Kinetics,Isotherms, and Thermodynamics of Hg(II) Biosorption onto Carica papaya

Carica papaya, a novel sorbent, was evaluated for sorption of Hg(II) from synthetic aqueous solutions using various pseudo-second order kinetic models as well as equilibrium sorption models. Batch kinetic and equilibrium experiments were carried out for the sorption of Hg(II) onto C. papaya at pH 6.5 and solid to liquid ratio (s/l) 1.0 g L^?1. The kinetic data were fitted to second order models of Sobkowsk and Czerwinski, Ritchie, Blanchard, Ho and McKay, whereas Langmuir, Freundlich, and Redlich-Peterson models were used for the equilibrium data. A comparative study on both linear and nonlinear regression showed that the Sobkowsk and Czerwinski and Ritchie's second order model were the same. Ho and McKay's pseudo-second order model fitted well to the experimental data when compared with the other second order kinetic expressions. Langmuir isotherm parameters obtained from the four Langmuir linear equations by using linear method were dissimilar, but were the same when nonlinear method was used. Additionally, various thermodynamic parameters, such as ΔG ⁰, ΔH ⁰, and ΔS ⁰, were calculated. The negative values of Gibbs free energy (ΔG ⁰) and ΔH ⁰ confirmed the intrinsic nature of biosorption process and exothermic, respectively. The negative value of ΔS ⁰ showed the decreased randomness at the solid-solution interface during biosorption.     

Adsorption kinetics,isotherm, and thermodynamics studies of acetyl-11-keto-β-boswellic acids (AKBA) from Boswellia serrata extract using macroporous resin

An acetyl-11-keto-β-boswellic acid (AKBA) is potent anti-inflammatory agent found in Boswellia serrata oleogum resin. Adsorption characteristics of AKBA from B. serrata were studied using macroporous adsorbent resin to understand separation and adsorption mechanism of targeted molecules. Different macroporous resins were screened for adsorption and desorption of AKBA and Indion 830 was screened as it showed higher adsorption capacity. The kinetic equations were studied and results showed that the adsorption of AKBA on Indion 830 was well fitted to the pseudo first-order kinetic model. The influence of two parameters such as temperature (298, 303, and 308?K) and pH (5–8) on the adsorption process was also studied. The experimental data was further investigated using Langmuir, Freundlich, and Temkin isotherm models. It was observed that Langmuir isotherm model was found to be the best fit for AKBA adsorption by Indion 830 and highest adsorption capacity (50.34?mg/g) was obtained at temperature of 303?K. The values of thermodynamic parameters such as the change of Gibbs free energy (ΔG*), entropy (ΔS*), and enthalpy (ΔH*), indicated that the process of adsorption was spontaneous, favourable, and exothermic.     

Adsorption of lead(II) from aqueous solution onto Hydrilla verticillata

The adsorption of Pb(II) onto Hydrilla verticillata was examined in aqueous solution with parameters of pH, adsorbent dosage, contact time and temperature. The linear Langmuir and Freundlich models were applied to describe equilibrium isotherms, and both models fitted well. The monolayer adsorption capacity of Pb(II) was found as 104.2 mg/g at pH 4 and 25°C. Dubinin–Radushkevich (D–R) isotherm model was also applied to the equilibrium data. The mean free energy of adsorption (15.81 kJ/mol) indicated that the adsorption of Pb(II) onto H. verticillata may be carried out via chemical ion-exchange mechanism. Thermodynamic parameters, free energy (ΔG ⁰), enthalpy (ΔH ⁰) and entropy (ΔS ⁰) of adsorption were also calculated. These parameters showed that the adsorption of Pb(II) onto H. verticillata was a feasible, spontaneous and exothermic process in nature. The influence of Cd²⁺, Cu²⁺ and Ni²⁺ on adsorption of Pb²⁺ onto H. verticillata was studied, too. In the investigated range of operating conditions, it was found that the existence of Cd ²⁺, Cu ²⁺ and Ni ²⁺ had no impact on the adsorption of Pb²⁺.     

