Direct calculation of interconversion barriers in dynamic chromatography and electrophoresis: Isomerization of captopril

Dynamic capillary electrophoresis (DCE) and direct calculation of the rate constants of isomerization has been applied to determine the cis-trans isomerization barriers of the angiotensin-converting enzyme inhibitor captopril. The separation of the rotational cis-trans isomeric drug has been performed in an aqueous 50 mM borate buffer at pH 9.3. Interconversion profiles featuring plateau formation, peak-broadening, and peak coalescence were observed. To determine the rate constants of the forward and backward reaction (k(cis-->trans) and k(trans-->cis)) of the isomerization process in dynamic capillary electrophoresis, a novel straightforward calculation method using the experimental parameters plateau height, h(plateau), peak width at half height w(h), the total migration times of the cis-trans isomers t(R) and the electroosmotic break-through time t(0) as well as the peak ratio of the cis-trans isomers is presented for the first time. From temperature dependent measurements the rate constants k(cis-->trans) and k(trans-->cis) and the kinetic activation parameters DeltaG( not equal), DeltaH( not equal), and DeltaS( not equal) of the cis-trans isomerization of captopril were obtained. From the activation parameters the isomerization barriers of captopril at 37 degrees C under basic conditions were calculated to be DeltaG( not equal) (cis-->trans) = 90.3 kJ.mol(-1)and DeltaG( not equal) (trans-->cis) = 90.0 kJ.mol(-1*).     

Kinetic study of the thermal inactivation of glucose oxidase in the presence of denaturant and stabilizer by means of bioelectrocatalysis method

The thermal inactivation of glucose oxidase (GOD) in aqueous solution has been studied by the electrochemical method to follow the bioelectrocatalytic current due to the oxidation of glucose by GOD. Exponential time-dependent decrease in bioelectrocatalytic current, that is, the decrease in the enzymatic activity of GOD, was observed at given temperatures to determine the rate constant (k) of a simple inactivation process: GOD (active) → GOD (inactive). The ln[k] vs. T(-1) plots gave straight lines with all solution conditions tested, so that the resulting Arrhenius activation parameters including ΔH(?) and ΔS(?) can be compared with each other. In the 50 mmol/L phosphate buffer at 70°C, k was determined to be (6.6 ± 1.6)× 10(-4) s(-1), and ΔH(?) and ΔS(?) were calculated to be 202 ± 13 kJ mol(-1) and 282 ± 39 J K(-1) mol(-1), respectively. By addition of 3 mol/L guanidine hydrochloride, the k was increased to (4.7 ± 0.6)× 10(-3) s(-1), indicating that the denaturant accelerates the thermal inactivation. In this case, ΔH(?) was significantly reduced. By addition of 1 g/L ε-poly-L-lysine, which may adsorb onto the GOD surface to reduce the local disorder, k was decreased to (1.8 ± 0.6)× 10(-4) s(-1). In this case, ΔS(?) was reduced but ΔH(?) was not decreased much. This can be used as an important indication for selection of the enzyme stabilizer in solution.     

Adsorption of a hazardous dye, erythrosine, over hen feathers

Erythrosine is a popular dye that is widely used in cosmetics, foodstuffs, medicines, and textiles. It is highly toxic to mankind and can lead to many diseases including carcinogenicity. Removal of erythrosine has been carried out using waste material--hen feathers--as adsorbent. The effects of pH, concentration of the dye, temperature, and adsorbent dosage have been studied. Adsorption of erythrosine over hen feathers has been correlated with Freundlich and Langmuir isotherms and satisfies both models. The adsorption process has been found endothermic in nature and thermodynamic parameters, Gibb's free energy (DeltaG(0)), change in enthalpy (DeltaH(0)), and change in entropy (DeltaS(0)) have been calculated. The paper also includes results on the kinetic measurements of adsorption of the dye on hen feathers at different temperatures. The adsorption follows a first-order kinetics at all the temperatures and values of the rate constant (k(ad)) have been calculated as 0.0179, 0.0177, and 0.0172 s(-1) at 30, 40, and 50 degrees C, respectively. By rate expression and treatment of data it has been ascertained that the adsorption of erythrosine over hen feathers follows a particle diffusion mechanism.     

