Effect of sucrose on BSA denatured aggregation at high concentration studied by the iso-conversional method and the master plots method

The kinetics of protein thermal transition is of a significant interest from the standpoint of medical treatment. The effect of sucrose (0–15 mass%) on bovine serum albumin denatured aggregation kinetics at high concentration was studied by the iso-conversional method and the master plots method using differential scanning calorimetry. The observed aggregation was irreversible and conformed to the simple order reaction. The denaturation temperature (T _m), the kinetic triplets all increased as the sucrose concentration increased, which indicated the remarkable stabilization effect of sucrose. The study purpose is to provide new opportunities in exploring aggregation kinetics mechanisms in the presence of additive.     

Non-isothermal oxidation kinetics of single- and multi-walled carbon nanotubes up to 1273 K in ambient

Non-isothermal oxidation kinetics of single- and multi-walled carbon nanotubes (CNTs) have been studied using thermogravimetry up to 1273 K in ambient using multiple heating rates. One single heating rate based model-fitting technique and four multiple heating rates based model-free isoconversional methods were used for this purpose. Depending on nanotube structure and impurity content, average activation energy (E _a), pre-exponential factor (A), reaction order (n), and degradation mechanism changed considerably. For multi-walled CNTs, E _a and A evaluated using model-fitting technique were ranged from 142.31 to 178.19 kJ mol⁻¹, respectively, and from 1.71 × 10⁵ to 5.81 × 10⁷ s⁻¹, respectively, whereas, E _a for single-walled CNTs ranged from 83.84 to 148.68 kJ mol⁻¹ and A from 2.55 × 10² to 1.18 × 10⁷ s⁻¹. Although, irrespective of CNT type, the model-fitting method resulted in a single kinetic triplet i.e., E _a, A, and reaction mechanism, model-free isoconversional methods suggested that thermal oxidation of these nanotubes could be either a simple single-step mechanism with almost constant activation energy throughout the reaction span or a complex process involving multiple mechanisms that offered varying E _a with extent of conversion. Criado method was employed to predict degradation mechanism(s) of these CNTs.     

Studies of the interaction between apigenin and bovine serum albumin by spectroscopic methods

The interaction between apigenin (Ap) and bovine serum albumin (BSA) in physiological buffer (pH = 7.4) is investigated by fluorescence quenching technique and UV-vis absorption spectra. The results reveal that Ap could strongly quench the intrinsic fluorescence of BSA. The quenching mechanism of Ap for BSA varies with the change of Ap concentration. when Ap concentration is lower, it is a static quenching procedure, when Ap concentration is higher, a combined quenching (both static and dynamic) would operate. The apparent binding constants Ka and number of binding sites n of Ap with BSA are obtained by fluorescence quenching method. The thermodynamic parameters, enthalpy change (Δ_r H ^m and entropy change (Δ_r S ^m ), are calculated to be −15.382 kJ mol⁻¹ K⁻¹ < 0 and 104.888 J mol⁻¹ K⁻¹ > 0, respectively, which indicate that the interaction of Ap with BSA is driven mainly by hydrogen bonding and hydrophobic interactions. The distance r between BSA and Ap is calculated to be 1.89 nm based on F?rster’s non-radiative energy transfer theory. The results of synchronous fluorescence spectra show that binding of Ap with BSA cannot induce conformational changes in BSA.     

Study of the thermooxidative degradation kinetics of poly(tetrafluoroethene) using iso-conversional calculation procedure

The thermooxidative degradation kinetics of poly(tetrafluoroethene) (PTFE) in air flow has been studied at different heating rates (6, 10, 12 and 15 K min⁻¹) by non-isothermal differential thermal analysis (DTA). Six calculation procedures based on single TG curves and iso-conversional method, as well as 27 mechanism functions were used. The comparison of the results obtained with these calculation procedures showed that they strongly depend on the selection of proper mechanism function for the process. Therefore, it is very important to determine the most probable mechanism function. In this respect the iso-conversional calculation procedure turned out to be more appropriate. In the present work, the values of apparent activation energy E, pre-exponential factor A in Arrhenius equation, as well as the changes of entropy ΔS ^≠, enthalpy ΔH ^≠ and free Gibbs energy ΔG ^≠ for the formation of the activated complex from the reagent are calculated. All calculations were performed using programs compiled by ourselves.     

