocytes, as well as cells around germinal centers in lymph nodes and spleen, showed strong reactivity by immunofluorescence and/or immunoperoxidase. Expression of the OVB1 antigen in the myeloid cells of normal human bone marrow occurred from the promyelocyte stage through to more mature cells in a subpopulation of myeloblasts. Indirect immunofluorescence of live peripheral blood cells showed localization to the surface of PMNs, eosinophils, and certain lymphocytes. Double-immunofluorescence studies (with a direct fluorescein-anti-lactoferrin antibody conjugate) showed co-localization of OVB1 and OKM1 (anti-C3bi receptor) antibodies to specific granules of PMNs. Localization of OVB1 and OKM1 antibodies to granular structures in the PMN was confirmed by electron microscopy using the ferritin bridge technique. The antigen reacting with the OVB1 antibody was shown to be neuraminidase sensitive, but protease insensitive. The OVB1 monoclonal antibody may be useful in identification of ovarian tumors and subclassification of myeloid leukemias.

Antibodies, Monoclonal↗
PubMedDate not supplied

[Characteristics of the glutamate decarboxylase reaction in homogenates of various regions of the rat brain].

The glutamate decarboxylase activity in rough homogenates of cerebellum, cortex and truncal part of the rat brain was studied under different conditions of incubation: in the presence of 25 mM glutamate sodium, 0.4 mM pyridoxal-5'-phosphate and both these components. It is found that the initial glutamate decarboxylase activity in cerebellum homogenates is approximately twice as high as in the cortex and trunk homogenates. Addition of the substrate and cofactor, especially in the combination, stimulates considerably the yield of gamma-aminobutyric acid (GABA) in the glutamate decarboxylase reaction, the most pronounced activation being observed in the truncal homogenates. The glutamate/GABA relation both initial and after the completion of the reaction is the maximal in the cortex and minimal in the truncal part of the brain. The data obtained evidence for the differences in the content of the GABA-producing enzyme rather than for the presence of the specific mechanisms of the enzyme regulation in different brain areas.

Animals↗