Quantitative comparison of kinetic stabilities of metallomacrocycle-based rotaxanes

Four mononuclear metallomacrocycles with identical cavities but different transition metals (Os(VI), Pd(II), Pt(II), and Re(I)) were prepared. With these metallomacrocycles, the corresponding rotaxanes 2-Os, 2-Pd, 2-Pt, and 2-Re were self-assembled by hydrogen-bonding interactions. The kinetic stabilities of the rotaxanes were determined quantitatively and compared with each other by (1)H NMR spectroscopic techniques, including two-dimensional exchange spectroscopy (2D-EXSY) experiments. The activation free energies (DeltaG( not equal )) for the exchange between the rotaxanes 2-Os, 2-Pd and 2-Pt and their free components were determined to be 15.5, 16.0, and 16.4 kcal mol(-1), respectively. These magnitudes imply that the rotaxanes 2-Os, 2-Pd and 2-Pt are kinetically labile at room temperature and exist only as equilibrium mixtures with free components in solution. In contrast, the rotaxane 2-Re is kinetically stable enough to be isolated in pure form by silica gel chromatography under ordinary laboratory conditions. However, at higher temperatures (>60 degrees C) 2-Re was slowly disassembled into its components until the equilibrium was established. The rate constants were measured at three different temperatures, and the Eyring plot yielded the activation enthalpy DeltaH(not equal)=35 kcal mol(-1) and the activation entropy DeltaS(not equal)=27 eu for the disassembly of the rotaxane 2-Re in Cl(2)CDCDCl(2). These thermodynamic parameters gave the activation free energy DeltaG(not equal)(off)=27.1 kcal mol(-1) at 25 degrees C. Consequently, 2-Re is one example of a novel metallomacrocycle-based rotaxane that contains a coordination bond with enough strength to allow both for isolation in pure form around room temperature and for self-assembly at higher temperatures.     

Kinetics of Dissociation of Iron(III) Complexes of Tiron in Aqueous Acid

The kinetics of dissociation of the mono, bis, and tris complexes of Tiron (1,2-dihydroxy-3,5-benzenedisulfonate) have been studied in acidic aqueous solutions in 1.0 M HClO(4)/NaClO(4), as a function of [H(+)] and temperature. In general, the kinetics can be explained by two reactions, (H(2)O)Fe(L)(n)(-1) + H(2)L right arrow over left arrow (H(2)O)Fe(L(n)H) + H(+) (k(n), k(-n)) and (HO)Fe(L)(n)(-1) + H(2)L right arrow over left arrow (H(2)O)Fe(L(n)H) (k(n)', k(-n)'), a rapid equilibrium, (H(2)O)Fe(L(n)H) right arrow over left arrow (H(2)O)Fe(L)(n) + H(+) (K(cn)), and the formation constant (H(2)O)Fe(L)(n)(-1) + H(2)L right arrow over left arrow (H(2)O)Fe(L)(n) + 2H(+). For n = 1, the reaction was observed at 670 nm, and at [H(+)] of 0.05-0.5 M at temperatures of 2.0, 14.0, 25.0, and 36.7 degrees C. For n = 2, the analogous conditions are 562 nm, at [H(+)] of 1.5 x 10(-3) to 1.4 x 10(-2) M at temperatures of 2.0, 9.0, and 14.0 degrees C. For n = 3, the conditions are 482 nm, at pH 4.5-5.7 in 0.02 M acetate buffer at temperatures of 1.8, 8.0, and 14.5 degrees C. The rate or equilibrium constants (25 degrees C) with DeltaH or DeltaH degrees (kcal mol(-1)) and DeltaS or DeltaS degrees (cal mol(-1) K(-1)) in brackets are as follows: for n = 1, k(1) = 2.3 M(-1) s(-1) (8.9, -27.1), k(-1) = 1.18 M(-1) s(-1) (4.04, -44.8), K(c1) = 0.96 M (-9.99, -33.6), K(f1) = 2.01 M (-5.14, -15.85); for n = 2, k(-2)/K(c2) = 1.9 x 10(7) (19.9, 41.5) and k(-2)'/K(c2) = 1.85 x 10(3) (1.4, -38.8) and a lower limit of K(c2) > 0.015 M; for n = 3, k(3) = 7.7 x 10(3) (15.8, 12.3), k(-3) = 1.7 x 10(7) (16.2, 28.9), K(c3) = 7.4 x 10(-5) M (4.1, -5.1), and K(f3) = 3.35 x 10(-8) (3.7, -21.7). From the variations in rate constants and activation parameters, it is suggested that the Fe(L)(2) and Fe(L)(3) complexes undergo substitution by dissociative activation, promoted by the catecholate ligands.     

