Hydration Heat Capacity Research Articles

Enthalpy and heat capacity of hydration of the cytostatic Dioxadet

Thermochemical data on hydration of the well-established anticancer agent Dioxadet were obtained. Its enthalpies of solution and sublimation at various temperatures were determined calorimetrically, and on their basis the standard enthalpy (ΔhydrH0 = −154.6 ± 3.6 kJ mol−1) and the standard heat capacity (ΔhydrCp0 = 384 ± 80 J mol−1 K−1) of the transfer of the solute from the state of an ideal gas to an aqueous solution at infinite dilution at 298.15 K were calculated.

Uncovering Differences in Hydration Free Energies and Structures for Model Compound Mimics of Charged Side Chains of Amino Acids.

Free energies of hydration are of fundamental interest for modeling and understanding conformational and phase equilibria of macromolecular solutes in aqueous phases. Of particular relevance to systems such as intrinsically disordered proteins are the free energies of hydration and hydration structures of model compounds that mimic charged side chains of Arg, Lys, Asp, and Glu. Here, we deploy a Thermodynamic Cycle-based Proton Dissociation (TCPD) approach in conjunction with data from direct measurements to obtain estimates for the free energies of hydration for model compounds that mimic the side chains of Arg+, Lys+, Asp–, and Glu–. Irrespective of the choice made for the hydration free energy of the proton, the TCPD approach reveals clear trends regarding the free energies of hydration for Arg+, Lys+, Asp–, and Glu–. These trends include asymmetries between the hydration free energies of acidic (Asp– and Glu–) and basic (Arg+ and Lys+) residues. Further, the TCPD analysis, which relies on a combination of experimental data, shows that the free energy of hydration of Arg+ is less favorable than that of Lys+. We sought a physical explanation for the TCPD-derived trends in free energies of hydration. To this end, we performed temperature-dependent calculations of free energies of hydration and analyzed hydration structures from simulations that use the polarizable Atomic Multipole Optimized Energetics for Biomolecular Applications (AMOEBA) force field and water model. At 298 K, the AMOEBA model generates estimates of free energies of hydration that are consistent with TCPD values with a free energy of hydration for the proton of ca. −259 kcal/mol. Analysis of temperature-dependent simulations leads to a structural explanation for the observed differences in free energies of hydration of ionizable residues and reveals that the heat capacity of hydration is positive for Arg+ and Lys+ and negative for Asp– and Glu–.

Origin of heat capacity increment in DNA folding: The hydration effect

BackgroundUnderstanding DNA folding thermodynamics is crucial for prediction of DNA thermal stability. It is now well established that DNA folding is accompanied by a decrease of the heat capacity ∆cp, F, however its molecular origin is not understood. In analogy to protein folding it has been assumed that this is due to dehydration of DNA constituents, however no evidence exists to support this conclusion. MethodsHere we analyze partial molar heat capacity of nucleic bases and nucleosides in aqueous solutions obtained from calorimetric experiments and calculate the hydration heat capacity contribution ∆cphyd. ResultsWe present hydration heat capacity contributions of DNA constituents and show that they correlate with the solvent accessible surface area. The average contribution for nucleic base dehydration is +0.56 J mol−1 K−1 Å−2 and can be used to estimate the ∆cp, F contribution for DNA folding. ConclusionsWe show that dehydration is one of the major sources contributing to the observed ∆cp, F increment in DNA folding. Other possible sources contributing to the overall ∆cp, F should be significant but appear to compensate each other to high degree. The calculated ∆cphyd for duplexes and noncanonical DNA structures agree excellently with the overall experimental ∆cp, F values. By contrast, empirical parametrizations developed for proteins result in poor ∆cphyd predictions and should not be applied to DNA folding. General significanceHeat capacity is one of the main thermodynamic quantities that strongly affects thermal stability of macromolecules. At the molecular level the heat capacity in DNA folding stems from removal of water from nucleobases.

Temperature, Pressure, and Concentration Derivatives of Nonpolar Gas Hydration: Impact on the Heat Capacity, Temperature of Maximum Density, and Speed of Sound of Aqueous Mixtures.