The Suitability of Jatropha Seed Press Cake as a Biosorbent for Removal of Hexavalent Chromium from Aqueous Solutions

Jatropha seed press cake (JPC), a biodeisel waste, was investigated for its use as biosorbent for Cr(VI) removal from wastewater. The acid-pretreated biomass exhibited 1.9-fold higher biosorption efficiency for Cr(VI). The Cr(VI) biosorption efficiency was found to increase with decrease in pH of aqueous medium. The adsorption capacity of biosorbent for Cr(VI) increased with increasing concentration of Cr(VI). The biosorption of Cr(VI) by acid-treated JPC followed a pseudo-second-order kinetics. The results of equilibrium studies showed that the biosorption process fitted the Langmuir isotherm model, with a maximum adsorption capacity of 22.727 mg of Cr(VI)/g of biosorbent at 30°C. The activation energy was found to be 27.114 kJ/mol, suggesting that the adsorption process was mainly a physical process. The important thermodynamic parameters of adsorption (ΔG, ΔH, andΔS) were determined, which indicated that the Cr(VI) sorption by JPC is a spontaneous and endothermic process.     

Spectroscopic investigation of water‐soluble alloyed QDs with bovine serum albumin

We present here a systematic investigation on the interaction between a water‐soluble alloyed semiconductor quantum dot and bovine serum albumin using various spectroscopic techniques i.e. fluorescence quenching, resonance light scattering and synchronous fluorescence spectroscopy. The analysis of fluorescence spectrum and fluorescence intensity indicates that the intrinsic fluorescence of bovine serum albumin (BSA) gets quenched by both static and dynamic quenching mechanism. The Stern‐Volmer quenching constants, energy transfer efficiency parameters, binding parameters and corresponding thermodynamic parameters (ΔH⁰, ΔS⁰ and ΔG⁰) have been evaluated by using van 't Hoff equation at different temperatures. A positive entropy change with a positive enthalpy change was observed suggesting that the binding process was an entropy‐driven, endothermic process associated with the hydrophobic effect. The intermolecular distance (r) between donor (BSA) and acceptor (CdSeS/ZnS quantum dots) was estimated according to Förster's theory of non‐radiative energy transfer. The synchronous fluorescence spectra revealed a blue shift in the emission maxima of tryptophan which is indicative of increasing hydrophobicity. Negative ΔG⁰ values implied that the binding process was spontaneous. It was found that hydrophobic forces played a role in the quenching process. Copyright © 2016 John Wiley & Sons, Ltd.     

Lipase adsorption on woven nylon-6 membrane: Optimization,kinetic and thermodynamic analyses

Abstract

Candida rugosa lipase was immobilized on the activated nylon-6 membrane using 0.5% glutaraldehyde as cross-linking agent. The immobilized lipase had an improved temperature and pH stability and exhibited good reusability. The adsorption process followed pseudo-second order rate equation and the biosorption isotherms correlated well with the Langmuir isotherm model. Negative ΔG indicated the adsorption process was spontaneous and the positive ΔH indicated that the adsorption was endothermic. The standard entropy ΔS value was also positive, which suggested an increase in the freedom of the system.     

Determination of enantiomerization barriers of hypericin and pseudohypericin by dynamic high‐performance liquid chromatography on immobilized polysaccharide‐type chiral stationary phases and off‐column racemization experiments

Direct enantiomer separation of hypericin, pseudohypericin, and protohypericin was accomplished by high‐performance liquid chromatography (HPLC) using immobilized polysaccharide‐type chiral stationary phases (CSPs). Enantioselectivities up to 1.30 were obtained in the polar‐organic elution mode whereby for hypericin and pseudohypericin Chiralpak IC [chiral selector being cellulose tris(3,5‐dichlorophenylcarbamate)] and for protohypericin Chiralpak IA (chiral selector being the 3,5‐dimethylphenylcarbamate of amylose) gave favorable results. Enantiomers were distinguished by on‐line electronic circular dichroism detection. Optimized enantioselective chromatographic conditions were the basis for determining stereodynamic parameters of the enantiomer interconversion process of hypericin and pseudohypericin. Rate constants delivered by computational simulation of dynamic HPLC elution profiles (stochastic model, consideration of peak tailing) were used to calculate averaged enantiomerization barriers (ΔG) of 97.6–99.6 kJ/mol for both compounds (investigated temperature range 25–45°C). Complementary variable temperature off‐column (i.e., in solution) racemization experiments delivered ΔG = 97.1–98.0 kJ/mol (27–45°C) for hypericin and ΔG = 98.9–101.4 kJ/mol (25–55°C) for pseudohypericin. An activation enthalpy of ΔH^# = 86.0 kJ/mol and an activation entropy of ΔS^# = ?37.7 J/(K mol) were calculated from hypericin racemization kinetics in solution, whereas for pseudohypericin these figures amounted to 74.1 kJ/mol and ?82.6 J/(K mol), respectively. Although the natural phenanthroperylene quinone pigments hypericin and pseudohypericin as well as their biological precursor protohypericin are chiral and can be separated by enantioselective HPLC low enantiomerization barriers seem to prevent the occurrence of an excess of one enantiomer under typical physiological conditions—at least as long as stereoselective intermolecular interactions with other chiral entities are absent. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