Intramolecular electron transfer in lysozyme studied by time-resolved chemically induced dynamic nuclear polarization

The kinetics of the chemically induced dynamic nuclear polarization (CIDNP) produced in reactions of hen lysozyme with photosensitizers have been studied for the native state of the protein at pH 3.8 and for two denatured states. The latter were generated by raising the temperature to 80 degrees C or by combining a temperature rise (to 50 degrees C) with the addition of chemical denaturant (10 M urea). Detailed analysis of the CIDNP time dependence on a microsecond time scale revealed that, in both denatured states, intramolecular electron transfer (IET) from a tyrosine residue to the cation radical of a tryptophan residue (rate constant k(f)) is highly efficient and plays a decisive role in the evolution of the nuclear polarization. To describe the observed CIDNP kinetics with a self-consistent set of parameters, IET in the reverse direction, from a tryptophan residue to a tyrosine residue radical (rate constant k(r)), has also to be taken into account. The IET rate constants determined by analysis of the CIDNP kinetics are, at 80 degrees C: k(f) = 1 x 10(5) s(-1) and k(r) = 1 x 10(4) s(-1); at 50 degrees C in the presence of 10 M urea: k(f) = 7 x 10(4) s(-1), k(r) = 1 x 10(4) s(-1). IET does not appear to influence the CIDNP kinetics of the native state.     

Retro Diels-Alder reactions of 5,6-disubstituted-7-oxabicyclo

Several 5,6-disubstituted-7-oxabicyclo[2.2.1]hept-2-enes (1-4) were synthesized on > or = 0.1 mol scale. The heat-induced retro Diels-Alder (rDA) decomposition of these derivatives was studied by thermal analysis, and the kinetics of the rDA were measured for 4. First-order rate constants (k = 1.91-14.2 x 10(-5) s(-1)), measured at four temperatures between 124 and 150 degrees C, were used to calculate Arrhenius activation parameters Ea (34.5 +/- 0.5 kcal/mol) and ln A (1.77 +/- 0.03 x 10(4)). The observed activation energy was significantly larger (by 9.5 kcal/mol) than that previously measured for the maleic anhydride adduct 1, and this was attributed to the difference in LUMO energies for the two dienophiles. Modeling of the activation parameters found for 4 with density functional theory (DFT) calculations for similar compounds 5 and 6 gave close quantitative correlations for deltaH double dagger, deltaG double dagger, deltaS double dagger. The rDA reactions studied were found to be entropy-driven.     

Thermal inactivation of immobilized enzymes: A kinetic study

The thermal inactivation kinetics of chymotrypsin, trypsin, and—-amylase bound to silica, polyacrylamide, and polystyrene were studied at different temperatures. The inactivation curves were analyzed by a kinetic model, assuming a first-order reaction of differently stable enzyme fractions on the matrix. In all cases the assumption of two enzyme fractions with distinctly different inactivation constants was sufficient for describing the inactivation progress (standard deviations between experimental and calculated inactivation curves, 1-4%). Both the inactivation constants as well as the relative concentrations of the enzyme fractions were found to change in dependence on temperature.     

Surface enzyme kinetics for biopolymer microarrays: a combination of Langmuir and Michaelis-Menten concepts

Real-time surface plasmon resonance (SPR) imaging measurements of surface enzymatic reactions on DNA microarrays are analyzed using a kinetics model that couples the contributions of both enzyme adsorption and surface enzyme reaction kinetics. For the case of a 1:1 binding of an enzyme molecule (E) to a surface-immobilized substrate (S), the overall enzymatic reaction can be described in terms of classical Langmuir adsorption and Michaelis-Menten concepts and three rate constants: enzyme adsorption (k(a)), enzyme desorption (k(d)) and enzyme catalysis (k(cat)). In contrast to solution enzyme kinetics, the amount of enzyme in solution is in excess as compared to the amount of substrate on the surface. Moreover, the surface concentration of the intermediary enzyme-substrate complex (ES) is not constant with time, but goes to zero as the reaction is completed. However, kinetic simulations show that the fractional surface coverage of ES on the remaining unreacted sites does reach a steady-state value throughout the course of the surface reaction. This steady-state value approaches the Langmuir equilibrium value for cases where k(a)[E] > k(cat). Experiments using the 3' --> 5' exodeoxyribonuclease activity of Exonuclease III on double-stranded DNA microarrays as a function of temperature and enzyme concentration are used to demonstrate how this model can be applied to quantitatively analyze the SPR imaging data.     

