Enthalpy-entropy Compensation Effect Research Articles

Thermodynamic properties of novel phenylene-bridged periodic mesoporous organosilica as high performance liquid chromatographic stationary phase

The thermodynamic properties of spherical phenylene-bridged periodic mesoporous organosilica (PMOs) as high performance liquid chromatographic stationary phase were investigated and compared with commercial grafted C18 and phenyl stationary phases. The retention behavior of several compounds of polycyclic aromatic hydrocarbons (PAHs) was investigated. The thermodynamic parameters of transfer from mobile phase to stationary phase were discussed. The results showed that the enthalpy-entropy compensation effect does not exist for homemade phenylene-bridged PMOs. The retention mechanism of solutes on the prepared stationary phase involves synergistic interaction of hydrophobic, pi-pi and inclusion interactions, which is different with that on the commercial stationary phases.

Grain Boundary Engineering and Alterations in Anisotropy of Interfacial Properties

Literature data on grain boundary properties show surprisingly in some cases reversed courses of the structural dependences than expected, or directly prove their qualitative changes with changing temperature. This “reversed anisotropy” is demonstrated for example of grain boundary segregation. It is shown that this reversed anisotropy of grain boundary properties can be explained by the enthalpy–entropy compensation effect. Serious consequences of altered structural dependence of grain boundary properties for Grain Boundary Engineering are discussed.

Reactivity of sodium arenesulfinates in the substitution reaction to γ‐functionalized allyl bromides

AbstractThe kinetics of nucleophilic bimolecular substitution reactions of γ‐functionalized allyl bromides with non‐substituted and p‐substituted sodium arenesulfinates has been studied. Both the structure of allyl bromides and nucleophilicity of arenesulfinate ions exerted a significant effect on the values of the kinetic parameters such as the second‐order rate constants k, activation energy EA, and changes in the entropy ΔS≠, enthalpy ΔH≠, and free energy ΔG≠ of the formation of the activated complex from reactants. Based on the evaluation of kinetic parameters, the reactants could be arranged, according to their decreasing reactivity in the SN2‐reactions as follows: p‐toluenesulfinate ion > benzenesulfinate ion > p‐chlorobenzenesulfinate ion and 4‐bromo‐2‐butenenitrile > 1,3‐ dibromopropene, respectively. Comparison was also made between the kinetic data obtained and some delocalization reactivity indexes for both the substrates and nucleophiles. The enthalpy–entropy compensation effect was observed for the reactions of sodium arenesulfinates with γ‐functionalized allyl bromides. Copyright © 2009 John Wiley & Sons, Ltd.

Studies of Enthalpy−Entropy Compensation, Partial Entropies, and Kirkwood−Buff Integrals for Aqueous Solutions of Glycine,l-Leucine, and Glycylglycine at 298.15 K

Densities and osmotic coefficient measurements for dilute aqueous solutions of glycine, l-leucine, and glycylglycine have been reported at 298.15 K. The partial molar volumes and activity coefficients of solute as well as solvent have been estimated using the density and osmotic coefficient data, respectively. Excess and mixing thermodynamic properties, such as Gibbs free energy, enthalpy, and entropy changes, have been obtained using the activity data from this study and the heat data reported in the literature. The concentration enthalpy-entropy compensation effects have been observed for the studied systems, and the compensation temperatures are reported. It has been observed that the excess free energy change for all the studied systems is almost the same over the studied concentration range, showing that the differences in properties of such solutions are largely decided by the enthalpy-entropy effects. These results, along with partial entropy data, show the effects of the presence of hydrophobic interactions and water structure making effect in the case of aqueous solutions of l-leucine. The application of the Starikov-Norden enthalpy-entropy compensation model yielded information about a "hidden Carnot cycle" and the existence of multiple microphases. Application of the Kirkwood-Buff (KB) theory of solutions for the studied systems yields pair correlation functions between the components. The variation of Kirkwood-Buff integrals with concentration further signifies the concentration dependence of the hydrophobic hydration and interactions in the solution phase. The osmotic second virial coefficients have also been obtained using the KB theory and show good agreement with those obtained using the McMillan-Mayer theory of solutions. The mean square concentration fluctuations is estimated using the KB theory, which gives information about the microheterogeneity in the solution phase, which further reflects the presence of hydration and solute association.

