Values Of Second-order Rate Constants Research Articles

TEMPERATURE-DEPENDENT KINETICS OF ANTHOCYANINS EXTRACTION FROM RED ROSE PETALS

The red rose is one of the perspective natural resources of anthocyanins, however kinetic study for anthocyanins extraction of red rose petals is not yet elucidated. Red rose petals were dried at 303 K and ground (particle size ~ 5 mm). The petals were extracted by aqueous acid solutions using conventional solid-liquid extraction. The efficiency of 0.1 M aqueous solutions of acids as extractants towards red rose anthocyanins was studied and it can be arranged in the following row: citric acid < acetic acid < hydrochloric acid. Kinetic investigations focusing on the temperature effect on solid-liquid extraction of anthocyanins from petals of red rose were performed at three temperatures (293, 313, and 333 K). Kinetic curves were obtained using an aqueous solution of hydrochloric acid. It was found that the rate of extraction of anthocyanins was very fast at the beginning, but it began to slow down after 60 minutes for all temperatures. The experimental kinetic curves of the extraction of anthocyanins at different temperatures were obtained and analyzed using common empirical kinetic models (the first-order, second-order kinetic models, the Peleg model, and the power-law model). The second-order kinetic model provided the best agreement with the experimental results in terms of statistical parameters (coefficient of determination (R2) and average relative error (ARE)) than other studied models. As temperature increases, the second-order rate constant values and the initial extraction rate values also increased. The second-order rate constants were used to evaluate the activation energy of extraction. The calculated activation energy of the anthocyanin extraction is 39.3 kJ/mol, which is an indication of an endothermic process. Obtained data can be used to predict the extraction of anthocyanins from red rose petals.

Inhibition of glutathione S-transferases by photoactive calix[4]arene α-ketophosphonic acids

Calix[4]arenes bearing photoactive α-ketophosphonic acid groups at the upper rim of the macrocycle were synthesized and evaluated as inhibitors of glutathione S-transferases. Irradiation at 365 nm increased the inhibition effects of some macrocyclic compounds on GSTP1-1 by more than two orders of magnitude. Calix[4]arene bis-α-ketophosphonic acids substituted at the lower rim by n-propyl or n-butyl groups showed IC50 values in the low micromolar range. Kinetics of the irreversible inhibition was described by pseudo-first-order rate constants dependent on inhibitor concentration. The values of second-order rate constants were higher for glutathione S-transferase from human placenta than for the enzyme from equine liver. Molecular docking suggested that photoactive macrocyclic compounds cover the active site of glutathione S-transferase, providing the possibility to modify the catalytically important amino acid residues during irradiation.

Synthesis, Antioxidant Activity, and Determination of Binding Parameters of Meso-Tetra-4-Actophenyl-Porphyrin and its Palladium (II) Complex with Superoxide Anion Radicals

Background: Meso-tetra-4-actophenyl-porphyrin (TAcPPH2) was synthesized by reacting 4- acetyl-benzaldehyde with pyrrole in propionic acid, and used as a ligand for the synthesis of palladium (II) complex (PdTAcPP). The structure of the ligand and the complex were characterized by NMR and electronic spectroscopy. Methods: he antioxidant activity and the binding parameters of both the ligand and its complex with superoxide anion radical . (O2 -) were measured using cyclic voltammetry based assay. The assays were based on the measurement of the anodic peak current density of . O2− electrochemically generated by reduction of molecular oxygen in DMF. Results: The complex PdTAcPP showed the highest antioxidant activity (0.73 ± 0.01 mg/mL) which is four times higher than that of the standard antioxidant α-tocopherol (3.04 ± 0.03 mg/mL). Discussion: Binding parameters like binding constants, the ratio of binding constants and binding free energies were also measured. Conclusion: The value of the binding free energy ranging from -7.89 kJmol-1 for TAcPPH2 to -17.59 kJ.mol-1for PdTAcPP suggests an electrostatic interaction of . O2− with TAcPPH2 and PdTAcPP which has been found to be the dominant interaction mode. The kinetics of the interaction reaction of the ligand and complex was quantified having second-order rate constant values equal to 0.2 and 1.3 M-1 s-1, respectively.