Kinetic studies of AMP-dependent phosphorolysis of amylopectin catalyzed by phosphorylase b on a 27 MHz quartz-crystal microbalance

Catalytic cleavage reactions of phosphorylase b were monitored directly on an amylopectin-immobilized 27 MHz quartz-crystal microbalance (QCM). When the inactivated phosphorylase b was injected into a phosphate buffer solution of amylopectin-immobilized QCM (method A), the binding of the enzyme to amylopectin was observed as a frequency decrease (mass increase). Then, when AMP (adenosine monophosphate) was added to activate the enzyme, the frequency gradually increased (mass decreased) due to the phosphorolysis of amylopectin in the presence of phosphates as buffers. When the AMP-activated phosphorylase b was employed (method B), the continuous reaction was observed which includes both the mass increase due to the enzyme binding to amylopectin at first and then the following mass decrease due to the phosphorolysis by the AMP-activated enzyme. All kinetic parameters for the enzyme binding to the substrate (binding and dissociation rate constants, k(on) and k(off), and dissociation constant, K(d)), the AMP binding to the enzyme as activator (K(AMP)), the catalytic rate constant (k(cat)) were obtained from curve fittings of time-courses of frequency (mass) changes. The obtained kinetic parameters were compared with those from Michaelis-Menten kinetics.     

Proton-transfer reactions between nitroalkanes and hydroxide ion under non-steady-state conditions. Apparent and real kinetic isotope effects.

The kinetics of the proton-transfer reactions between 1-nitro-1-(4-nitrophenyl)ethane (NNPE(H(D))) and hydroxide ion in water/acetonitrile (50/50 vol %) were studied at temperatures ranging from 289 to 319 K. The equilibrium constants for the reactions are large under these conditions, ensuring that the back reaction is not significant. The extent of reaction/time profiles during the first half-lives are compared with theoretical data for the simple single-step mechanism and a 2-step mechanism involving initial donor/acceptor complex formation followed by unimolecular proton transfer and dissociation of ions. In all cases, the profiles for the reactions of both NNPE(H) and NNPE(D) deviate significantly from those expected for the simple single-step mechanism. Excellent fits of experimental data with theoretical data for the complex mechanism, in the pre-steady-state time period, were observed in all cases. At all base concentrations (0.5 to 5.0 mM) and at all temperatures the apparent kinetic isotope effects (KIE(app)) were observed to increase with increasing extent of reaction. Resolution of the kinetics into microscopic rate constants at 298 K resulted in a real kinetic isotope effect (KIE(real)) for the proton-transfer step equal to 22. Significant proton tunneling was further indicated by the temperature dependence of the rate constants for proton and deuteron transfers: KIE(real) ranging from 17 to 26, E(a)(D) -- E(a)(H) equal 2.8 kcal/mol, and A(D)/A(H) equal to 4.95.     

Surface characterization and direct electrochemistry of redox copper centers of bilirubin oxidase from fungi Myrothecium verrucaria

The key characteristics of multicopper oxidases are redox potentials of Type 1, Type 2 and Type 3 copper centers of enzymes. However, there is still a challenge to obtain a value of the redox "signature" of the enzymes. In this study, the electrochemical behavior of T1 and T2/T3 redox copper centers of bilirubin oxidase (BOD) from the fungi Myrothecium verrucaria was studied based on direct bioelectrocatalysis. Two distinct redox peaks corresponding to reduction and oxidation of T1 and T2/T3 redox centers of enzymes have been clearly detected in anaerobic conditions. The bioelectrocatalytic activity of the enzyme was studied in the presence of oxygen and redox mediators. The electron-transfer rate constant for BOD immobilized on carbon electrode (CE) is 1.5 s(-1). The mechanism of enzyme inactivation by ABTS has been proposed. The physical architecture of BOD layers immobilized on the electrode surface, including elemental and chemical composition, relative thickness and assembly of layers was investigated by Angle Resolved X-ray photoelectron spectroscopy. Unique peaks of BOD at 288.5 eV and of CE at 284.6 eV were used in a substrate over layer model for estimation of the thickness of the of BOD film on the carbon electrode surface.     

