Kinetic Behaviour (kinetic + behaviour)

Distribution by Scientific Domains

Selected Abstracts

Fractionated extraction of polycyclic aromatic hydrocarbons (PAHs) from polluted soils: estimation of the PAH fraction degradable through bioremediation

S. Thiele-Bruhn
Summary Chemical extractions were tested to estimate the fraction of 15 polycyclic aromatic hydrocarbons (PAHs) in polluted soils degradable through bioremediation. Of 30 soil samples, differing in PAH concentrations, from 10 long-term polluted sites, four were used in screening tests conducted with 14 different extractants. PAH extractability increased in the order salt solutions < surfactant solutions < organic solvents and organic solvent,water mixtures. Closest correlations (r = 0.73,0.91) and ratios of c. 1:1 of the extracted to the degradable PAH concentrations were found with aqueous solutions of two surfactants (Genapol UDD 88, Synperonic LF/RA 30). The PAH extractability by surfactant solutions increased with surfactant concentration (0,50 mg ml,1) and with decreasing degree of PAH condensation (6,2 rings). Kinetic behaviour of PAH solubilization by surfactants resembled the kinetics of PAH degradation (r = 0.84,0.99). With repeated extractions, a finite fraction of PAHs was extracted, which is thought to be the mobile and mobilizable PAH fraction, the latter being controlled by back-diffusion from intra- and inter-particle sites. Experiments with all 30 soil samples and a priori testing with an additional 10 soil samples confirmed that single extractions with surfactant solutions (5 mg ml,1; soil:solution ratio 1:20) were suitable for estimating the degradable fraction of PAHs in soil. With separate linear regression equations for PAHs grouped according to their degree of condensation, the extracted concentrations described the degradable concentrations with correlation coefficients (r) ranging from 0.62 to 0.80 (P < 0.001). PAH extractions with the surfactant solutions can be used to assess the prospects of bioremediation of PAH-polluted sites. [source]

Kinetic behaviour of the adsorption and photocatalytic degradation of salicylic acid in aqueous TiO2 microsphere suspension

Abstract A new photocatalyst, named TiO2 microspheres, prepared by a sol-spraying-calcination method, can freely suspend with air bubbling in its aqueous suspension and easily settle down from a water phase under gravity. The experimental results demonstrated that TiO2 microspheres had better adsorption capacity than conventional TiO2 powders, due to large surface area, large pore volume, and also a porous structure. The photocatalytic activity of TiO2 microspheres in aqueous suspension was evaluated using salicylic acid (SA) as a model substrate. It was found that the Langmuir,Hinshelwood model in its integral form described the kinetics of SA photocatalytic degradation in the TiO2 microsphere suspensions better than its simplified form as a first-order reaction model, since the significant substrate adsorption on the catalysts was not negligible. The kinetics of SA photocatalytic degradation with different initial concentrations and pH was further investigated. The experiments demonstrated that the change of pH could significantly affect the adsorption of SA in the TiO2 microsphere suspensions. The effects of substrate adsorption rate and photoreaction rate on the overall performance of photocatalytic degradation is also discussed on the basis of experimental data. Copyright 2004 Society of Chemical Industry [source]

Kinetic behaviour and stability of Escherichia coli ATCC27257 alkaline phosphatase immobilised in soil humates

Mara C Pilar
Abstract Alkaline phosphatase (EC extracted from Escherichia coli ATCC27257 was immobilised by co-flocculation with soil humates in the presence of Ca2+. The effects of time, temperature, pH and concentration of enzyme and support on immobilisation were studied. Between 58 and 92% of the added phosphatase was strongly bound to the humates, depending on the conditions of immobilisation used. Some characteristics of the humate,phosphatase complexes and of the free enzyme were compared. The enzymatic complexes showed values of Km (2.22,mM) and activation energy (33.4,kJ,mol,1) similar to those of the free enzyme (2.00,mM and 27.6,kJ,mol,1). The pH/activity profiles revealed no change in terms of shape or optimum pH (10.5) upon immobilisation of alkaline phosphatase. However, the immobilised enzyme showed maximal activity in the range of 80,100,C, while the free enzyme had its highest activity at 60,C. The thermal stability of alkaline phosphatase was enhanced by complexation to the soil humates. 2003 Society of Chemical Industry [source]