Collective removal of phenol and ammonia in a moving bed biofilm reactor using modified bio-carriers: Process optimization and kinetic study

The performance of a moving bed biofilm reactor (MBBR) with bio-carriers made of polypropylene-polyurethane foam (PP-PUF) was evaluated for the collective removal of phenol and ammonia. Three independent variables, including pH (5.0–8.0), retention time (2.0–12.0 h), and airflow rate (0.8–3.5 L/min) were optimized using central composite design (CCD) of response surface methodology (RSM). The maximum removal of phenol and ammonia was obtained to be 92.6, and 91.8%, respectively, in addition to the removal of 72.3% in the chemical oxygen demand (COD) level at optimum conditions. First-order and second-order kinetic models were analyzed to evaluate the pollutants removal kinetics in a MBBR. Finally, a second-order model was found to be appropriate for predicting reaction kinetics. The values of second-order rate constants were obtained to be 2.35, 0.25, and 1.85 L2/gVSS gCOD h for phenol, COD, and ammonia removal, respectively.

Antioxidant Activities, Binding Parameters, and Electrochemical Behavior of Superoxide Anion Radicals Twords 1-Ferrocenylmethylthymine and 1-Ferrocenylmethylcytosine

Background: An electrochemical method based on cyclic voltametry techniques was used to measure the antioxidant activity of two ferrocene-nucleobases. Objective: The present study aims to measure the antioxidant activity of two ferrocene derivatives bearing nucleobases; the technique is based on the reaction of the in-situ electrochemical generated superoxide anion radical with ferrocene-nucleobases. Methods: The decrease in the anodic peak current density of the O2 / O2 .− redox couple following the addition of ferrocene-nucleobases was used to measure the antioxidant activity and binding parameters of 1-ferrocenylmethylthymine and 1-ferrocenylmethylcytosine with superoxide anion radical. Conclusion: Both 1-Ferrocenylmethylthymine and 1-Ferrocenylmethylcytosine showed higher antioxidant activity (0.34 ± 0.03 and 0.045 ± 0.02 mg/mL respectively) than that of standard antioxidant α-tocopherol (3.04 ± 0.03 mg/mL). The value of the binding free energy ranging from -16.1 kJ.mol-1 for 1-ferrocenylmethylthymine to -21.8 kJ.mol-1 for 1- ferrocenylmethylcytosine suggests an electrostatic interaction of superoxide anion radical with both compounds which has been found to be the dominant interaction mode. The kinetics of the interaction reaction of the compounds was quantified having second-order rate constant values equal to 4.0 and 16.5 M-1 s-1 respectively.

Statistical optimization of process parameters for improvement of phycobiliproteins (PBPs) yield using ultrasound-assisted extraction and its kinetic study.

The present study is mainly concerned for the development of an optimal ultrasound-assisted extraction (UAE) condition for phycobiliproteins (PBPs) from Oscillatoria sp. (BTA 170) using Taguchi methodology. Four process parameters viz. solid to liquid ratio, duty cycle, electrical acoustic intensity, and pH, for UAE were optimized using Taguchi methodology for enhanced PBPs extraction. The ratio of signal to noise (S/N) was used to compute the optimized condition required to attain a higher yield of PBPs, the average performance of individual parameter and corresponding interactive effects. The statistically significant parameters with their contribution were assessed using Analysis of variance (ANOVA). Results showed that duty cycle contributed the maximum influence (30.81%) on phycocyanin (PC) extraction followed by a solid liquid ratio (28.62%), pH (22.46%) and electrical acoustic intensity (18.10%). The highest contribution on the extraction of phycoerythrin (PE) was found from pH (33.16%), followed by duty cycle (31.57%), solid to liquid ratio (22.83%) and electrical acoustic intensity (12.45%). For extraction of allophycocyanin (APC), the duty cycle, solid to liquid ratio, pH and electrical acoustic intensity contributed 29.47, 29.07, 29.03, and 12.43% respectively. Results obtained from Taguchi methodology indicated that enhanced PC (94.10%), PE (95.20%) and APC (90.54%) can be achieved with solid-liquid ratio (0.2 g/ml), electrical acoustic intensity (16.99 w/cm2), duty cycle (75%), and pH 7 than the yield of PBPs obtained under unoptimized condition. In the present study, higher yield of PC (38.99%), PE (20.84%), and APC (11.93%) were attained with UAE compared to yield obtained from homogenized Oscillatoria sp. BTA 170 using 0.05 M phosphate buffer. Batch extraction data of PBPs under UAE was fitted well with the second order model. The values of second-order rate constant (k) were computed as 6.66 × 10-4, 64.09 × 10-4 and 1.49 × 10-4 L/mg/min for extraction of PC, PE and APC respectively. The PBPs exhibited significant antioxidant property and hydrogen peroxide scavenging activity, which were increased with the enhancement of PBPs concentration.