The hydrophobic effect is an umbrella term encompassing a number of solvation phenomena associated with solutions of nonpolar species in water, including the following: a meager solubility opposed by entropy at room temperature; large positive hydration heat capacities; positive shifts in the temperature of maximum density of aqueous mixtures; increases in the speed of sound of dilute aqueous mixtures; and negative volumes of association between interacting solutes. Here we present a molecular simulation study of nonpolar gas hydration over the temperature range 273.15-373.15 K and a pressure range -500 to 1000 bar to investigate the interrelationships between distinct hydrophobic phenomena. We develop a new free energy correlation for the solute chemical potentials founded on the Tait equation description of the equation-of-state of liquid water. This analytical correlation is shown to provide a quantitatively accurate description of nonpolar gas hydration over the entire range of thermodynamic state points simulated, with an error of ∼0.02 kBT or lower in the fitted chemical potentials. Our simulations and the correlation accurately reproduce many of the available experimental results for the hydration of the solutes examined here. Moreover, the correlation reproduces the characteristic entropies of hydration, temperature dependence of the hydration heat capacity, perturbations in the temperature of maximum density, and changes in the speed of sound. While negative volumes of association result from pairwise interactions in solution, beyond the limits of our simulations performed at infinite dilution, we discuss how our correlation could be supplemented with second virial coefficient information to expand to finite concentrations. In total, this work demonstrates that many distinct phenomena associated with the hydrophobic effect can be captured within a single thermodynamically consistent correlation for solute hydration free energies.

Heat capacity effects associated with urea and tetramethylurea hydration: insight from computer simulation

Molecular dynamic simulation of the hydrophilic urea and hydrophobic tetramethylurea aqueous solutions using the random network model of liquid water reveals that apolar Me groups induce an increase in the population of water molecules with linear and shorter H-bonds in their first hydration shell, whereas the carbonyl oxygen atom causes an opposite effect with elevation in the population of high angle/distance water molecules pairs. This behavior of water is the major reason for opposite changes in the heat capacity of hydration for apolar and polar species.

Air–Water Partitioning of C5 and C6 Alkanones: Measurement, Critical Compilation, Correlation, and Recommended Data

Air–water partitioning of all nine C5 and C6 isomeric alkanones (namely, pentan-2-one, pentan-3-one, 3-methylbutan-2-one, hexan-2-one, hexan-3-one, 3-methylpentan-2-one, 4-methylpentan-2-one, 2-methylpentan-3-one, and 3,3-dimethylbutan-2-one) in water was examined at several temperatures in the range from (273 to 333) K using the inert gas stripping technique. Results are reported in the form of air–water partition coefficients Kaw and limiting (infinite dilution) activity coefficients γ1∞. A comprehensive critical review is further presented for such literature data as well as data on respective derivative thermal properties (enthalpies and heat capacities of hydration and dissolution). For each alkanone, the data measured in this work together with those reported previously in the literature were correlated with a suitable model equation providing an adequate simultaneous description of the equilibrium measurements and the calorimetric information. As a result, recommended Kaw(T) and γ1∞(T) temperature ...

D2O–H2O solvent isotope effects on the enthalpy of 1,1,3,3-tetramethyl-2-thiourea hydration at temperatures from (278.15 to 313.15)K and ambient pressure

Abstract The enthalpies of solution of 1,1,3,3-tetramethyl-2-thiourea (TMTU) in ordinary (H2O) and heavy (D2O) water were measured at (278.15, 283.15, 288.15, 298.15, and 313.15) K and atmospheric pressure. Standard molar enthalpies and heat capacities of solution and hydration, together with D2O–H2O solvent isotope effects (IEs) on these quantities, were calculated. It was established that, unlike the process of forming aqueous 1,1,3,3-tetramethyl-2-urea (TMU), the dissolution of TMTU in both H2O and D2O is an endothermic effect over the whole temperature range studied, and the standard enthalpy-isotopic effect undergoes a negative-to-positive sign inversion nearby of T = 304 K. Going from TMTU to TMU, the standard heat capacity of solution (hydration) and corresponding IE become less positive.

Prediction of Heat Capacities of Hydration of Various Organic Compounds Using Partial Least Squares and Artificial Neural Network

A quantitative structure–property relationship study based on artificial neural network (ANN) was carried out for the prediction of the heat capacities of hydration of a set of 289 organic compounds of very different chemical natures. The genetic algorithm-partial least squares (GA-PLS) method was applied as a variable selection tool. A PLS method was used to select the best descriptors, and the selected descriptors were then used as input neurons in a neural network model. These descriptors are: number of H atoms (NHA), maximum partial charge in the molecule (Q max), atomic charge weighted PPSA (PPSA3), relative positive charge (RPCG), minimum net atomic charge (Q min), fractional PPSA (FPSA3), and Randic index (order 1) (1 χ). The results obtained show the ability of the developed artificial neural network model to predict heat capacities of hydration of various organic compounds. Also, the results reveal the superiority of the ANN over the PLS model.