Thermodynamic parameters for the helix-coil thermal transition of ribonuclease-S-peptide and derivatives from 1H-NMR data

Values for the thermodynamic quantities, ΔH° = 11.8 ± 2.0 Kcal/mole and ΔS° = 43.6 ± 6.0 e.u., of the 3-13 helix–coil equilibrium of isolated S-peptide (19 residue N-terminal fragment of ribonuclease A) in aqueous solution (3 m M, 1M NaCl, pD 5.4) have been determined from a joint analysis of the Thr 3γ, Ala 6β, Phe 8meta, and Phe 8para ¹H chemical shift vs temperature curves (?7 to 80°C) in several aqueous–trifluorethanol mixtures. Chemical shifts in the coil and in the helix have been determined for up to 16 protons belonging to the 3-13 fragment. Thermodynamic parameters have also been determined for C-peptide (13 residue fragment) and a number of S-peptide derivatives. From the variation of the values of the thermodynamic parameters at pD 2.5, 5.4, and 8.0, a quantitation of the two helix-stabilizing side-chain interactions can be made: (1) Δ(ΔH°) ? 5 Kcal/mole and Δ(ΔS°) ? 18 e.u. for the salt bridge Glu 2^? … Arg 10⁺ and (2) Δ(ΔH°) ? 3 Kcal/mole and Δ(ΔS°) = 9 e.u. for the one in which the His 12⁺ imidazolium group is involved, presumably a partial stacking with the Phe 8 side chain.     

Adsorption of ethidium bromide from aqueous solution onto nutraceutical industrial fennel seed spent: Kinetics and thermodynamics modeling studies

Dye pollutants from research laboratories are one of the major sources for environmental contamination. In the present study, a nutraceutical industrial fennel seed spent (NIFSS) was explored as potential adsorbent for removal of ethidium bromide (EtBr) from aqueous solution. The adsorbent was characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Through batch experiments, the operating variables like initial dye concentration, adsorbent dosage, temperature, contact time, and pH were optimized. Equilibrium data were analyzed using three number of two-parameter and six number of three-parameter isotherm models. The adsorption kinetics was studied using pseudo-first order and pseudo-second order. The diffusion effects were studied by film diffusion, Webber–Morris, and Dumwald–Wagner diffusion models. The thermodynamic parameters; change in enthalpy (ΔHº), entropy (ΔSº), and Gibbs free energy (ΔGº) of adsorption system were also determined and evaluated.     

Cadmium(II) sorption from water samples by powdered marble wastes

Abstract

A series of batch adsorption experiments were carried out, with the aim of removing cadmium ions from aqueous solutions and water samples using powdered marble wastes (PMW) as an effective inorganic sorbent. PMW is inexpensive, widespread, and may be considered as environmental problem. The main parameters (i.e. solution pH, sorbent and cadmium concentrations, stirring time, and temperature) influencing the sorption process were investigated. The results obtained for sorption of cadmium ions onto PMW are well described by the Freundlich and Langmuir models. The Dubinin-Radushkevick (D–R) isotherm model was applied to describe the nature of the adsorption of the metal ion; it was found that the adsorption process was chemical in nature. The thermodynamic parameters were also calculated from the Gibbs free energy change (ΔG°), enthalpy (AH°) and entropy (ΔS°). These parameters indicated that the adsorption process of cadmium(II) ions on PMW was spontaneous and endothermic in nature. Under the optimum experimental conditions employed the removal of ca ~100% of Cd²⁺ ions was attained. The procedure was successfully applied to removal of the cadmium ions from aqueous and various natural water samples. The adsorption mechanism is discussed.     