Synthesis,spectroscopic, thermal and biological aspect of mixed ligand copper(II) complexes

Derivative of 8-hydroxyquinoline i.e. Clioquinol is well known for its antibiotic properties, drug design and coordinating ability towards metal ion such as Copper(II). The structure of mixed ligand complexes has been investigated using spectral, elemental and thermal analysis. In vitro anti microbial activity against four bacterial species were performed i.e. Escherichia coli, Pseudomonas aeruginosa, Serratia marcescens, Bacillus substilis and found that synthesized complexes (15–37 mm) were found to be significant potent compared to standard drugs (clioquinol i.e. 10–26 mm), parental ligands and metal salts employed for complexation. The kinetic parameters such as order of reaction (n = 0.96–1.49), and the energy of activation (E _a = 3.065–142.9 kJ mol⁻¹), have been calculated using Freeman–Carroll method. The range found for the pre-exponential factor (A), the activation entropy (S* = −91.03 to−102.6 JK⁻¹ mol⁻¹), the activation enthalpy (H* = 0.380–135.15 kJ mol⁻¹), and the free energy (G* = 33.52–222.4 kJ mol⁻¹) of activation reveals that the complexes are more stable. Order of stability of complexes were found to be [Cu(A⁴)(CQ)OH] · 4H₂O > [Cu(A³)(CQ)OH] · 5H₂O > [Cu(A¹)(CQ)OH] · H₂O > [Cu(A²)(CQ)OH] · 3H₂O     

Kinetics of the exothermic decomposition of 1-nitro-3-(β,β,β-trinitroethyl)-4,5-dinitroiminoimidazolidine-2-one

The kinetic parameters of the exothermic decomposition of the title compound in a temperatureprogrammed mode have been studied by means of DSC. The DSC data obtained are fitted to the integral, differential, and exothermic rate equations by the linear least-squares, iterative, combined dichotomous, and least-squares methods, respectively. After establishing the most probable general expression of differential and integral mechanism functions by the logical choice method, the corresponding values of the apparent activation energy (E _a), preexponential factor (A), and reaction order (n) are obtained by the exothermic rate equation. The results show that the empirical kinetic model function in differential form and the values of E _a and A of this reaction are (1 − α)^−4.08, 149.95 kJ mol⁻¹, and 10^14.06 s⁻¹, respectively. With the help of the heating rate and kinetic parameters obtained, the kinetic equation of the exothermic decomposition of the title compound is proposed. The critical temperature of thermal explosion of the compound is 155.71°C. The above-mentioned kinetic parameters are quite useful for analyzing and evaluating the stability and thermal explosion rule of the title compound. The text was submitted by the authors in English.     

A kinetic analysis of thermal decomposition of polyaniline/ZrO<Subscript>2</Subscript> composite

Synthesis, characterization and thermal analysis of polyaniline (PANI)/ZrO₂ composite and PANI was reported in our early work. In this present, the kinetic analysis of decomposition process for these two materials was performed under non-isothermal conditions. The activation energies were calculated through Friedman and Ozawa-Flynn-Wall methods, and the possible kinetic model functions have been estimated through the multiple linear regression method. The results show that the kinetic models for the decomposition process of PANI/ZrO₂ composite and PANI are all D₃, and the corresponding function is ƒ(α)=1.5(1−α)^2/3[1−(1-α)^1/3]−1. The correlated kinetic parameters are E _a=112.7±9.2 kJ mol⁻¹, lnA=13.9 and E _a=81.8±5.6 kJ mol⁻¹, lnA=8.8 for PANI/ZrO₂ composite and PANI, respectively.     

Electrochemical study on screen-printed carbon electrodes with modification by iron nanoparticles in Fe(CN)<Subscript>6</Subscript><Superscript>4−/3−</Superscript> redox system

The remarkable enhancement of electron transfer on screen-printed carbon electrodes (SPCEs) with modification by iron nanoparticles (Fe_nano), coupled with Fe(CN)₆ ^4−/3− redox species, was characterized with an increase of electroactive area (A _ea) at electrode surface together with a decrease of heterogeneous electron transfer rate constant (k°) in the system. Hence, Fe_nano-Fe(CN)₆ ³⁻ SPCEs with deposition of glucose oxidase (GOD) demonstrated a higher sensitivity to various glucose concentrations than Fe(CN)₆ ³⁻/GOD-deposited SPCEs. In addition, an inhibited diffusion current from cyclic voltammograms was also observed with an increase in redox concentration and complicated the estimation of A _ea. Further analysis by the electrochemical impedance method, it was shown that this effect might be resulted from the electrode surface blocking by the products of activated complex decomposition.     