Effects of long-chain alcohols on the micellar properties of anionic surfactants in non-aqueous solutions by titration microcalorimetry

The power-time curves of micellar formation of two anionic surfactants, sodium laurate (SLA) and sodium dodecyl sulfate (SDS), in N,N-dimethyl acetamide (DMA) in the presence of various long-chain alcohols (1-heptanol, 1-octanol, 1-nonanol and 1-decanol) were measured by titration microcalorimetry at 298 K. The critical micelle concentrations (CMCs) of SLA and SDS under various conditions at 298 K were obtained based on the power-time curves. Thermodynamic parameters (DH � , DSand DG � ) for micellar systems at 298 K were evaluated according to the power-time curves and the mass action model. The influences of the number of car- bon-atom and the concentration of alcohol were investi- gated. Moreover, combined the thermodynamic parameters at 303, 308 and 313 K in our previous work and those of 298 K in the present work for SLA and SDS in DMA in the presence of long-chain alcohols, an enthalpy-entropy compensation effect was observed. The values of the enthalpy of micellization calculated by direct and indirect methods were made a comparison.

Delineation of Alternative Conformational States in Escherichia coli Peptide Deformylase via Thermodynamic Studies for the Binding of Actinonin

We investigated the binding of a naturally occurring antibiotic, actinonin, to the Ni(2+)-reconstituted recombinant form of Escherichia coli peptide deformylase (PDF(Ec)) via isothermal titration microcalorimetry. The binding data conformed to both exothermic and endothermic phases with magnitudes of DeltaG degrees , DeltaH degrees , and TDeltaS degrees being equal to -12, -2.7, and 9.3 kcal/mol and -8.7, 3.9, and 12.6 kcal/mol, respectively. Evidently, although both phases are dominated by favorable entropic changes, the exothermic phase has about 6.7 kcal/mol enthalpic advantage over the endothermic phase. We observed that the removal of bound Ni(2+) from PDF(Ec) abolished the exothermic phase without affecting the endothermic phase, but it was regained upon addition of Zn(2+). In conjunction with metal analysis data, we propose that the recombinant form of PDF(Ec) is expressed in two stable conformational states that yield markedly distinct ITC profiles (i.e., exothermic versus endothermic) upon interaction with actinonin. The existence of two conformational states of PDF(Ec) is further supported by the observation of two distinct and independent transitions during the thermal unfolding of the enzyme. In addition, the thermodynamic data reveal that the formation of the PDF(Ec)-actinonin complex results in the transfer of one H(+) from the enzyme phase to the bulk solvent at pH 6.3. Both exothermic and endothermic phases produce highly negative DeltaC(p) degrees values, but there is no apparent enthalpy-entropy compensation effect upon formation of the PDF(Ec)-actinonin complex. In view of the known structural features of the enzyme, arguments are presented that the alternative conformational states of PDF(Ec) are modulated by the metal ligation at the enzyme site.

Binding equilibria between β-cyclodextrin and p-nitro-aniline derivatives: the first systematic study in mixed water–methanol solvent systems

Complexation equilibria, in mixed water–methanol solvent media, between native β-cyclodextrin and a set of suitably selected p-nitro-aniline derivatives were studied by means of polarimetry. The effects exerted by the organic co-solvent on the conditional inclusion free energies Δ G cond 0 and the differential molar optical rotations Δ Θ were thoroughly analyzed under the perspective of the enthalpy–entropy compensation effect. Experimental data suggest an intimate participation (‘dynamic co-inclusion’) of solvent molecules in the formation and in the conformational dynamics of the host–guest inclusion complex.

Thermodynamics of mixtures containing polycyclic aromatic hydrocarbons

The DISQUAC group contribution model has been applied to the study of mixtures containing a polycyclic aromatic compound, naphthalene, phenanthrene, anthracene or pyrene, and an organic solvent, n-alkane, cyclohexane or 1-alkanol. The corresponding interaction parameters are reported. Systems with alkanes are characterized by dispersive parameters only. For naphthalene + 1-alkanol mixtures, the aromatic/OH contacts are represented by dispersive and quasichemical parameters. The latter are equal to those encountered in 1-alkanol + benzene, or + alkylbenzene mixtures for such contacts. DISQUAC provides good results on molar excess enthalpies, H E , solid–liquid equilibria, SLE, and vapor–liquid equilibria over a wide range of temperature for the investigated mixtures. In PAH + n-alkane systems, there is an enthalpy-entropy compensation effect in solutions with long alkanes, and no Patterson's effect is observed. Solubility of naphthalene in hexane or 1-hexanol is similar, and lower than that of 1-alkanol in alkane. This seems to indicate that interactions between unlike molecules are not so relevant as in 1-alkanol + benzene mixtures.