Roles of diffusion and activation barrier on the appearance of Marcus inversion behavior: a study of a photoinduced electron-transfer reaction in aqueous triblock copolymer (p123) micellar solutions

Photoinduced electron transfer (ET) reactions between amines and a series of coumarins have been investigated using fluorescence-quenching measurements in aqueous P123 triblock copolymer micellar solutions. Fluorescence spectral characteristics and fluorescence anisotropy measurements indicated a nearly similar microenvironment for all of the coumarins used in P123 micelles. Substantial quenching of coumarin fluorescence in the presence of amines has been observed. The quenching rates (k(q)(TR)) are largely reduced in the P123 micelle as compared to those in other micelles (sodium dodecyl sulfate (SDS), Triton-X 100 (TX-100), cetyl trimethyl ammonium bromide (CTAB), and dodecyl trimethyl ammonium bromide (DTAB)), which is probably due to larger coumarin-amine separations in the micellar phase. The k(q)(TR) values, when plotted against free energy changes (DeltaG degrees), follow a Marcus predicted bell-shaped correlation. The estimated activation energy for the ET reactions follow an inverse bell-shaped correlation with DeltaG degrees. Present results indicate that the appearance of Marcus inversion is primarily related to the changes in the activation barrier, as predicted from the Marcus ET theory. As the k(q)(TR) values are higher than the estimated bimolecular diffusional rate constant, the role of reactant diffusion on the quenching kinetics in the P123 micelle is negligible. The appearance of Marcus inversion at unexpectedly lower exergonicity has been rationalized on the basis of slow solvent relaxation and by the application of the two-dimensional ET (2DET) theory. Critical analysis of the present results establishes that the inversion in the ET rates at high exergonicity is not due to the alteration in the diffusion parameters of the reactants, as has been suggested in some recent reports. Instead, it is evident that the inversion in quenching rates at high exergonicity is due to the alteration in the activation barrier for the ET reactions.     

Hydride reduction of NAD+ analogues by isopropyl alcohol: kinetics, deuterium isotope effects and mechanism

Observed pseudo-first-order rate constants (k(obs)) of the hydride-transfer reactions from isopropyl alcohol (i-PrOH) to two NAD(+) analogues, 9-phenylxanthylium ion (PhXn(+)) and 10-methylacridinium ion (MA(+)), were determined at temperatures ranging from 49 to 82 degrees C in i-PrOH containing various amounts of AN or water. Formations of the alcohol-cation ether adducts (ROPr-i) were observed as side equilibria. The equilibrium constants for the conversion of PhXn(+) to PhXnOPr-i in i-PrOH/AN (v/v = 1) were determined, and the equilibrium isotope effect (EIE = K(i-PrOH)/K(i-PrOD)) at 62 degrees C was calculated to be 2.67. The k(H) of the hydride-transfer step for both reactions were calculated on the basis of the k(obs) and K. The corresponding deuterium kinetic isotope effects (e.g., KIE(OD)(H) = k(H)(i-PrOH)/k(H)(i-PrOD) and KIE(beta-D6)(H) = k(obs)(i-PrOH)/k(obs)((CD3)2CHOH)), as well as the activation parameters, were derived. For the reaction of PhXn(+) (62 degrees C) and MA(+) (67 degrees C), primary KIE(alpha-D)(H) (4.4 and 2.1, respectively) as well as secondary KIE(OD)(H) (1.07 and 1.18) and KIE(beta-D6)(H) (1.1 and 1.5) were observed. The observed EIE and KIE(OD)(H) were explained in terms of the fractionation factors for deuterium between OH and OH(+)(OH(delta+)) sites. The observed inverse kinetic solvent isotope effect for the reaction of PhXn(+) (k(obs)(i-PrOH)/k(obs)(i-PrOD) = 0.39) is consistent with the intermolecular hydride-transfer mechanism. The dramatic reduction of the reaction rate for MA(+), when the water or i-PrOH cosolvent was replaced by AN, suggests that the hydride-transfer T.S. is stabilized by H-bonding between O of the solvent OH and the substrate alcohol OH(delta+). This result suggests an H-bonding stabilization effect on the T.S. of the alcohol dehydrogenase reactions.     