Melting studies of short DNA hairpins: Influence of loop sequence and adjoining base pair identity on hairpin thermodynamic stability

Spectroscopic and calorimetric melting studies of 28 DNA hairpins were performed. These hairpins form by intramolecular folding of 16 base self‐complementary DNA oligomer sequences. Sequence design dictated that the hairpin structures have a six base pair duplex linked by a four base loop and that the first five base pairs in the stem are the same in every molecule. Only loop sequence and identity of the duplex base pair closing the loop vary for the set of hairpins. For these DNA samples, melting studies were carried out to investigate effects of the variables on hairpin stability. Stability of the 28 oligomers was ascertained from their temperature‐induced melting transitions in buffered 115 mM Na⁺ solvent, monitored by ultraviolet absorbance and differential scanning calorimetry (DSC). Experiments revealed the melting temperatures of these molecules range from 32.4 to 60.5°C and are concentration independent over strand concentrations of 0.5 to 260 μM; thus, as expected for hairpins, the melting transitions are apparently unimolecular. Model independent thermodynamic transition parameters, ΔH_cal, ΔS_cal, and ΔG_cal, were determined from DSC measurements. Model dependent transition parameters, ΔH_vH, ΔS_vH, and ΔG_vH were estimated from a van't Hoff (two‐state) analysis of optical melting transitions. Results of these studies reveal a significant sequence dependence to DNA hairpin stability. Thermodynamic parameters evaluated by either procedure reveal the transition enthalpy, ΔH_cal (ΔH_vH) can differ by as much as 20 kcal/mol depending on sequence. Similarly, values of the transition entropy ΔS_cal (ΔS_vH) can differ by as much as 60 cal/Kmol (eu) for different molecules. Differences in free energies ΔG_cal (ΔG_vH) are as large as 4 kcal/mol for hairpins with different sequences. Comparisons between the model independent calorimetric values and the thermodynamic parameters evaluated assuming a two‐state model reveal that 10 of the 28 hairpins display non‐two‐state melting behavior. The database of sequence‐dependent melting free energies obtained for the hairpins was employed to extract a set of n‐n (nearest‐neighbor) sequence dependent loop parameters that were able to reproduce the input data within error (with only two exceptions). Surprisingly, this suggests that the thermodynamic stability of the DNA hairpins can in large part be reasonably represented in terms of sums of appropriate nearest‐neighbor loop sequence parameters. © 1999 John Wiley & Sons, Inc. Biopoly 50: 425–442, 1999     

A study of the growth of normal and iodinated collagen fibrils in vitro using electron microscope autoradiography

Electron microscopic autoradiographic observations on collagen fibrils grown in vitro allow growth rates in the N- and C-terminal directions to be measured on individual fibrils. Such observations, made on normal and iodinated collagen, show that normal fibrils grow at both ends (although rather more rapidly at the N-terminal end), whereas fully-iodinated collagen fibrils grow only at the N-terminal end. Measurements of growth rates at different temperatures provide estimates of the activation enthalpy (ΔH^≠) and entropy (ΔS^≠) of precipitation for the two types of collagen. Solubility measurements have also yielded values for the thermodynamic enthalpy (ΔH) and entropy (ΔS) of precipitation. Results show that the activated (rate-limiting) state is characterized by a large positive ΔH^≠ and ΔS^≠ similar in magnitude to the ΔH and ΔS of transition from solution to fibril. It is also concluded that the different rates of precipitation of normal and iodinated collagen cannot be explained in terms of fibril formation requiring ionization of the tyrosine residues.     

Biosorption of Nickel(II) from aqueous solution by the fungal mat of Trametes versicolor (rainbow) biomass: equilibrium, kinetics, and thermodynamic studies

This study investigates the equilibrium, kinetics and thermodynamics of Nickel(II) biosorption from aqueous solution by the fungal mat of Trametes versicolor (rainbow) biomass. The optimum biosorption conditions like pH, contact time, biomass dosage, initial metal ion concentration and temperaturewere determined in the batch method. The biosorbent was characterized by FTIR, SEM and BET surface area analysis. The experimental data were analyzed in terms of pseudo-first-order, pseudo-secondorder and intraparticle diffusion kinetic models, further it was observed that the biosorption process of Ni(II) ions closely followed pseudo-second-order kinetics. The equilibrium data of Ni(II) ions at 303, 313, and 323 K were fitted to the Langmuir and Freundlich isotherm models. Langmuir isotherm provided a better fit to the equilibrium data andthe maximum monolayer biosorption capacity of the T. versicolor(rainbow) biomass for Ni(II) was 212.5 mg/g at pH 4.0. The calculated thermodynamic parameters, ΔG^○, ΔH^○, and ΔS^○, demonstrated that the biosorption of Ni(II) ions onto the T. versicolor (rainbow) biomass was feasible, spontaneous and endothermic at 303 ～ 323 K. The performance of the proposed fungal biosorbent was also compared with that of many other reported sorbents for Nickel(II) removal and it was observed that the proposed biosorbent is effective in terms of its high sorption capacity.     