Characterization of carbonic anhydrase from Neisseria gonorrhoeae

FEBS JOURNAL, Issue 6 2001
Bjrn Elleby
We have investigated the steady state and equilibrium kinetic properties of carbonic anhydrase from Neisseria gonorrhoeae (NGCA). Qualitatively, the enzyme shows the same kinetic behaviour as the well studied human carbonic anhydrase II (HCA II). This is reflected in the similar pH dependencies of the kinetic parameters for CO2 hydration and the similar behaviour of the kinetics of 18O exchange between CO2 and water at chemical equilibrium. The pH profile of the turnover number, kcat, can be described as a titration curve with an exceptionally high maximal value of 1.7 106 s,1 at alkaline pH and a pKa of 7.2. At pH 9, kcat is buffer dependent in a saturable manner, suggesting a ping-pong mechanism with buffer as the second substrate. The ratio kcat/Km is dependent on two ionizations with pKa values of 6.4 and 8.2. However, an 18O-exchange assay identified only one ionizable group in the pH profile of kcat/Km with an apparent pKa of 6.5. The results of a kinetic analysis of a His66,Ala variant of the bacterial enzyme suggest that His66 in NGCA has the same function as a proton shuttle as His64 in HCA II. The kinetic defect in the mutant can partially be overcome by certain buffers, such as imidazole and 1,2-dimethylimidazole. The bacterial enzyme shows similar Ki values for the inhibitors NCO,, SCN, and N3, as HCA II, while CN, and the sulfonamide ethoxzolamide are considerably weaker inhibitors of the bacterial enzyme than of HCA II. The absorption spectra of the adducts of Co(II)-substituted NGCA with acetazolamide, NCO,, SCN,, CN, and N3, resemble the corresponding spectra obtained with human Co(II)-isozymes I and II. Measurements of guanidine hydrochloride (GdnHCl)-induced denaturation reveal a sensitivity of the CO2 hydration activity to the reducing agent tris(2-carboxyethyl)phosphine (TCEP). However, the A292/A260 ratio was not affected by the presence of TCEP, and a structural transition at 2.8,2.9 m GdnHCl was observed. [source]

A Coherent Mechanistic Rationale for Additive Effects and Autoinductive Behaviour in Proline-Mediated Reactions

Natalia Zotova
Abstract Differences in the kinetic behaviour of aldol reactions compared to aminoxylation and amination reactions are rationalized by consideration of the rate-determining step in each case. Both autoinductive behaviour and the rate-enhancing effect of additives are attributed to an effect on the enamine formation step. [source]

One-Pot Multienzymatic Synthesis of 12-Ketoursodeoxycholic Acid: Subtle Cofactor Specificities Rule the Reaction Equilibria of Five Biocatalysts Working in a Row

Daniela Monti
Abstract The hydroxysteroid dehydrogenases (HSDHs)-catalyzed one-pot enzymatic synthesis of 12-ketoursodeoxycholic acid (3,,7,-dihydroxy-12-oxo-5,-cholanoic acid), a key intermediate for the synthesis of ursodeoxycholic acid, from cholic acid has been investigated. This goal has been achieved by alternating oxidative and reductive steps in a one-pot system employing HSDHs with different cofactor specificity, namely NADH-dependent HSDHs in the oxidative step and an NADPH-dependent 7,-HSDH in the reductive one. Coupled in situ regeneration systems have been exploited not only to allow the use of catalytic amounts of the cofactors, but also to provide the necessary driving force to opposite reactions (i.e., oxidation and reduction) acting on different sites of the substrate molecule. Biocatalysts suitable for the set-up of this process have been selected and their kinetic behaviour in respect of the reactions of interest has been evaluated. Finally, the process has been studied employing the enzymes both in free and compartmentalized form. [source]

Application of a carbon sorbent for the removal of cadmium and other heavy metal ions from aqueous solution

I El-Shafey
Abstract Treatment of flax shive with sulfuric acid produces a carbonaceous material that has been used to remove metal ions from aqueous solution. Metal ions including Cd(II), Cu(II), Cr(III), Co(II), Ni(II), Zn(II) and Pb(II) have been investigated for kinetic behaviour and sorption capacities. These metal ions show fast sorption kinetics following a first order rate equation. Cadmium was chosen as representative of these metal ions and a detailed study was carried out. The effect of pH on sorption was studied and it was found that maximum uptake occurred above pH 3,7, sorption was accompanied by release of protons into the solution and a ratio of [H+] released to [Cd2+] sorbed of approximately 2 was found. The sorption capacity showed no significant increase with increase of temperature. The presence of other metal ions such as K+, Na+, Mg2+ and Ca2+ decreases the Cd(II) capacity, indicating competition for the ion exchange sites. Successive sorption of Cd(II) shows that the capacity exceeds the monolayer capacity calculated from the Langmuir equation. Column studies showed good performance over a total of seven cycles of loading/stripping. These studies indicate that the sorption mechanism for these metal ions is related to a reversible ion exchange process on the carbon surface. 2002 Society of Chemical Industry [source]

Estimation of absolute oral bioavailability of moxidectin in dogs using a semi-simultaneous method: influence of lipid co-administration

Moxidectin is a long-acting anthelmintic drug for which little is known about its kinetic behaviour in dogs and its oral absolute bioavailability has never been reported. We studied the pharmacokinetics of moxidectin in dogs, with a special emphasis on oral bioavailability and the influence of lipid co-administration, by using a semi-simultaneous method of administration. Ten Beagle dogs were dosed orally and then intravenously (i.v.) with 0.2 mg/kg moxidectin. The oral application was conducted with or without corn oil co-administration. Moxidectin concentration,time profiles in plasma were analysed using a compartmental modelling approach, designed to fit the oral and i.v. kinetic disposition curves simultaneously. In contrast to what happens in other species, our study indicates that the bioavailability of orally given moxidectin in dogs is nearly total (90.2 7.4%), and is not enhanced by lipid co-administration. The clearance, the volume of distribution, the mean residence time and the terminal half-life were similar to what was already described for other species. Finally our trial suggests that the body condition (degree of obesity) is likely to be a major determinant of moxidectin kinetics in dogs because of its modulation of the volume of distribution that indirectly controls the terminal half-life of the drug. [source]

