Interaction of insecticides with human plasma lipoproteins

The binding of chlorinated hydrocarbon, carbamate and organophosphate insecticides to human low density plasma lipoproteins (LDL) and high density plasma lipoproteins (HDL) was studied at pH 7.0 and 16°C and 26°C by equilibrium dialysis, difference spectra and fluorescence. The results suggest interaction to be a partitioning rather than a stoichiometric binding process. Distribution is related to lipid content and composition of the lipoproteins. The K-values vary from 3 × 10⁵ M^?1 for 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) to less than 10 M^?1 for nicotine and aldicarb, and ΔG_tr° is in the range of 7400 cal for DDT to less than 1000 cal for aldicarb and nicotine. The K and ΔG_tr° are inversely related to the water solubility of the insecticides. A significant role of plasma lipoproteins in the transport of slightly water soluble insecticides is suggested.     

Thermodynamics and coordination characteristics of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 extraction complex

The extraction equilibrium of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 complex was carried out in the crown ether1,2-dichloroethaneHCl aqueous solution system at different temperatures. The extraction complex has the overall composition (L)₂·(H₃O⁺·χH₂O)₂·UO₂Cl₄²⁻ (L = dicyclohexano-24-crown-8). The values of the extraction equilibrium constants (K_ex) increase steadily with a decrease in temperature: 13.5 (298 K), 7.96 (301 K), 4.20 (303 K) and 2.07 (305 K). A plot of log K_ex against 1/T shows a straight line. The value of the enthalpy change, ΔH°, was calculated from the slope and equals −212 kJ mol⁻¹. The value of the entropy change, ΔS°, was calculated from ΔH° and K_ex and equals −690 J K⁻¹ mol⁻¹, whereas ΔG° = −6.45 kJ mol⁻¹. Comparing these thermodynamic parameters with those of the dicyclohexano-18-crown-6 isomer A [1] (ΔS° = −314 J K⁻¹ mol⁻¹, ΔH° = −101 kJ mol⁻¹ and ΔG° = −8.37 kJ mol⁻¹), it can be seen that ΔH° and ΔS° are more negative for the former than for the latter, and both are enthalpy-stabilized complexes. The molecular structure of the complex has the feature that there are two H₅O₂⁺ ions in it, in contrast to the H₃O⁺ ions in the dicyclohexano-18-crown-6 isomer A complex [1]. Each of the H₅O₂⁺ ions is held in the crown ether cavity by four hydrogen bonds. The H₅O₂⁺ ion has a central bond. The uranium atom forms UO₂Cl₄²⁻ as a counterion away from the crown ether. The formation of this complex is in good agreement with more negative entropy change and less negative free energy change, as mentioned above.     

Calorimetric measurement of enthalpy change in the isothermal helix-coil transition of poly(L-ornithine) in aqueous solution.

The enthalpy change associated with the isothermal pH-induced uncharged coil-to-helix transition ΔH_h° in poly(L -ornithine) in 0.1 N KCl has been determnined calorimetrically to be ?1530 ± 210 and ?1270 ± 530 cal/mol at 10° and 25°C, respectively. Titration data provided information about the state of charge of the polymer in the calorimetric experiments, and optical rotatory dispersion data about its conformation. In order to compute ΔH_h°, the observed calorimetric heat was corrected for the heat of breaking the sample cell, the heat of dilution of HCl, the heat of neutralization of the OH^? ion, and the heat of ionization of the δ-amino group in the random coil. The latter was obtained from similar calorimetric measurements on poly(D ,L -ornithine). Since it was discovered that poly(L -ornithine) undergoes chain cleavage at high pH, the calorimetric measurements were carried out under conditions where no degradation occurred. From the thermally induced uncharged helix–coil transition curve for poly(L -ornithine) at pH 11.68 in 0.1 N KCl in the 0°–40°C region, the transition temperature T_tr and the quantity (?θ_h/?T)_Ttr have been obtained. From these values, together with the measured values of ΔH_h°, the changes in the standard free energy ΔG_h° and entropy ΔG_h°, associated with the uncharged coil-to-helix transition at 10°C have been calculated to be ?33 cal/mol and ?5.3 cal/mol deg, respectively. The value of the Zimm–Bragg helix–coil stability constant σ has been calculated to be 1.4 × 10^?2 and the value of s calculated to be 1.06 at 10°C, and between 0.60 and 0.92 at 25°C.     