Spectroelectrochemistry of ferrioxamine B,ferrichrome, and ferrichrome A

Thin-layer spectroelectrochemical techniques were used to determine the entropy change for the reduction of the three siderophores ferrioxamine B, ferrichrome, and ferrichrome A. The entropy changes were found to be large and negative. The _ΔS° values obtained are: ferrioxamine B. pH 10.2, _ΔS° = ?33.3 ± 0.4 eu; pH 9.0, _ΔS° = ?26.9 ± 0.9 eu; pH 8.0, _ΔS° = ?23.3 ± 1.2 eu; ferrichrome, pH 10.0, ΔS° = ?42.6 ± 0.5 eu; pH 9.1, ΔS° = ?35.8 ± 0.4 eu; pH 7.3, ΔS° = ?74.5 ± 3.4 eu; ferrichrome A, pH 10.1, _ΔS° = ?35.6 ± 0.9 eu; pH 9.1, _ΔS° = ?34.3 ± 0.9 eu; pH 7.9, _ΔS° = ?31.7 ± 0.9 eu. These values are adjusted to the scale on which S°_H + = 0. The large decreases in entropy upon reduction are attributed to an increase in the solvent ordering around the ferrous complex. Upon reduction, the rigid structure of the ferric chelate is loosened and previously sequestered amide groups are made available for solvent interactions. This increased interaction with solvent causes an increase in the order of the water around the molecule and this is responsible for the observed entropy changes. Variations in ΔS° values and the pH dependencies of these values are attributed to structural peculiarities of the individual siderophores.     

Studies of lysozyme binding to histamine as a ligand for hydrophobic charge induction chromatography

Histamine was immobilized on Sepharose CL‐6B (Sepharose) for use as a ligand of hydrophobic charge induction chromatography (HCIC) of proteins. Lysozyme adsorption onto Histamine‐Sepharose (HA‐S) was studied by adsorption equilibrium and calorimetry to uncover the thermodynamic mechanism of the protein binding. In both the experiments, the influence of salt (ammonium sulfate and sodium sulfate) was examined. Adsorption isotherms showed that HA‐S exhibited a high salt tolerance in lysozyme adsorption. This property was well explained by the combined contributions of hydrophobic interaction and aromatic stacking. The isotherms were well fitted to the Langmuir equation, and the equilibrium parameters for lysozyme adsorption were obtained. In addition, thermodynamic parameters (ΔH_ads, ΔS_ads, and ΔG_ads) for the adsorption were obtained by isothermal titration calorimetry by titrating lysozyme solutions into the adsorbent suspension. Furthermore, free histamine was titrated into lysozyme solution in the same salt‐buffers. Compared with the binding of lysozyme to free histamine, lysozyme adsorption onto HA‐S was characterized by a less favorable ΔG_ads and an unfavorable ΔS_ads because histamine was covalently attached to Sepharose via a three‐carbon‐chain spacer. Consequently, the immobilized histamine could only associate with the residues on the protein surface rather than those in the hydrophobic pocket, causing a less favorable orientation between histamine and lysozyme. Further comparison of thermodynamic parameters indicated that the unfavorable ΔS_ads was offset by a favorable ΔH_ads, thus exhibiting typical enthalpy‐entropy compensation. Moreover, thermodynamic analyses indicated the importance of the dehydration of lysozyme molecule and HA‐S during the adsorption and a substantial conformational change of the protein during adsorption. The results have provided clear insights into the adsorption mechanisms of lysozyme onto the new HCIC material. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2010     

Cross-linked succinyl chitosan as an adsorbent for the removal of Methylene Blue from aqueous solution

Removal of a basic dye (Methylene Blue) from aqueous solution was investigated using a cross-linked succinyl-chitosan (SCCS) as sorbent. The chemical structures of chitosan and its derivatives were testified by FT-IR. X-ray diffraction, DTG analysis and swelling measurements were conducted to clarify the characteristics of the chemically modified chitosan. The effect of process parameters, such as pH of the initial solution, and concentrations of dyes on the extent of Methylene Blue (MB) adsorption was investigated. The Langmuir isotherm model was used to fit the equilibrium experimental data, giving a maximum sorption capacity of 289.02 mg/g at 298 K. Kinetic studies showed that the kinetic data were well described by the pseudo-second-order kinetic model. Thermodynamic parameters such as enthalpy change (ΔH°), free energy change (ΔG°) and entropy change (ΔS°) were determined to be −25.32 kJ mol⁻¹, −6.76 kJ mol⁻¹ and −62.36 J mol⁻¹ K⁻¹, respectively, which leads to a conclusion that the adsorption process is spontaneous and exothermic.     