Kinetics and mechanism of the base-catalyzed oxygenation of flavonol in DMSO-H(2)O solution.

The kinetics of the base-catalyzed oxygenation of flavonol have been investigated in 50% DMSO-H(2)O solution in the pH range 6.4-10.8 and an ionic strength of 0.1 mol L(-1) using spectrophotometric techniques at temperatures between 70 and 90 degrees C. The rate law -d[flaH]/dt = k(obs) [OH(-)][flaH][O(2)] (k(obs) = kK(1)/[H(2)O]) describes the kinetic data. The rate constant, activation enthalpy, and entropy at 353.16 K are as follows: k/mol(-1) L s(-1) = (4.53 +/- 0.07) x 10(-2), DeltaH/kJ mol(-1)= 59 +/- 4, DeltaS/J mol(-1) K(-1) = -110 +/- 11. The reaction showed specific base catalysis. It fits a Hammett linear free energy relationship for 4'-substituted flavonols and electron-releasing substituents enhanced the reaction rate. The linear correlation between the oxidation potential of the flavonols and the rate constants supports that a higher electron density on the flavonolate ion makes them more nucleophilic and the electrophilic attack of O(2) easier.     

Quenching mechanism of Rose Bengal triplet state involved in photosensitization of oxygen in ethylene glycol

The quenching rate constants of the excited triplet state of Rose Bengal (RB) by oxygen (k(obs)) were measured in ethylene glycol (EG) at different temperatures using nanosecond laser flash photolysis. Although a plot of the quenching rate constant k(obs) for RB triplet state vs oxygen concentration is linear at 20 degrees C, the oxygen dependence of k(obs) does not exhibit linearity but upward curvature at high temperatures from 130 to 140 degrees C. The upward curvature at high temperatures is not well-described by a kinetic scheme first postulated by Gijzeman et al., which is characterized by exciplex formation and a unimolecular dissociation of the exciplex to products, but instead by a more comprehensive mechanism involving a bimolecular dissociation in addition to a unimolecular one. The measurements of the oxygen dependence of k(obs) for RB triplet state at different temperatures yielded a reaction enthalpy for the exciplex formation of 150 kJ mol(-1). Due to the large exothermic reaction enthalpy, equilibrium was obtained for the exciplex at 20 degrees C even at low oxygen concentration and the bimolecular quenching by oxygen became the major dissociation process. The equilibrium attainment and bimolecular dissociation provide a linear oxygen dependence of k(obs) to all outward appearances. Therefore, linearity does not always mean that exciplex dissociation proceeds solely through a unimolecular mechanism.     

Formation of Three-Membered Rings by S(H)i Displacement. Reverse of Cyclopropyl Ring Opening(1)

The general methods, photoinitiated or peroxide-initiated free radical chain additions of halomethanes to olefins, yield 1,2-addition products at temperatures ranging from 20 to 100 degrees C. At lower temperatures, -42 to -104 degrees C, a competitive reaction, subsequent to the addition of CCl(2)X(*), yields alkylcyclopropanes. The reactions of 1-octene or 1-hexene and 1-methylcyclohexene with atomic hydrogen carried out in the presence of several transfer agents (CCl(4), CCl(3)Br, CCl(2)Br(2)) initiate a radical chain addition of CCl(2)X(*) and yield cyclized materials resulting from the S(H)i displacement of halogen by a carbon-centered radical. The radical displacement of a halogen on carbon, the reverse of homolytic displacement on cyclopropyl carbon, is dominant at low temperatures. The rate constants for cyclization (k(c)) vs transfer with halomethane (k(t)) showed isokinetic temperatures of -46 degrees C (CCl(4), 1-hexene); -35 degrees C (CCl(4), 1-methylcyclohexene). The isokinetic temperatures for the reactions of the two substrates carried out in the presence of BrCCl(3) were calculated as -204 degrees C (1-octene) and -109 degrees C (1-methylcyclohexene).     