Steric effects on activation energy for the electrochemical oxidation of organocobaloximes

Heterogeneous electron transfer rate constants were determined as a function of electrode potential for one-electron oxidation in acetonitrile (AN) at O °C of a series of organocobaloximes [R-Co(DH)₂L] bearing widely different organic groups. Reaction entropies were determined by voltammetric half-wave potential (E^r_{$\text{1}\text{2}$}) measurements in a non-isothermal cell. The electron transfer coefficients and reorganization parameters were calculated following the Marcus theory. The reaction free energies relative to a reference couple ΔG° are linearly correlated with the polar Taft constant of the organic substituent R.The steric effects on ΔG° are shown by the correlation of E^r_{$\text{sol1}\text{2}$} with the CoC bond distance.Assuming constancy of double layer effects along the series in the given solution composition, the trends of the apparent rate constants k_app were considered in order to evaluate the effects of the nature of the organic ligand on the activation energy ΔG³ of the electron transfer. The steric effects on ΔG³ are pointed out i.a. by consideration of the relationship between ΔG³ and ΔG°.     

Temperature and pressure dependent equilibrium studies of 1, 4, 8, 11-tetramethyl-1, 4, 8, 11-tetraazacyclotetradecane nickel(ii) in coordinating solvents

In coordinating solvents, the complex 1, 4, 8, 11- tetramethyl-1, 4, 8, 11-tetraazacyclotetradecane nickel(II) bisperchlorate exists as an equilibrium mixture involving four coordinate R,S,R,S-[Ni(tmc)]²⁺ and five coordinate R,S,R,S-[Ni(tmc)(solvent)]²⁺ species. Spectrophotometric measurements of this equilibrium in a number of solvents have been conducted over a range of temperatures and pressures. The stability order for the five coordinate complex in the solvents investigated is CH₃CN>DMF>DMSO>C₆H₅CN> H₂O>ClCH₂CN at 25 °C. Differences in stability are considered in terms of the measured thermodynamic parameters ΔH° and ΔS°. Both steric and electronic factors were found to influence solvent coordination with the macrocyclic complex.For the equilibrium in CH₃CN, C₆H₅CN, DMF and H₂O, reaction volumes, ΔV°, of −3.2±0.5, −4.2±0.5, −0.2±0.5 and −0.5±0.5 cm³ mol⁻¹ respectively have been determined. Each is significantly smaller than the corresponding solvent molar volume. The ΔV° for the equilibrium in CH₃CN is comparable with the previously determined activation volume for exchange of this solvent on R, S, R, S- [Ni(tmc)(CH₃CN)]²⁺. The equilibrium and measured volume parameters are discussed in relation to the mechanism for solvent exchange.     