High‐Performance Liquid Chromatographic Enantioseparation of Cyclic β‐Amino Acids on Zwitterionic Chiral Stationary Phases Based on Cinchona Alkaloids

Stereoselective high‐performance liquid chromatographic separations of eight sterically constrained cyclic β‐amino acid enantiomer pairs were carried out using the newly developed Cinchona alkaloid‐based zwitterionic chiral stationary phases Chiralpak ZWIX(+) and ZWIX(?). The effects of the mobile phase composition, the nature and concentrations of the acid and base additives, the counterions and temperature on the separations were investigated. The changes in standard enthalpy, Δ(ΔH°), entropy, Δ(ΔS°), and free energy, Δ(ΔG°), were calculated from the linear van't Hoff plots derived from the ln α vs. 1/T curves in the studied temperature range (10–50°C). The values of the thermodynamic parameters depended on the nature of the selectors and the structures of the analytes. Unusual temperature behavior was observed on the ZWIX(?) column: decreased retention times were accompanied by increased separation factors with increasing temperature. On the ZWIX(+) column only enthalpically, whereas on the ZWIX(?) column both enthalpically and entropically driven separations were observed. The elution sequence was determined in all cases and was observed to be the opposite on ZWIX(+) and on ZWIX(?). Chirality 27:563–570, 2015. © 2015 Wiley Periodicals, Inc.     

Biosorption of uranium by magnetically modified Rhodotorula glutinis

Adsorption of uranium from aqueous solution onto the magnetically modified yeast cell, Rhodotorula glutinis, was investigated in a batch system. Factors influencing sorption such as initial solution pH, biomass dosage, contact time, temperature, initial uranium concentration and other common cations were analyzed. Sorption isotherm, kinetic and thermodynamic studies of uranium on magnetically modified R. glutinis were also carried out. The temperature dependent equilibrium data agreed well with the Langmuir model. Kinetic data obtained at different temperatures were simulated using pseudo-first-order and pseudo-second-order kinetic models, the pseudo-second-order kinetic model was found to describe the data better with correlation coefficients near 1.0. The thermodynamic parameters, ΔH°, ΔS° and ΔG° were calculated from the sorption data gained at different temperatures. These thermodynamic parameters showed that the sorption process was endothermic and spontaneous. All results indicated that magnetically modified R. glutinis can be a potential sorbent for uranium wastewater treatment.     

Thermodynamic descriptors,profiles and driving forces in membrane receptor-ligand interactions

Extension of the (isothermal) Gibbs–Helmholtz equation for the heat capacity terms (ΔC_p) allows formulating a temperature function of the free (Gibbs) energy change (ΔG). An approximation of the virtually unknown ΔC_p temperature function enables then to determine and numerically solve temperature functions of thermodynamic parameters ΔH and ΔS (enthalpy and entropy change, respectively). Analytical solutions and respective numeric procedures for several such approximation formulas are suggested in the presented paper. Agreement between results obtained by this analysis with direct microcalorimetric measurements of ΔH (and ΔC_p derived from them) was approved on selected cases of biochemical interactions presented in the literature. Analysis of several ligand-membrane receptor systems indicates that temperature profiles of ΔH and ΔS are parallel, largely not monotonic, and frequently attain both positive and negative values within the current temperature range of biochemical reactions. Their course is determined by the reaction change of heat capacity: temperature extremes (maximum or minimum) of both ΔH and ΔS occur at ΔC_p?=?0, for most of these systems at roughly 285–305 K. Thus, the driving forces of these interactions may change from enthalpy-, entropy-, or enthalpy-entropy-driven in a narrow temperature interval. In contrast, thermodynamic parameters of ligand-macromolecule interactions in solutions (not bound to a membrane) mostly display a monotonic course. In the case of membrane receptors, thermodynamic discrimination between pharmacologically defined groups—agonists, partial agonists, antagonists—is in general not specified and can be achieved, in the best, solely within single receptor groups.     

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.