MESO-TETRA-p-TOLYLPORPHYRIN: SYNTHESIS, CHARACTERISATION, ANTIOXIDANT ACTIVITY AND DETERMINATION OF BINDING PARAMETERS OF WITH SUPEROXIDE ANION RADICAL

In this paper we present the synthesis and the evaluation of the antioxidant activity of meso-tetra-p-tolylporphyrin (TPPH2(p-methyl)) using superoxide anion radical in DMF as a solvant. The binding parameters of the interaction between the synthesised compound and the radical were also measured using cyclic voltammetry techniques. TPPH2(p-methyl) shows lower antioxidant activity (30.7 ± 0.08 mg/mL) than that of α-tocopherol standard (3.04 ± 0.03 mg/mL). The value of the binding Gibbs energy of -8.15 KJ.mol-1 calculated for TPPH2(p-methyl) suggests an electrostatic interaction of with TPPH2(p-methyl) which has been found to be the dominant interaction mode. The kinetics of the interaction reaction was also mesured having second-order rate constant values equal to 0.2 M-1 s-1.

Structure, magnetism and reactivity of a {MnIII(μ-O)2MnIV}3+ core towards oxidation of phenols

Two complexes [(L)2MnII(OClO3)(CH3CN)](ClO4)·CH3CN (1) and [(L)4MnIII/IV2(μ-O)2](ClO4)3·2CH3CN·Et2O (2), where L is 2-((1H-pyrazol-1-yl)methyl)-1-methyl-1H-benzimidazole, have been synthesized, structurally characterized, UV–Vis and EPR spectral properties, variable-temperature (2–300 K) magnetic susceptibility and redox behavior investigated. Structural analysis reveals that the {MnIII(μ-O)2MnIV}3+ core in 2 is an example of delocalized class III system. Complex 1 has S = 5/2 ground-state. The MnIII and MnIV centres in 2 are antiferromagnetically coupled (J = −192 cm−1) and has S = 1/2 ground-state. Complex 2 exhibits characteristic 16-line EPR spectrum (120 K) centered at g ≈ 2. Reactivity of 2 towards p-X-2-tert-butylphenols (X = H, Me, OMe, tBu) has been investigated. The oxidation of phenols generate phenoxyl radical (2,4,6-tri-tert-butylphenol exhibits EPR signal due to radical)/radical-coupled bis-phenol product (in the case of 2,4-di-tert-butylphenol, DTBP). Kinetic experiments have allowed us to evaluate second-order rate constant values for the faster initial step (k2 × 10−2 M−1 s−1) for p-X-2-tert-butylphenols: 0.6 (X = H), 1.95 (Me), 3.0 (OMe), 1.85 (tBu) and for the slower second step (k2 × 10−3 M−1 s−1) for p-X-2-tert-butylphenols: 0.9 (X = H), 2.5 (Me), 4.0 (OMe). Reaction between {MnIII(μ-O)2MnIV}3+ of 2 and phenols proceeds via hydrogen atom transfer mechanism to produce oxidation products. Initial reaction supposedly generates {MnIII(O)(OH)MnIII}3+ species, which finally ends up as (L)-coordinated MnII species (ESI-MS spectrum for the reaction between 2 and DTBP).