Complexation of niflumic acid with native and hydroxypropylated α‐ and β‐cyclodextrins in aqueous solution

AbstractInteractions between niflumic acid and native and hydroxypropylated α‐ and β‐cyclodextrins (CDs) were investigated by 1H NMR, UV‐vis spectroscopy, densimetry, and calorimetry at pH = 7.4 (phosphate buffer) and T = 298.15 K. Thermodynamic parameters of 1:1 complex formation were calculated and discussed in terms of influence of cavity size and availability of hydroxypropyl substituents on the complex stability. The 1H NMR data indicated the inclusion of niflumic acid into macrocyclic cavity of all CDs under study. It was found that both phenyl and pyridine rings of niflumic acid molecule can be included in the cyclodextrin cavity. The co‐existence of two different kinds of 1:1 inclusion complexes in the solution was suggested. In spite of the fact that binding of niflumic acid with α‐cyclodextrin is more enthalpically favorable, stability of the inclusion complexes is very low due to the enthalpy–entropy compensation effect. Complex formation of β‐CDs with niflumic acid is characterized by the higher enthalpy and entropy changes caused by more intense dehydration. Introduction of hydroxypropyl groups in the cyclodextrin molecule was found to promote the binding with niflumic acid. Copyright © 2008 John Wiley & Sons, Ltd.

PDBcal: A Comprehensive Dataset for Receptor–Ligand Interactions with Three‐dimensional Structures and Binding Thermodynamics from Isothermal Titration Calorimetry

Compounds designed solely based on structure often do not result in any improvement of the binding affinity because of entropy-enthalpy compensation. Thermodynamic data along with structure provide an opportunity to gain a deeper understanding of this effect and aid in the refinement of scoring functions used in computational drug design. Here, we scoured the literature and constructed the most comprehensive hand-curated calorimetry dataset to date. It contains thermodynamic and structural data for more than 400 receptor-ligand complexes. The dataset can be accessed through a web interface at http://www.pdbcal.org. The thermodynamic data consists of free energy, enthalpy, entropy and heat capacity as measured by isothermal titration calorimetry (ITC). The dataset also contains the experimental conditions that were used to carry out the ITC experiments. The chemical structures of the ligands are also provided. Analysis of the data confirms the existence of enthalpy-entropy compensation effect for the first time using strictly ITC data.

Kinetics of the reaction of substituted 4-nitrophenyl benzoates with benzenethiol in the presence of potassium carbonate in dimethylformamide

The effect of the substituent nature on the rate and activation parameters of transesterification of a series of 4-nitrophenyl benzoates with benzenethiol in dimethylformamide in the presence of potassium carbonate was studied by the competing reaction method. In all cases, change of the Gibbs energy of activation is determined mainly by variation of the enthalpy of activation. 4-Nitrophenyl benzoates having electron-withdrawing substituents in the benzoyl fragment were found to fit an isokinetic relation with an isokinetic temperature β of 318 K. Enthalpy-entropy compensation effect was observed in the reactions with all the examined 4-nitrophenyl benzoates. The relation between the reactivity and polarizability of nucleophilic center in S-and O-nucleophiles is discussed.

Enthalpy entropy compensation during thermal degradation of chlorophyll in mint and coriander puree

Enthalpy entropy compensation effect has been a debatable topic for explaining the thermodynamic parameters in chemical reactions. Kinetic data for thermal degradation of chlorophylls ( a, b and total) at 105–145 °C in coriander and mint leaf puree at pH 5.5–8.5 was used for this study. By applying the modified transition state theory and Krug’s unbiased statistical regression procedure, the existence of thermodynamic compensation was tested for chlorophyll degradation. Enthalpy of activation (Δ H #) for the coriander and mint purees ranged from 2.36 to 91.99 kJ/mol while entropy (Δ S #) ranged from −0.047 to −0.713 kJ/(mol K). Though the enthalpy and entropy values exhibited an excellent linear relationship ( R 2 > 0.96), a critical analysis of the results indicated absence of any extra-thermodynamic effect or thermodynamic compensation for chlorophylls degradation. The presence of isokinetic point was also not detected.