Protein denaturation kinetic processes of a simple and a complex reaction mechanism analyzed by an iso-conversional method

The protein denaturation kinetic processes of a simple and a complex reaction mechanism represented by bovine serum albumin and hen egg-white lysozyme were analyzed by an iso-conversional method using differential scanning calorimetry. After differential scanning calorimetry using the iso-conversional method, the results were found to pose distinct contrasts between the two proteins. Bovine serum albumin showed an increasing peak temperature of the transition as the scan rate and protein concentration increased, whereas hen egg-white lysozyme exhibited almost constant peak temperature. The differential scanning calorimetry transition of bovine serum albumin was calorimetrically irreversible, while one part of hen egg-white lysozyme denaturation process was irreversible during which aggregation occurred and the other part was reversible. The iso-conversional method indicated that the value of bovine serum albumin apparent activation energy hardly varied with the degree of conversion, which showed that the denaturation kinetic process should conform to single reaction model. Using the master plots method, the most possible kinetic model for bovine serum albumin denaturation might be described by F _n kinetic model. On the contrary, the hen egg-white lysozyme value of apparent activation energy decreased with the increase of degree of conversion. It was not a process involving the two standard reversible states, and can be described by the simple Lumry–Eyring model. The iso-conversional method provides new opportunities in exploring a simple and a complex reaction mechanism of protein denaturation.     

Mass Spectrometric Determination of Association Constants of Bovine Serum Albumin (BSA) with para-Sulphonato-Calix[n]arene Derivatives

Electrospray Ionization Mass Spectrometry (ESI/MS) has been used to determine the association constants (K_As) and binding stoichiometries for parent para-Sulphonato-calix[n]arenes and their derivatives with bovine serum albumin (BSA). K_A values were determined by titration experiments using a constant concentration of protein. K_A measurements were carried out in a methanol–formic acid solution. 5,11,17,23–tetra-Sulphonato-calix[4]arene (1a) and 25-mono-(2-aminoethoxy)-5,11,17,23-tetra-Sulphonato-calix[4]arene (1d) interact strongly with BSA showing 3 non-equivalent binding sites with K_A1 = 7.69 × 10⁵ M⁻¹, K_A2 = 3.85 × 10⁵ M⁻¹, K_A3 = 0.33 × 10⁵ M⁻¹ and K_A1 = 1.69 × 10⁵ M⁻¹, K_A2 = 2.94 × 10⁵ M⁻¹, K_A3 = 0.60 × 10⁵ M⁻¹, respectively. The strength of the interactions between the calixarene and BSA is inversely proportional to the size of macrocyclic ring: n = 4 > n=6>>n=8.     

Synthesis,molecular structure,non-isothermal decomposition kinetics and adiabatic time to explosion of 3,3-dinitroazetidinium 3,5-dinitrosalicylate

The title compound 3,3-dinitroazetidinium (DNAZ) 3,5-dinitrosalicylate (3,5-DNSA) was prepared and the crystal structure has been determined by a four-circle X-ray diffractometer. The thermal behavior of the title compound was studied under a non-isothermal condition by DSC and TG/DTG techniques. The kinetic parameters were obtained from analysis of the TG curves by Kissinger method, Ozawa method, the differential method and the integral method. The kinetic model function in differential form and the value of E _a and A of the decomposition reaction of the title compound are f(α)=4α^3/4, 130.83 kJ mol⁻¹ and 10_13.80s⁻¹, respectively. The critical temperature of thermal explosion of the title compound is 147.55 °C. The values of ΔS ^≠, ΔH ^≠ and ΔG ^≠ of this reaction are −1.35 J mol⁻¹ K₋₁, 122.42 and 122.97 kJ mol⁻¹, respectively. The specific heat capacity of the title compound was determined with a continuous C _p mode of mircocalorimeter. Using the relationship between C _p and T and the thermal decomposition parameters, the time of the thermal decomposition from initiation to thermal explosion (adiabatic time-to-explosion) was obtained.     