Electrochemical behavior of superoxide anion radical towards quinones: a mechanistic approach

Among the reactive oxygen species, the superoxide anion radical (O 2 ·− ) has a fundamental role in several biological functions. Consequently, its interaction with several chemical moieties is equally important both from a biological and chemical point of view. Considering this importance, the present study demonstrates the electrochemical generation of O 2 ·− followed by its interaction with four different quinones. The fate of electrochemically generated O 2 ·− was probed in the presence of a quinone while monitoring the changes in the shape of its voltammogram. The nature of interaction was interpreted in terms of electrochemical mechanism and the interaction moiety was identified both from voltammetric and computational data (Mullikan charges). The chemical coupled reactions of the quinones with O 2 ·− were identified as hydrogen atom transfer, protonation and electron transfer in different cases. The strength of interaction was quantified in terms of association constant (Kao) and IC50 values for antiradical activity of quinones and compared with standards of vitamin C and quercetin. The kinetics of the interaction reaction was quantitated having second-order rate constant values 0.1 to 3.0 × 103 M−1 s−1.

Strong influence of intramolecular Si⋯O proximity on reactivity: Systematic molecular structure, solvolysis, and mechanistic study of cyclic N -trimethylsilyl carboxamide derivatives

A comparative alcoholysis study of N-silylated derivatives of simple heterocyclic carboxamides (lactams, imides, ureas) is presented. The second-order rate constant values span a range as wide as three orders of magnitude. On the basis of DFT calculations, a good correlation between reactivity and the Si⋯O distance was found within each family of compounds. The viability of two different reaction pathways was evaluated using a detailed computational mechanistic study of the methanolysis of cyclic urea homologues. Peculiarities in the single-crystal X-ray diffraction structures of the trimethylsilyl and trimethylsiloxy phthalimides are also discussed.

Absorption of CO2 into novel aqueous bis(3-aminopropyl)amine and enhancement of CO2 absorption into its blends with N-methyldiethanolamine

A study towards the kinetics of carbon dioxide (CO2) absorption into a novel aqueous solution of bis(3-aminopropyle)amine (APA) was carried out at 303, 308, 313 and 323K over a concentration range of 0.1–0.5kmolm−3 and different CO2 partial pressure. The reaction mechanism of CO2 with primary and secondary amines (zwitterionic mechanism) is described and based on this, experimentally obtained kinetic data are interpreted. A qualitative nuclear magnetic resonance [NMR (1D and 2D)] spectroscopy method has been applied to develop the reaction scheme for novel aqueous APA with CO2. The kinetic rate parameters were investigated according to the pseudo-first-order condition. The values of second-order rate constant, k2-APA and reaction rate with CO2 reported in this study were higher than piperazine (PZ), 2-(1-piperazinl)-ethylamine (PZEA), etc. Furthermore, enhancement factor increases significantly in comparison to aqueous MDEA solutions when the APA concentrations in the blends with MDEA varied from 0 to 0.5kmolm−3.

A novel strategy for chromogenic chemosensors highly selective toward cyanide based on its reaction with 4-(2,4-dinitrobenzylideneamino)benzenes or 2,4-dinitrostilbenes

N-(2,4-dinitrobenzylidene)-4-methoxyaniline (1), 4-(N,N-dimethylamine)-N-(2,4-dinitrobenzylidene)aniline (2), 2,4-dinitro-4′-methoxystilbene (3), and 2,4-dinitro-4′-(dimethylamino)stilbene (4) were synthesized and studied in dimethyl sulfoxide in a novel strategy as anionic chromogenic chemosensors. The color of the solutions of these compounds changed only in the presence of cyanide. The kinetic studies were performed with compounds 1–3 in an excess of cyanide. Higher second-order rate constant values were obtained for the compounds containing a methoxy group in relation to the compounds with a dimethylamino substituent, since the methoxy group donates electronic density to the 2,4-dinitrophenyl electron-accepting group less easily compared with the dimethylamino group. Stilbenes generally have greater structural rigidity than imines, facilitating the action of the substituents through the mesomeric effect. The data obtained indicate that the anion acts as a nucleophile, being responsible for CN bond breaking. The CC bridge is not broken in the stilbene dyes, but cyanide performs a nucleophilic attack on the 2,4-dinitrophenyl group.