Micellization behavior of cationic surfactant dodecyldimethylethylammonium bromide (DDAB) in the presence of papain

Abstract The effect of water-soluble, globular protein papain, on the cmc of a cationic surfactant dodecyldimethylethylammonium bromide (DDAB) in aqueous solution has been studied by using conductivity, viscosity and absorbance spectroscopy. The results obtained show that the interactions between surfactant and protein depend upon the concentration of protein and the temperature. The cmc values of DDAB have been estimated at various concentrations of papain as well as at different temperatures ranging from 283.15 to 303.15 K. The thermodynamic parameters of micellization have been determined from cmc values and an enthalpy–entropy compensation effect also observed for the systems. Thermodynamic parameters ( Δ G m ° , Δ H m ° and Δ S m ° ) for the micelle systems in the presence of papain have been obtained by applying the mass action model. Viscosity measurements have been performed to evaluate the rheological and conformational changes of protein–surfactant solution. To confirm the interactions between surfactant and protein, absorbance spectroscopy has been performed.

Fluorescence enhancement of the aflatoxin B 1 by forming inclusion complexes with some cyclodextrins and molecular modeling study

The interaction between the aflatoxin B 1 (AFB 1) and three cyclodextrins, α-cyclodextrin ( α-CD), β-cyclodextrin ( β-CD) and heptakis-2,6-dimethyl- o- β-cyclodextrin (ome-CD), was studied by spectrofluorescence technique. It was found that the inclusion association behavior occurs for the complexes of cyclodextrins with AFB 1. The fluorescence of AFB 1 is generally enhanced in the complexes with cyclodextrins in aqueous solutions. The inclusion complex constants of the three types of cyclodextrins at different temperatures were evaluated from Benesi–Hildebrand plot and also by non-linear regression analysis. These cyclodextrins can only form the 1:1 (host:guest) inclusion complex in the studied temperature range of 20–50 °C. The enthalpy (Δ H°) and entropy (Δ S°) changes of complexation were extracted from the temperature dependency of complex formation constants ( K). Temperature-dependent measurements showed that the association step is controlled by enthalpy–entropy compensation effect. The use of ome-CD generally resulted in the greatest fluorescence intensity. On the other hand, the discrepancy between the exhibited enhanced fluorescence and thermodynamic parameters (Δ G°) is proposed to be different only by the orientation of the AFB 1 within the cyclodextrin cavity. To find the most favorable structure, the geometry of complex was investigated by molecular modeling approach employing the semiemperical HF–SCF calculations.

Carica papaya lipase: An effective biocatalyst for esterification resolution of ( RS)-2-(chlorophenoxy)propionic acid

A partially purified Carica papaya (pCPL) is explored as an effective biocatalyst for the esterification resolution of ( RS)-2-(chlorophenoxy)propionic acid (CPA) with trimethylsilylmethanol in anhydrous isooctane. In comparison with a crude Candida rugosa lipase (CRL), pCPL has higher enantioselectivity ( E > 100) for 2- and 4-chloro but not 3-chloro substituted CPA, in which more than an order-of-magnitude lower reactivity for the hindered ( RS)-2-(2-chlorophenoxy)propionic acid is found for both lipases. The thermodynamic analysis indicates that the enantiomeric discrimination is mainly driven by the difference of activation enthalpy (ΔΔ H) in the present reaction conditions. A linear enthalpy-entropy compensation effect between ΔΔ H and the difference of activation entropy (ΔΔ S) is demonstrated and elucidated from the regio-effect of 2-, 3- and 4-chloro substituent. The kinetic analysis indicates that pCPL is strongly inhibited by the acid substrate, but to a less extent by the alcohol substrate. With the disadvantage of decreasing the enzyme activity, the E value can be greatly enhanced when trimethylsilylmethanol concentration higher than 40 mM in isooctane is employed.

Thermodynamic enthalpy–entropy compensation effects observed in the complexation of basic drug substrates with β-cyclodextrin