Thermal decomposition study of bismuth (III) trichloride complex with 1,10-phenanthroline as the ligand

The complex BiCl₃·L (L = 1,10-phenanthroline) was synthesized and characterized by elemental analysis and infrared spectroscopy (IR). Infrared spectroscopy data suggested that the nitrogen atom of the aromatic ring is bonded to the bismuth atom. The kinetic study of thermal degradation was determined by non-isothermal thermogravimetry. Two methods based on integral equation of Coats-Redfern, were necessary for determining the kinetic trip: the fitting method, known as the checking model and an iso-conversional method. The latter gives the activation energy for each degree of conversion and the first, the kinetic model gives activation energy and the pre-exponential factor for thermal decomposition processes that occur through a single simple mechanism. The kinetic parameters, E _a and log A for the heating rates of 5, 10, and 15 min K⁻¹, were determined considering the decomposition model denoted by F0/R1 in the range of degree of conversion between 0.065 and 0.71.     

Non-isothermal kinetics of free-radical polymerization of 2,2-dinitro-1-butyl acrylate

The free-radical bulk polymerization of 2,2-dinitro-1-butyl-acrylate (DNBA) in the presence of 2,2′-azobisisobutyronitrile (AIBN) as the initiator was investigated by DSC in the non-isothermal mode. Kissinger and Ozawa methods were applied to determine the activation energy (E _a) and the reaction order of free-radical polymerization. The results showed that the temperature of exothermic polymerization peaks increased with increasing the heating rate. The reaction order of non-isothermal polymerization of DNBA in the presence of AIBN is approximately 1. The average activation energy (92.91±1.88 kJ mol ⁻¹) obtained was smaller slightly than the value of E _a=96.82 kJ mol⁻¹ found with the Barrett method.     

Thermal behavior study and decomposition kinetics of salbutamol under isothermal and non-isothermal conditions

The thermal decomposition of salbutamol (β₂ — selective adrenoreceptor) was studied using differential scanning calorimetry (DSC) and thermogravimetry/derivative thermogravimetry (TG/DTG). It was observed that the commercial sample showed a different thermal profile than the standard sample caused by the presence of excipients. These compounds increase the thermal stability of the drug. Moreover, higher activation energy was calculated for the pharmaceutical sample, which was estimated by isothermal and non-isothermal methods for the first stage of the thermal decomposition process. For isothermal experiments the average values were E _act=130 kJ mol⁻¹ (for standard sample) and E _act=252 kJ mol⁻¹ (for pharmaceutical sample) in a dynamic nitrogen atmosphere (50 mL min⁻¹). For non-isothermal method, activation energy was obtained from the plot of log heating rates vs. 1/T in dynamic air atmosphere (50 mL min⁻¹). The calculated values were E _act=134 kJ mol⁻¹ (for standard sample) and E _act=139 kJ mol⁻¹ (for pharmaceutical sample).     

Effects of CuCl2��6H2O and ZnCl2��6H2O on the viscosity of aqueous ethanol mixtures

The effects of CuCl₂ and ZnCl₂ on the viscosity in aqueous ethanol mixtures (10%–50% v/v) were studied in the concentration range 1.0×10⁻²–8.0×10⁻² mol·dm⁻³ at different temperatures. It was found that the viscosities increased with an increase in the concentration of the salts and percent composition of ethanol content, whereas it decreased with an increase in temperature. Ion-ion and ion-solvent interactions are determined with the help of A- and B-coefficients of Jones-Dole equation. The values of A- and B-coefficients are irregular and increase with a rise in temperature and also with an increase in ethanol contents for both salts. Negative values of B-coefficients show that ion solvent interactions is comparatively small and suggest that CuCl₂ and ZnCl₂ behave as structure breakers in aqueous ethanol mixtures. Thermodynamic parameters like the energy of activation (E _η ) and change in entropy of activation (ΔS*) were also evaluated which confirm the structure breaker behavior of salts in aqueous ethanol mixtures.     