Diuron Multilayer Adsorption on Activated Carbon from CO2 Activation of Grape Seeds

Granular activated carbons were obtained from grape seeds by pyrolysis at 600°C and subsequent physical activation with CO2 (750–900°C, 1–3 h, 25–74% burn-off). The carbon and ash content increased during the activation, reaching values of 79.0% and 11.4%, respectively. Essentially microporous materials with BET surface areas between 380 and 714 m2/g were obtained. The performance of the activated carbon in the adsorption of diuron in aqueous phase was studied within the 15–45°C temperature range. Equilibrium data showed that the maximum uptake increased with temperature from 120 to 470 µmol/g, also evidencing some dependence of the adsorption mechanism on temperature. Data were fitted to five isotherm models [Langmuir, Freundlich, Dubinin–Radushkevich, BET, and GAB (Guggenheim, Anderson, and de Boer)]. Kinetic data were analyzed using first- and second-order rate equations and intraparticle diffusion model. The second-order rate constant values obtained (2.8–13.5 × 10−3 g/µmol min) showed that the hollow core morphology of the material favors the adsorption kinetics.

Stabilizing effect of citrate buffer on the photolysis of riboflavin in aqueous solution

In the present investigation the photolysis of riboflavin (RF) in the presence of citrate species at pH 4.0–7.0 has been studied. A specific multicomponent spectrophotometric method has been used to assay RF in the presence of photoproducts during the reactions. The overall first-order rate constants (kobs) for the photolysis of RF range from 0.42 to 1.08×10–2min−1 in the region. The values of kobs have been found to decrease with an increase in citrate concentration indicating an inhibitory effect of these species on the rate of reaction. The second-order rate constants for the interaction of RF with total citrate species causing inhibition range from 1.79 to 5.65×10–3M−1min−1 at pH 4.0–7.0. The logk–pH profiles for the reactions at 0.2–1.0M citrate concentration show a gradual decrease in kobs and the value at 1.0M is more than half compared to that of k0, i.e., in the absence of buffer, at pH 5.0. Divalent citrate ions cause a decrease in RF fluorescence due to the quenching of the excited singlet state resulting in a decrease in the rate of reaction and consequently leading to the stabilization of RF solutions. The greater quenching of fluorescence at pH 4.0 compared to that of 7.0 is in accordance with the greater concentration of divalent citrate ions (99.6%) at that pH. The trivalent citrate ions exert a greater inhibitory effect on the rate of RF photolysis compared to that of the divalent citrate ions probably as a result of excited triplet state quenching. The values of second-order rate constants for the interaction of divalent and trivalent citrate ions are 0.44×10–2 and 1.06×10–3M–1min–1, respectively, indicating that the trivalent ions exert a greater stabilizing effect, compared to the divalent ions, on RF solutions.

Electrochemical characterization of ortho and meta-nitrotoluene derivatives in different electrolytic media. Free radical formation

This paper reports a comprehensive study by tast polarography, d.p.p., and cyclic voltammetry on the electrochemical reduction in different electrolytic media of ortho- and meta-nitrotoluene derivatives. Controlled potential electrolysis was used to generate the nitro radical anions and its detection by cyclic voltammetry and UV–Vis spectroscopy was performed. In protic media (30% ethanol/0.1 M Britton–Robinson buffer pH 2–12) both derivatives gave a sharp irreversible well-defined peak in all the pH range on Hg in a reaction involving four electrons to give the hydroxylamine derivative. In this medium meta-nitrotoluene is easier reduced in approximately 80 mV than that of the ortho-nitrotoluene. In mixed aqueous organic media (0.015 M aqueous citrate/DMF: 60:40, 0.3 M KCl and 0.1 M TBAI) at pH>8, the isolation and the electrochemical characterization of the one-electron reduction product, the nitro radical anion was achieved. At a 1 mM of nitrotoluene concentration, the average dismutation second-order rate constant values, k 2, were: 11,000±170 and 6900±72 M −1 s −1 for ortho-and meta-nitrotoluene, respectively. In aprotic media (0.1 M TBAI in DMF), the nitro radical anions were more stable than that of mixed media, with the following dismutation second-order rate constant values, k 2: 5800±35 and 4700±42 M −1 s −1 for ortho- and meta-nitrotoluene, respectively. Also, a comparison between nitrotoluene derivatives and some nitrocompounds of pharmacological relevance relating the effects of substituents on nitrobenzene and the electrolytic media composition on both the easiness of formation and stability of radicals is presented.