Measurement of the variation of inherent drug solubility (So) and 1:1 drug/cyclodextrin complex formation constants (K11) with temperature were used to estimate the thermodynamic parameters (ΔHo, ΔSo and ΔGo). A plot of TΔSo against ΔHo indicates the extent of enthalpy–entropy compensation; that is, how much of the enthalpic gain is cancelled by entropy loss or vice versa (the slope indicates the fraction of conformational change contribution to enthalpy gain that is cancelled by an accompanying entropy loss). The remaining fraction of enthalpy gain contributes to complex formation. The intercept is the inherent contribution to complex stability, which is due to desovation. Extensive phase solubility studies combined with rigorous analysis were conducted in the temperature range 20–45°C for the following basic drugs complexing with β-cyclodextrin (β-CD): astemizole (Astm), cisapride (Cisp), dipyridamole (Dipy), ketotifen (Keto), pizotifen (Pizo), terfenadine (Terf), fexofenadine (Fexo), sildenafil (Sild), and celecoxib (Celox). The results indicate that inherent drug solubility is accompanied by unfavorable conformational changes to the extent of 86%, which are counterbalanced by opposite favorable entropy changes. Only 14% of the favorable enthalpy change contributes to drug solubility. The extent of solvation (hydration) accompanying solubility amounts to −30 kJ/mol, which retards solubility as an unfavorable entropy change. In contrast, 1:1 drug/β-CD complex formation is accompanied by favorable conformational changes to the extent of 94%, which are counterbalanced by unfavorable entropy changes. Only about 6% of enthalpy changes contribute to complex stability. However, the extent of favorable entropy change (desolvation) accompanying complex formation amounts to 26 kJ/mol.

Self-assembly of tetradecyltrimethylammonium bromide in glycerol aqueous mixtures: A thermodynamic and structural study

In this paper, we have studied the effect of glycerol on the micelle formation of tetradecyltrimethylammonium bromide. Changes in both the critical micelle concentration and the degree of counterion binding of the surfactant upon the addition of glycerol across a temperature range (20–40 °C) were examined by using the conductance method. The equilibrium model of micelle formation was applied to obtain the thermodynamic parameters of micellization. An enthalpy–entropy compensation effect was observed in all the solvent systems, but whereas the micellization of the surfactant in the medium with 20% glycerol occurs under the same structural conditions as in pure water, in glycerol rich mixtures the results suggest that the lower aggregation in these media is due to the minor cohesive energy of the solvent system in relation to water. It was also observed that the micellar aggregation number, as obtained by the static quenching method, decreases with the glycerol content. This fact was attributed to an increase in the surface area per headgroup of the surfactant as a consequence of an enhanced solvation, probably induced by the incorporation of some glycerol molecules in the micellar solvation layer. Although the pyrene 1:3 ratio index does not indicate significant changes in the micropolarity at the micelle–bulk interface, the data of fluorescence anisotropy of coumarin 6 and fluorescein are compatible with the formation of a more compact solvation layer.

Enantioseparation of Amino Acid Derivatives with a Cellulose-Based Chiral Stationary Phase

Five structurally related amino acid derivatives were enantioseparated by HPLC with a commercially available chiral stationary phase, Chiralcel OD-H. The chromatographic experiments were performed in the normal phase mode. n-Hexane/polar alcohol was used as mobile phase. Excellent baseline enantioseparations could be obtained for all these solutes. The effects of the concentration of polar alcohol and the column temperature on the retentions and enantioseparations were studied in detail. From the van't Hoff plots the corresponding apparent thermodynamic parameters were derived. Mechanism aspects of chiral recognition were discussed based on the relationship between the thermodynamic parameters and the structures of the solutes. It was found that the substituent of the phenyl group on the residual group of the amino acid derivatives was close relevant to thermodynamic origin of enantioseparation. Much better enthalpy–entropy compensation effect was obtained by plotting the differential, rather than the original, thermodynamic parameters.

Kinetics of the reaction of 4-nitrophenyl benzoates with 4-chlorophenol in the presence of potassium carbonate in dimethylformamide

The effect of the substituent in the benzoyl group on the relative rate and activation parameters of transesterification of substituted 4-nitrophenyl benzoates with 4-chlorophenol in dimethylformamide in the presence of potassium carbonate was studied by the competing reaction technique. The whole series of benzoates showed the enthalpy-entropy compensation effect. 4-Nitrophenyl benzoates having electronacceptor substituents give rise to isokinetic relationship with an isokinetic temperature β of 382 K. The mechanism of the transesterification process is discussed.

Enthalpy–entropy compensation effect in grain boundary phenomena

Abstract The existence of the so-called “enthalpy – entropy compensation effect” as an inherent property of a system is proven on the basis of a pure thermodynamic analysis. The compensation effect is documented for the example of grain boundary segregation and migration. It is shown that the changes of the values of characteristic enthalpy and entropy that fulfil the compensation effect are caused by the changes of grain boundary energy via the changes of grain boundary structure and system chemistry: Despite of the nature of such changes, a single linear dependence between the enthalpy and entropy should exist under the same mechanism of the process.