A detailed assessment of the pattern of moxidectin tissue distribution after pour-on treatment in calves

J. M. Sallovitz
The use of topical (pour-on) administration of endectocide drugs in cattle has reached world-wide acceptance. However, only limited information is available on the kinetic behaviour for topically administered moxidectin (MXD). To improve our understanding of the relationship between pharmacokinetics and efficacy for pour-on preparations, MXD concentration profiles were measured in tissues of endo- and ectoparasites location over 35 days postadministration. MXD distribution to the fluid content and mucosal tissue of the abomasum and different intestinal sections (duodenum, ileum, caecum and colon) was assessed. The comparative patterns of MXD distribution to skin and hypodermic tissue from different anatomical sites (backline, rib cage, thigh and face) were also investigated following the pour-on administration. Wide tissue distribution and long residence time characterized the kinetics of topically administered MXD. MXD was recovered between 1 and 35 days post-treatment in all the tissues investigated. The highest MXD availabilities were observed in the skin layers at the site of administration (backline) and in the fat tissue. The fluid contents of different intestinal sections showed MXD concentrations higher than those measured in their respective mucosal tissues, particularly at day 1 post-treatment. MXD concentrations in the skin (epidermis + dermis) were higher than those measured in the hypodermic tissue. Large differences in the availability of MXD in skin from different anatomical regions (backline > rib cage > thigh > face) were observed. The low plasma and the high skin availability indicate the formation of a skin depot of the drug, being released slowly to the plasma and reaching concentrations in systemic tissues (abomasal mucosa, lungs, etc.) similar to those measured after subcutaneous administration. These findings demonstrate that target parasites may be exposed to markedly different drug concentrations according to their location sites, which is particularly relevant for ectoparasites located in different anatomical regions. Knowledge of the tissue distribution of topically administered endectocides contributes to understand the differences observed in efficacy and/or persistence of activity and to optimize their use in cattle. [source]

The Fouling Behaviour of Sodium Aluminosilicate Polytypes in High Ionic Strength Caustic Media Heat Exchangers

J. Addai-Mensah
Sodium aluminosilicate (SAS) scale formation, simulating high ionic strength caustic media process heat exchanger fouling, has been investigated for a temperature range of 30,240C. Findings from several studies performed with a variety of liquor, substrate-type and agitation conditions show that SAS polytypes; i.e. amorphous, zeolite A, sodalite and cancrinite phases, differing in thermodynamic stability and kinetic behaviour may deposit. The polytypes formed and subsequent transformations to more thermodynamically stable phases are strongly dependent on liquor composition, temperature and time. The scale deposition process is substrate- mediated heterogeneous nucleation and particle growth (precipitation foulin&, which may be accompanied by bulk liquor-nucleated solids adsorption (particulate fouling) at suficiently high supersaturations. Liquor seeding with stable SAS solid phases is found to be efective in mitigating fouling. The precipitation fouling behaviour is principally determined by the clystallo-chemical nature of the SAS polytype and the solution conditions. The substrate (mild steel, 316 stainless steel, nickel and tef2on) surface physico-chemical structure has a signlficant impact on the scale particle morphology and layer structure, particularly at high temperatures. [source]

Comparative disposition of ricobendazole enantiomers after intravenous and subcutaneous administration of a racemic formulation to calves

Carles Cristfol
Abstract The enantioselective disposition kinetics of the benzimidazole anthelmintic, ricobendazole (RBZ), have been characterized after its intravenous (iv) and subcutaneous (sc) administration as a racemic formulation to cattle. The (+) and (,) RBZ enantiomeric forms were recovered in plasma after iv and sc administration of the racemic RBZ formulation, using a chiral phase based HPLC method. A biexponential plasma concentration versus time curve was observed for both RBZ enantiomers following the iv treatment. Total body clearance was higher for (,) RBZ (150.4 mL/h,,kg) compared with that obtained for the (+) RBZ antipode (78.1 mL/h,,kg). The elimination half-life of the (,) RBZ enantiomer was shorter (T1/2,: 2.67 h) compared with the (+) enantiomer (T1/2,: 5.41 h). The plasma availability (expressed as AUC) was significantly higher for (+) RBZ compared with that obtained for the (,) antipode following both treatments. The enantiomeric ratio in plasma at T0 was close to unity (50% of each enantiomer); the analysis of the concentration ratios (+) RBZ/(,) RBZ, demonstrated an increase in the proportion of (+) RBZ during the time course of the kinetics after both iv and sc treatments. The results presented herein show the enantioselective disposition kinetics of RBZ in cattle and are a further contribution to the understanding of the kinetic behaviour of these sulphoxide-containing benzimidazole anthelmintics in ruminants. Copyright 2000 John Wiley & Sons, Ltd. [source]