Kinetics of selenite uptake by mononuclear cells from peripheral human blood

The present study deals with the kinetics and thermodynamics of the uptake of⁷⁵Se-labeled SeO ₃ ^2? from incubation media to lymphocytes cultivated from eight normal individuals (14–55 years of age, two females). The uptake of SeO ₃ ^2? was evaluated on the assumption of pseudo-first-order kinetics with regard to a reacting cellular receptor pool. On the basis of the experimental observations, it was assumed that the suggested pool of receptor molecules-symbolically represented by “£H₄”—reacts with SeO ₃ ^2? in the hypothetical reaction: $$\pounds H_4 + SeO_3^{2 - } + 2H^ + \underset{{ - k_1 }}{\overset{{k_1 }}{\longleftrightarrow}}\pounds Se + 3H_2 O$$ The mean value of the change in standard free energy at 25°C was calculated to be ΔG ^o=?141.6±1.3 kJ/mol, while the corresponding mean value of the free energy of activation at 25°C was calculated to be ΔG ²⁺=?7.8±0.9 kJ/mol for the forward reaction. The calculated values of the corresponding individual changes in the respective standard enthalpies and entropies were mutually interdependent for all eight donors. ΔH ^o=?152+315ΔS ^o(kJ/mol) corresponding to the common value ΔG ^o??152 kJ/mol at 315°K. These mutual interdependencies are possibly the effect of variable conformational states (e.g., the macromolecular compactness) of the cellular receptor pools. This suggestion may furthermore be supported by the correlation traced between ΔH ^o vs the biological age in years of the donors: △H ^°?76.7?1.0 (age)kJ/mol (r = ?0.92) The calculated values of activation enthalpy ΔH ²⁺ kJ/mol and activation entropy ΔS ²⁺ (kJ/mol K) also mutually correlated linearly (r=0.998); the regression line was: △H ²⁺ = ?8.9 + 305△S²⁺ (kJ/mol) corresponding to the common value △H ²⁺ △ ?8.9 (kJ/mol) at 305°K Similarly the activation enthalpy ΔH ²⁺ vs the biological age in years correlated linearly: ΔH ²⁺=67.4?0.73(age) (kJ/mol) (r=?0.76) The range of ΔH ²⁺ studied was from 13.8 to 53.9 kJ/mol with a linearly corresponding range in ΔS ²⁺ from 73 to 205 J/mol K. The thermodynamic data reveal the selenite uptake during the hypothetical standard reaction to be exergonic and endothermic. Critical pH dependencies of the selenite uptake were explained.     

Evidence for a high proton translocation stoichiometry of the H+-ATPase complex in well coupled proteoliposomes reconstituted from a thermophilic cyanobacterium

Evidence is presented for a high proton translocation stoichiometry (H+/ATP) of approx. 9 in ATPase proteoliposomes with extremely low permeability for ions, reconstituted from a thermophilic cyanobacterium. A proportional relation between the phosphate potential (ΔG_fp) and the proton-motive force (Δp) was observed in thermodynamic equilibrium. A bulk-to-bulk Δp was imposed by valinomycin-induced K⁻ diffusion potentials of different size while the initial ΔG_fp was varied. In all cases equilibrium was reached in about 1.5 h. A high H⁻/ATP ratio was also deduced from the relation between the initial rates of ATP synthesis or hydrolysis at varying ΔG_fp and Δp. The implications of these results for the mechanism of energy transduction in energy-conserving membranes are discussed.     

Thermodynamic parameters of stabilization of the Yersinia pestis Caf113-149 subunit in compact form

Several experimental methods (circular dichroism, viscosity, intrinsic fluorescence, and fluorescence labeling) were used to study the conformational folding/unfolding transitions in a compact monomeric form of the Caf1_13-149 subunit under the action of guanidine hydrochloride in the temperature range 5–45°C. It has been shown that transitions always occur between two major states (unfolded and compact). This has made it possible to determine all the main thermodynamic functions that characterize the compact state of the Caf1_13-149 subunit: stability temperature T _m, free energy of stabilization ΔG _st, enthalpy ΔH _tr, and heat capacity jump ΔC in collapse of the structure. These data have been confirmed by an independent experiment on melting of fluorescently labeled protein.     

The enthalpy of oxidation of flavin mononucleotide: Temperature dependence of in vitro bacterial luciferase bioluminescence

The enthalpy of the bioluminescent reaction

$\text{FMNH}{}_2\text{+ RCHO + O}{}_2\text{→}\text{luciferase}\text{FMN + RCOO + H}{}_3\text{O}{}^{\mathrm{+}}\text{+ hv}$