An electrochemical evidence of free radicals formation from flutamide and its reactivity with endo/xenobiotics of pharmacological relevance

This paper reports the feasibility of free radicals formation from flutamide by using cyclic voltammetry. The electrochemical characteristics and the reactivity of the one-electron reduction product from flutamide in mixed media with thiol compounds and the nuclei acid bases are characterized. Results from this paper show the thermodynamic feasibility of free radical formation expressed for both the cathodic peak potential and the second-order rate constant values. The reactivity of the radical towards thiol compounds (glutathione, cysteamine, N-acetylcysteine) and the nuclei acid base, adenine, thymine and uracil were quantitatively assessed through the calculation of the respective interaction rate constants. Based on these results, the following tentative order of reactivity towards the xeno/endobiotics is as follows: cysteamine>uracil>glutathione>adenine> N-acetylcysteine>thymine. The stability of the nitro radical anion electrochemically generated from flutamide showed a linear dependence with pH.

Kinetic and Mechanistic Studies of the Peroxynitrite-Mediated Oxidation of Oxymyoglobin and Oxyhemoglobin

Kinetic studies of the peroxynitrite-mediated oxidations of oxymyoglobin (MbFeO(2)) and oxyhemoglobin (HbFeO(2)) showed that the mechanisms of these reactions are more complex than what had previously been reported; both reactions proceed in two steps. For myoglobin, we found that the small amount of deoxymyoglobin (MbFe(II)) which is in equilibrium with MbFeO(2) is first oxidized by peroxynitrous acid to ferryl myoglobin (MbFe(IV)=O). Then, in the second step, MbFe(IV)=O is reduced by peroxynitrous acid to metmyoglobin (metMb). The second-order rate constant values obtained at pH 7.3 and 20 degrees C for the two steps are (5.4 +/- 0.2) x 10(4) and (2.2 +/- 0.1) x 10(4) M(-)(1) s(-)(1), respectively. Analogous studies with hemoglobin suggest that its reaction with peroxynitrite follows the same mechanism. In this case, the second-order rate constant values measured at pH 7.0 and 20 degrees C for the two steps are (8.8 +/- 0.4) x 10(4) and (9.4 +/- 0.7) x 10(4) M(-)(1) s(-)(1), respectively. A possible mechanism in the absence as well as in the presence of CO(2) and the relevance of these reactions in vivo are discussed.

Evolutionary Optimization of Enzyme Kinetic Parameters; Effect of Constraints

The distribution of the kinetic parameters of enzymic reactions is theoretically studied, on the assumption that, during evolution, the increase of reaction rates was an important target of natural selection. In extension of previous work on the optimization of enzyme kinetic parameters, the influence of constraints concerning upper limits of the individual rate constants is analyzed. The concept of "evolutionary effort" is applied to derive an expression for the cost function, leading to an overall upper limit for the values of the rate constants. The resulting optimization problem is solved for ordered mechanisms involving different numbers of elementary steps. It is shown that the optimum for the enzyme kinetic parameters strongly depends on the concentrations of the reactants. Low reactant concentrations lead generally to a tight binding of the reactants, while high concentrations result in a weak binding, favouring the rate constants of the other steps. In particular, states of maximum activity are not always characterized by maximal values of second-order rate constants. The results support the hypothesis that there is a mutual adaptation of Michaelis constants and reactant concentrations in an evolutionary timescale. In the limit of infinite values of the exponent of the cost function the results of the present "overall limit model" turn into the results of a model which takes into account individual upper limits for rate constants ("separate limit model"). The distributions of optimal rate constants are discussed in terms of free-energy profiles. The model is applied to the interpretation of the kinetic data of triosephosphate isomerase and inorganic pyrophosphatase.

Rate constants for the reactions of the hydroxyl radical with indane, indene and styrene

Values of second-order rate constants for the reactions [graphic omitted], have been measured using a discharge flow system operating at 295 K over the pressure range 65–1000 Pa. The OH concentration was monitored by resonance fluorescence and helium was used as the carrier gas. For indane the predominant mechanism is abstraction with a rate constant of (5.6 ± 0.8)× 109dm3 mol–1s–1. For both indene and styrene addition is the major reaction channel, giving rise to a pressure-dependent rate constant.