Adsorption of Acid Blue 193 from aqueous solutions onto Na-bentonite and DTMA-bentonite

Dodecyltrimethylammonium bromide-modified bentonite (DTMA-bentonite) was prepared and tested as an adsorbent for an acid dye (Acid Blue 193, AB193) removal from aqueous solution in comparison with Na-bentonite. The effect of various experimental parameters was investigated using a batch adsorption technique. In this manner, the adsorption isotherms, adsorption kinetics, and temperature and pH effects upon Acid Blue 193 adsorption on Na-bentonite and DTMA-bentonite were thoroughly examined. Results show that a pH value of 1.5 is favorable for the adsorption of Acid Blue 193. The isothermal data could be well described by the Freundlich equation. The dynamical data fit well with the pseudo-second-order kinetic model. The adsorption capacity of DTMA-bentonite (740.5 mg g(-1)) was found to be around 11 times higher than that of Na-bentonite (67.1 mg g(-1)) at 20 degrees C. Thermodynamic parameters such as activation energy (E(a)) and change in the free energy (DeltaG(0)), the enthalpy (DeltaH(0)), and the entropy (DeltaS(0)) were also evaluated. The overall adsorption process was exothermic but it is only spontaneous at 20 degrees C. The results indicate that Na-bentonite and DTMA-bentonite could be employed as low-cost alternatives to activated carbon in wastewater treatment for the removal of color which comes from textile dyes.     

A kinetic study of S-nitrosothiol decomposition

Under anaerobic conditions S-nitrosothiols 1a-e undergo thermal decomposition by homolytic cleavage of the S-N bond; the reaction leads to nitric oxide and sulfanyl radicals formed in a reversible manner. The rate constants, k(t), have been determined at different temperatures from kinetic measurements performed in refluxing alkane solvents. The tertiary nitrosothiols 1c (k1(69 degrees C) = 13 x 10(-3) min(-1)) and 1d (k1(69 degrees C) = 91 x 10(-3) min(-1)) decomposed faster than the primary nitrosothiols 1a (k1(69 degrees C) = 3.0 x 10(-3) min(-1)) and 1b (k1(69 degrees C) = 6.5 x 10(-3) min(-1)). The activation energies (E# = 20.5-22.8 Kcal mol(-1)) have been calculated from the Arrhenius equation. Under aerobic conditions the decay of S-nitrosothiols 1a-e takes place by an autocatalytic chain-decomposition process catalyzed by N2O3. The latter is formed by reaction of dioxygen with endogenous and/or exogenous nitric oxide. The autocatalytic decomposition is strongly inhibited by removing the endogenous nitric oxide or by the presence of antioxidants, such as p-cresol, beta-styrene, and BHT. The rate of the chain reaction is independent of the RSNO concentration and decreases with increasing bulkiness of the alkyl group; this shows that steric effects are crucial in the propagation step.     

Prediction of standard interfacial electron-transfer rate constants for the Ru(NH3)6(3+/2+) couple through omega-hydroxyalkanethiol self-assembled monolayers on gold electrodes

Two relatively simple approaches are developed and used to calculate (predict) the standard interfacial electron-transfer (ET) rate constants (k degrees) of the Ru(NH3)6(3+/2+) couple dissolved in aqueous electrolyte solutions in contact with Au electrodes coated with self-assembled monolayers (SAMs) composed of HS(CH2)nOH as functions of both n and temperature. These approaches are suggested by the conclusion reached by Smalley et al. (J. Electroanal. Chem. 2006, 589, 1-6) that the interfacial ET rate of a solution-dissolved redox couple in contact with a SAM is, within 1 order of magnitude, the same as the (normalized) interfacial ET rate of a similar attached (as a constituent of a similar SAM) couple. The calculations, therefore, employ the measured electronic coupling of the attached (to Au electrodes through alkanethiolate bridges) -PyRu(NH3)5(3+/2+) couple. The two approaches also both include dynamic solvent effects on the ET kinetics and the influence of electronic coupling on the activation barrier for the ET reaction. At T=298 K and n=3, 11, and 14, the predicted rate constants are in very good agreement with the existing measurements of k degrees. However, for n<3 at 298 K, the predicted rate constants are extremely large (i.e., >4.5 cm s(-1)) and do not tend toward a limiting value. Additionally, even if the electronic coupling between a Au electrode and a Ru(NH3)6(3+/2+) moiety located at the surface of the SAM is >0.1 eV, the calculated standard rate constant is not directly proportional to the inverse of the longitudinal dielectric time of the solvent. A primary reason for both the absence of a limiting value for the predicted k degrees's at 298 K and the attenuated influence of dynamic solvent effects is the activation energy barrier suppression caused by large values of the electronic coupling.