has been studied by direct calorimetric methods. Bacterial luciferase, isolated from Beneckea harveyi (formerly strain MAV) has been used to catalyze the oxidation of reduced flavin mononucleotide (FMNH₂) and a long chain aliphatic aldehyde (dodecanal, RCHO) by molecular oxygen to give the indicated products and blue-green light. The enthalpy measured for this process was found to be ΔH_L = ?338.9 k.J (mol FMN)^?1 (?81.0 kcal) at 25.00 °C and ?402.9 kJ (mol FMN)^?1 (?96.3 kcal) at 7.00 °C. Calculations based on redox electrode potentials indicate a corresponding value of the free energy change, ΔG_L = ?464.8 kJ (mol FMN)^?1 (?111.1 kcal), at 25 °C. Measurements were performed in 0.15 m phosphate buffer, pH 7.0 and the values were arrived at by correcting the observed heats for the heat associated with the autoxidation process: FMNH₂ + O₂ ? FMN + H₂O₂; ΔH_D = ?158.5 kJ (mol FMN)^?1 (?37.8). These data and a detailed thermodynamic analysis have demonstrated the need for two parameters, referred to as the intrinsic free energy, ΔG₁, and intrinsic enthalpy, ΔH₁, which are functionally defined by the relations ΔG_I = ΔG_L ? uhvΔH_I = ΔH_L ? uhv, where u is the quantum yield of the reaction expressed in einsteins mole^?1.These parameters reflect the thermochemistry of the bioluminescent reaction corrected for emitted photons. Thus, they are useful for comparing the thermochemistry of a chemiluminescent process. Their values for the bacterial luciferase system at 25 °C and pH 7.0 are ?391.6 and ?266.9 kJ (mol FMN)^?1 (?93.6 and ?63.8 kcal), respectively, assuming a value of 0.3 for the quantum yield. The calorimetric data also suggest the existence of a long-lived species which persists after photon emission.     

Calorimetric studies of the interactions of guanidinium hydrochloride and potassium iodide with model amides in aqueous solution.

The enthalpies of transfer, ΔH_tr, of a series of amides from water to aqueous solutions of either guanidinium hydrochloride (GuHCl) or potassium iodide were obtained from calorimetric measurements at 25°C. The amides were studied at molalities around 10^?2 m while salt molalities ranged from 0–10 m. The amides investigated were Ac-Gly-NHMe, Ac-Gly-Gly-NHMe, Ac-Ala-NHMe, and Ac-Leu-NHMe. Use of an additivity assumption allowed the calculation of group contributions to ΔH_tr in these two salt systems for the methyl group, leucyl side chain, and the peptide backbone unit. Values of the entropy of transfer were also obtained. The great ability of GuHCl to randomize protein structures appears to arise from effects on polar and nonpolar groups, which are characterized by enthalpies and entropies of transfer not substantially different from those with KI, a salt comprised of ions of comparable size and polarizability. The difference in the sign of the free energies of transfer of nonpolar groups from water to MX solutions, negative for GuHCl and positive for KI, is the result of these small differences in enthalpies and entropies of transfer. Variations in water structure produced by differences in ionic properties rather than a mode of action for GuHCl very different from that of other salts characterizes its superior denaturing ability.     

Stereoselective synthesis and characterization of the optically labile chiral Cr(III) complex Δ-(+)589{Cr[(−)bdtp]3}

The diastereoisomeric complex Δ-(+)-tris(cyclicO,O′, 1 (R), 2(R)(−)dimethylethylene dithiophosphato)chromium(III), was synthesized stereoselectively in tetrahydrofuran (THF) solution. The complex proves optically labile, [α]_D=+106, in CHCl₃, changing quickly to [α]_D=+211. The CD spectra in THF enable us to characterize the complex and show a configuration inversion which gives the diastereoisomeric equilibrium Λ⇌Δ with an excess of the Λ-(R,R)(R,R)(R,R) diastereoisomeric form. The equilibrium constant K=0.86 at 25 °C is indicative of a different thermodynamic stability between the two diastereoisomers in THF solution, Λ-(R,R)> Δ-(R,R), δΔH°=1.5 kJ mol⁻¹, δΔG°=0.3 kJ mol⁻¹, δΔS°=4 J mol⁻¹ K⁻¹. The kinetic diastereoisomer Δ-(R,R)(R,R)(R,R) is stabilized in CHCl₃, CH₂Cl₂, EtOH solvents where it is highly soluble and optically stable with a maximum negative chirality factor, g=−5×10⁻³, in CHCl₃.     