Comparative kinetic study of the non-isothermal thermal curing of <Emphasis Type="Italic">bis</Emphasis>-GMA/TEGDMA systems

The thermal polymerization kinetics of dimethacrylate monomers was studied by differential calorimetry using non-isothermal experiments. The kinetic analysis compared the following procedures: isoconversional method (model-free method), reduced master curves, the isokinetic relationship (IKR), the invariant kinetic parameters (IKP) method, the Coats-Redfern method and composite integral method I. Although the study focused on the integral methods, we compared them to differential methods. We saw that even relatively complex processes (in which the variations in the kinetic parameters were only slight) can be described reasonably well using a single kinetic model, so long as the mean value of the activation energy is known (E). It is also shown the usefulness of isoconversional kinetic methods, which provide with reliable kinetic information suitable for adequately choosing the kinetic model which best describes the curing process. For the system studied, we obtained the following kinetic triplet: f(α)=α^0.6(1−α)^2.4, E=120.9 kJ mol⁻¹ and lnA=38.28 min⁻¹.     

Mechanism of Oxidation of <Emphasis Type="SmallCaps">L</Emphasis>-Cysteine by Hexachloroiridate(IV) — A Kinetic Study

The kinetics of oxidation of L-Cysteine in aqueous HClO₄ medium were studied using a one-equivalent oxidant, hexachloroiridate(IV). The reaction exhibits second-order dependence with respect to hexachloroiridate(IV) and first-order in cysteine. The rate decreases with increase in hydrogen ion concentration indicating that the zwitterionic form of cysteine is more reactive. Cysteic acid is identified as the product of oxidation. A suitable mechanism involving the formation of [IrCl₆]²⁻ – sulphur bonded intermediate is proposed. The activation parameters of the reaction are computed using the linear least squares method and the values of E_a and ΔS^# are found to be 27.97±1.82 kJ mol⁻¹ and −51.30±6.0 J K⁻¹mol⁻¹, respectively.     

Studies of dehydration kinetics of Li2SO4·H2O by the master plots method

The kinetics of Li₂SO₄·H₂O dehydration in static air atmosphere was studied on the basis of nonisothermal measurements by differential scanning calorimetry. Dehydration data were subjected to an integral composite procedure, which includes an isoconversional method, a master plots method and a model-fitting method. Avrami-Erofeev equation was found to describe all the experimental data in the range of conversion degrees from 0.1 to 0.9. The determined activation energy equals 65.45 kJ·mol⁻¹ with standard deviation ±0.47 kJ·mol⁻¹. The estimated value of parameter m in Avrami-Erofeev equation is 2.15 with standard deviation ±0.11. Also, the obtained pre-exponential factor is 7.79×10⁵ s⁻¹ with standard deviation ±0.55×10⁵ s⁻¹. The results show that the present integral composite procedure gives self-consistent kinetic parameters.     

Kinetic model of poly(3-hydroxybutyrate) thermal degradation from experimental non-isothermal data

The non-isothermal data given by TG curves for poly(3-hydroxybutyrate) (PHB) were studied in order to obtain a consistent kinetic model that better represents the PHB thermal decomposition. Thus, data obtained from the dynamic TG curves were suitably managed in order to obtain the Arrhenius kinetic parameter E according to the isoconversional F-W-O method. Once the E parameters is found, a suitable logA and kinetic model (f(α)) could be calculated. Hence, the kinetic triplet (E±SD, logA±SD and f(α)) obtained for the thermal decomposition of PHB under non-isothermal conditions was E=152±4 kJ mol⁻¹, logA=14.1±0.2 s⁻¹ for the kinetic model, and the autocatalytic model function was: f(α)=α^m(1−α)ⁿ=α^0.42(1−α)^0.56.     

Thermal decomposition of complexes of cadmium(II) and mercury(II) with triphenylphosphanes

A series of substituted triphenylphosphane complexes of the type CdL₂X₂ (L= triorthotolylphosphane or trimetatolylphosphane; X=Cl⁻, Br⁻ or I⁻) and HgL₂X₂ (L=triphenylphosphane or triorthotolylphosphane) was prepared fresh. The thermal decomposition was carried out in air with heating rate programmed at 10°C min⁻¹ and it revealed that the complexes with ortho derivative were less stable and the triphenylphosphane moiety leaves along with halogen in the first step. All the complexes were stable up to 210°C. However, the stability order of the tetrahedral complexes was X=Cl>Br. Values of n, E, lnA and ΔS ^# have been approximated and compared. Complexes having Br have higher E _a, lnA and ΔS ^# values than that having Cl.