Urea interactions with protein groups: A volumetric study

We determined the partial molar volumes and adiabatic compressibilities of N‐acetyl amino acid amides, N‐acetyl amino acid methylamides, N‐acetyl amino acids, and short oligoglycines as a function of urea concentration. We analyze these data within the framework of a statistical thermodynamic formalism to determine the association constants for the reaction in which urea binds to the glycyl unit and each of the naturally occurring amino acid side chains replacing two waters of hydration. Our determined association constants, k, range from 0.04 to 0.39M. We derive a general equation that links k with changes in free energy, ΔG_tr, accompanying the transfer of functional groups from water to urea. In this equation, ΔG_tr is the sum of a change in the free energy of cavity formation, ΔΔG_C, and the differential free energy of solute–solvent interactions, ΔΔG_I, in urea and water. The observed range of affinity coefficients, k, corresponds to the values of ΔΔG_I ranging from highly favorable to slightly unfavorable. Taken together, our data support a direct interaction model in which urea denatures a protein by concerted action via favorable interactions with a wide range of protein groups. Our derived equation linking k to ΔG_tr suggests that ΔΔG_I and, hence, the net transfer free energy, ΔG_tr, are both strongly influenced by the concentration of a solute used in the experiment. We emphasize the need to exercise caution when two solutes differing in solubility are compared to determine the ΔG_tr contribution of a particular functional group. © 2010 Wiley Periodicals, Inc. Biopolymers 93: 866–879, 2010.     

Univalent ions in phospholipid model membranes: Thermodynamic and hydration aspects

By means of differential scanning calorimetry, effects of systematic series of Group I and VII ions on the phase state of model multibilayer dimyristoylphosphatidylcholine (di(14:0)PC) membranes have been studied at a lipid/ion molar ratio of 3/1. The sign-changing correlations between the ionic radii of cations and temperature shifts of di(14:0)PC phase transition were obtained. For cosmotropic Li⁺ and Na⁺, the observed shifts were positive (LiCl: ΔT _m = 0.6°C; ΔT _p = 1.9°C), whereas chaotropic K⁺ and Rb⁺ presence resulted in negative shifts (RbCl: ΔT _m = ?0.3°C; ΔT _p = ?2.5°C). The anions (Cl^?, Br^?, I^?) showed a similar effect increasing with the ion chaotropicity. An essentially weaker effect of Cs⁺ as compared to other alkali metal ions (CsCl: ΔT _m ≈ 0°C; ΔT _p = ?0.1°C) can be one of the reasons of its accumulation in living organisms. Generalization of all available data allowed us to specify some important factors of lipid-ion interactions that should be taken into account in further investigations in this field.     

Synthesis and structure of cis-dichlorobis(2-phenylazopyridine)chromium(II), an inert chromium(II) compound

Recent studies have shown 2-phenylazopyridine (Azpy) to be a useful departure from polypyridyl type ligands in coordination chemistry; frequently lower oxidation states are stabilized by this ligand. We have prepared and studied [Cr(Azpy)₂Cl₂] which is surprisingly inert for a chromium(II) complex. The compound crystallizes in the space group P2₁/c with a = 7.953(3), b = 21.189(11), c = 12.922(12) Å, β = 114.80(5)° and Z = 4. Based on 6777 reflections the structure has been refined to R = 0.030 and R_w = 0.038. The complex is six-coordinated with cis-chlorides, cis-azo and trans-pyridyl groups. The overall symmetry is close to C₂. The azo NN distances are elongated to 1.282 and 1.314 Å, significantly longer than observed in free azo groups.The magnetic moment of 2.8 BM is independent of temperature indicating an orbitally non-degenerate low-spin ground state. In acetonitrile the spectrum shows a band in the red at 1470 nm which is assigned as ligand-metal charge transfer, consistent with earlier interpretations on similar compounds. The compound is stable to air oxidation in acetonitrile or methylene chloride; cyclic voltammetry in these solvents yields potentials of 0.508 and −0.185 V, both appearing to be one electron transfers.The complex is a non-electrolyte in acetonitrile, nearly unchanged over 24 h. However, chloride ions are slowly replaced at 50 °C in this solvent by 2,2′-bipyridyl; this reaction is pseudo-first order in complex with a rate constant of 4.0 ± 0.3 × 10⁻⁷ s⁻¹. All data indicate the presence of a strong chromium-Azpy pi-interaction.     

Improved adsorption of cadmium ions from aqueous solution using chemically modified apple pomace: Mechanism,kinetics, and thermodynamics

Apple pomace (AP), before and after chemical modification (CM), was assessed for the removal of cadmium (Cd²⁺) ions from aqueous solution by equilibrium, kinetics, and thermodynamics studies. The chemical modification of AP was done with succinic anhydride by a simple ring opening mechanism for providing a large surface area for cadmium adsorption. The surface area of chemically modified apple pomace (CMAP) increased about 18% after the treatment. The amount of CMAP required for cadmium removal was 50 times less than the unmodified AP. The Langmuir adsorption isotherm equation was found to be more suitable for the AP and CMAP adsorption experimental data with a correlation coefficient of r² = 0.99 than was the Freundlich isotherm. The FTIR spectra of CMAP, with or without cadmium loading, indicated that ester (–COO), carboxyl (–CO), and amine (–NH₂) groups were involved in the cadmium adsorption mechanism. The adsorption of cadmium ions onto AP and CMAP followed pseudo-second-order kinetics. The ΔG^° value, at different temperatures, was calculated by applying the Van't Hoff equation and found to be negative, indicating that the reaction is spontaneous in nature. The value of ΔH^° indicated that the adsorption was exothermic (−6.93 KJ mol⁻¹) and endothermic (3.171 KJ mol⁻¹) for AP and CMAP, respectively. CMAP could be reutilized for up to three cycles with a removal efficiency of 76.6%, while AP efficiency lessened significantly after a single cycle.     

Enzymatic synthesis of β-lactam antibiotics: A thermodynamic background

The equilibrium constants and the respective standard Gibbs energy changes for hydrolysis of some β-lactam antibiotics have been determined. Native and immobilized penicillin amidase (EC 3.5.1.11) from Escherichia coli has been used as a catalyst. The values of standard Gibbs energy changes corresponding to the pH-independent product of equilibrium concentrations (ΔG⁰_c = ? RT ln K_c) have been calculated. The differences in the structure of the antibiotics nucleus hardly ever affect the value of the pH-independent component of the standard Gibbs energy change (ΔG⁰_c) and value of apparent standard Gibbs energy change at a fixed pH (ΔG^0′_c). At the same time, the value of ΔG⁰_c is more sensitive to the structure of the acyl moiety of the antibiotic; when ampicillin is used instead of benzylpenicillin, ΔG⁰_c increases by ～6.3 kJ mol^?1 (1.5 kcal mol^?1). pH-dependences of the apparent standard Gibbs energy changes for hydrolysis of β-lactam antibiotics have been calculated. The pH-dependences of ΔG^0′_c for hydrolysis of all β-lactam antibiotics have a similar pattern. The thermodynamic pH optimum of the synthesis of these compounds is in the acid pH range (pH < 5.0). The breakage of the β-lactam ring leads to a sharp decrease in the ΔG^0′_c value and a change in the pattern of the pH-dependence. For example, at pH 5.0 ΔG^0′_c decreases from 14.4 kJ mol^?1 for benzylpenicillin to ?1.45 kJ mol^?1 for benzylpenicilloic acid. The reason for these changes is mainly a considerable increase in the pK of the amino group of the nucleus of the antibiotic and, as a consequence, a decrease in the component of standard Gibbs energy change, corresponding to the ionization of the system. The thermodynamic potentials of the enzymatic synthesis of semisynthetic penicillins and cephalosporins on the basis of both free acids and their derivatives (N-acylated amino acids, esters) are discussed. It is shown that with esters of the acids, a high yield of the antibiotic can, in principle, be achieved at higher pH values.     

Purification,kinetic and thermodynamic characterization of soluble acid invertase from sugarcane (Saccharum officinarum L.)

We report for the first time kinetic and thermodynamic properties of soluble acid invertase (SAI) of sugarcane (Saccharum officinarum L.) salt sensitive local cultivar CP 77-400 (CP-77). The SAI was purified to apparent homogeneity on FPLC system. The crude enzyme was about 13 fold purified and recovery of SAI was 35%. The invertase was monomeric in nature and its native molecular mass on gel filtration and subunit mass on SDS-PAGE was 28 kDa. SAI was highly acidic having an optimum pH lower than 2. The acidic limb was missing. Proton transfer (donation and receiving) during catalysis was controlled by the basic limb having a pKa of 2.4. Carboxyl groups were involved in proton transfer during catalysis. The kinetic constants for sucrose hydrolysis by SAI were determined to be: k_m = 55 mg ml^?1, k_cat = 21 s^?1, k_cat/k_m = 0.38, while the thermodynamic parameters were: ΔH* = 52.6 kJ mol^?1, ΔG* = 71.2 kJ mol^?1, ΔS* = ?57 J mol^?1 K^?1, ΔG*_E–S = 10.8 kJ mol^?1 and ΔG*_E–T = 2.6 kJ mol^?1. The kinetics and thermodynamics of irreversible thermal denaturation at various temperatures 53–63 °C were also determined. The half -life of SAI at 53 and 63 °C was 112 and 10 min, respectively. At 55 °C, surprisingly the half -life increased to twice that at 53 °C. ΔG*, ΔH* and ΔS* of irreversible thermal stability of SAI at 55 °C were 107.7 kJ mol^?1, 276.04 kJ mol^?1 and 513 J mol^?1K^?1, respectively.     

Application of seaweeds for the removal of lead from aqueous solution

Ten different seaweed species were compared on the basis of lead uptake at different pH conditions. The brown seaweed, Turbinaria conoides, exhibited maximum lead uptake (at pH 4.5) and hence was selected for further studies. Sorption isotherms, obtained at different pH (4–5) and temperature (25–35 °C) conditions were fitted using Langmuir and Sips models. According to the Langmuir model, the maximum lead uptake of 439.4 mg/g was obtained at optimum pH (4.5) and temperature (30 °C). The Sips model better described the sorption isotherms with high correlation coefficients at all conditions examined. Various thermodynamic parameters such as ΔG°, ΔH° and ΔS° were calculated indicating that the present system was a spontaneous and endothermic process. Through potentiometric titrations, number of binding sites (carboxyl groups) and pK₁ were determined as 4.1 mmol/g and 4.4, respectively. The influence of co-ions (Na⁺, K⁺, Mg²⁺ and Ca²⁺) on lead uptake was well pronounced in the case of divalent ions compared to monovalent ions. The solution of 0.1 M HCl successfully eluted all lead ions from lead-loaded T. conoides biomass. The regeneration experiments revealed that the alga could be successfully reused for five cycles without any loss in lead biosorption capacity. A glass column (2 cm i.d. and 35 cm height) was used to study the continuous lead biosorption performance of T. conoides. At 25 cm (bed height), 5 ml/min (flow rate) and 100 mg/l (initial lead concentration), T. conoides exhibited lead uptake of 220.1 mg/g. The column was successfully eluted using 0.1 M HCl, with elution efficiency of 99.7%.     

Kinetics of imidazole addition to the axial site of the iron(II) complex of 2, 3, 9,10-tetraphenyl-1,4, 8,11-tetraaza-1,3, 8,10-cyclotetradecatetraene in dimethyl sulfoxide. Evidence for a dissociative-interchange mechanism

The axial adduct formation of the iron(II) complex of 2,3,9,10-tetraphenyl-l,4,8,11-tetraaza-1,3,8,10-cyclotetradecatetraene (L) with imidazole in dimethyl sulfoxide has been investigated spectrophotometrically at various temperatures and pressures. In the presence of a large excess of imidazole the reaction with the two phases has been observed. The first faster reaction is the formation of the monoimidazole complex of FeL²⁺, and the second slower reaction corresponds to the formation of the bisimidazole complex. Activation parameters are as follows: for the first step with k₁ (25.0°C) = (6.8 ±0.2)×10⁵ mol⁻¹ kg s⁻¹, ΔH³₁ = 47.5 ± 4.9 kJ mol⁻¹, ΔS³₁ = 26±16 J K⁻¹ mol⁻¹, and ΔV³₁ (30.0°C) = 27.2±1.5 cm³ mol⁻¹; for the second step with k₂ (25.0°C) = 26.8±0.8 mol⁻¹ kg s⁻¹, ΔH³₂ = 91.6± 0.8 kJ mol⁻¹, ΔS³₂ = 90±3 J K⁻¹ mol⁻¹, and ΔV³₂ (35.0°C) = 21.8±0.9 cm³ mol⁻¹. The large positive activation volumes strongly indicate a dissociative character of the activation process.