The hydrogen bond properties of water from 273 K to 573 K; equations for the prediction of gas-water partition coefficients

An equation due to Plyasunov and Shock for the calculation of gas to water partition coefficients, as log K(w), at high temperature requires knowledge of experimental values of log K, ΔH the standard enthalpy of hydration at 298 K, ΔCp the heat capacity of hydration at 298 K and b' that is a constant that refers to the variation of ΔCp(w) with temperature. Linear free energy relationships based on Abraham descriptors for solutes have been constructed for all of these four input quantities, so that log K(w), ΔH, ΔCp and b' can be estimated. Known values of the four input quantities, where available, together with estimated values enabled values of log K(w) from 273 to 573 K to be calculated for 555 solutes. At various fixed temperatures LFERs again based on Abraham descriptors were constructed from the obtained log K(w) values for the 555 solutes. These LFERs can then be used to estimate further values of log K(w) between 273 and 573 K for any solute with already determined descriptors, so that knowledge of experimental values of log K, ΔH, ΔCp and b' is no longer necessary. The obtained LFERs show how the solvation properties of water vary with temperature. Both water hydrogen bond acidity and hydrogen bond basicity towards solutes decrease substantially with increase of temperature until at 573 K the hydrogen bond properties of water resemble those for solvents such as nitrobenzene and acetonitrile at 298 K. A characteristic property of water at ambient temperatures is that water will not solvate non-polar solutes, thus leading to large Henry's law constants and small values of K(w). Although this property also decreases with increase of temperature, our results indicate that it still remains to some extent even at 573 K, so that the general solvation properties of water at 573 K do not resemble the solvation properties of organic solvents at 298 K.

Temperature and Length Scale Dependence of Tetraalkylammonium Ion Solvation in Water, Formamide, and Ethylene Glycol

This paper focuses on the temperature and length scale dependence of solvation of familiar hydrophobic solutes: tetraalkylammonium ions in highly associated solvents with H-bond networks. Standard enthalpies and heat capacities of solution of symmetrical tetraalkylammonium bromides in formamide (FA) and ethylene glycol (C(2)H(4)(OH)(2)) have been computed using experimental ΔH(m)(sol) values and compared with previously reported enthalpies and heat capacities in pure water. Heat capacities of hydration are found to be large, positive, and increase almost linearly up to tetrapentylammonium bromide. Hydration of tetrahexyl- and tetraheptylammonium bromides is accompanied by much smaller heat capacity changes than would be expected from the length scale dependence observed. The same behavior reveals enthalpies and heat capacities of solute transfer from FA and C(2)H(4)(OH)(2) to water. The results obtained rather indicate that solvation of large tetraalkylammonium ions containing more than twenty carbon atoms is accompanied by much less water orientational ordering than for solutes of a moderate size. It results in smaller fluctuations in the water-water pair energy in a hydration shell and consequently reduces the heat capacity of hydration. The comparison of tetraalkylammonium ion behavior in water and the nonaqueous solvents studied shows that the length scale dependence of solvophobic solvation is strongly pronounced in water but not in nonaqueous media.

Optimization of linear and branched alkane interactions with water to simulate hydrophobic hydration

Previous studies of simple gas hydration have demonstrated that the accuracy of molecular simulations at capturing the thermodynamic signatures of hydrophobic hydration is linked both to the fidelity of the water model at replicating the experimental liquid density at ambient pressure and an accounting of polarization interactions between the solute and water. We extend those studies to examine alkane hydration using the transferable potentials for phase equilibria united-atom model for linear and branched alkanes, developed to reproduce alkane phase behavior, and the TIP4P/2005 model for water, which provides one of the best descriptions of liquid water for the available fixed-point charge models. Alkane site/water oxygen Lennard-Jones cross interactions were optimized to reproduce the experimental alkane hydration free energies over a range of temperatures. The optimized model reproduces the hydration free energies of the fitted alkanes with a root mean square difference between simulation and experiment of 0.06 kcal/mol over a wide temperature range, compared to 0.44 kcal/mol for the parent model. The optimized model accurately reproduces the temperature dependence of hydrophobic hydration, as characterized by the hydration enthalpies, entropies, and heat capacities, as well as the pressure response, as characterized by partial molar volumes.

Assessing the thermodynamic signatures of hydrophobic hydration for several common water models

Following the conclusions of an information theory analysis that hydrophobic hydration is dictated by the equation of state of liquid water, we perform simulations of ten different water models to examine the correlation between the fidelity of each model to the experimental density of liquid water and the accuracy of its description of methane hydration. We find that the three- and five-point water models provide an inferior description of both the liquid density and methane solubility compared to the four-point water models. Of the four-point water models, TIP4P/2005 provides the best description of both the aqueous equation-of-state and methane hydration thermodynamics. When the optimized potentials for liquid simulation united-atom description for methane is used, we find that while the entropy and heat capacity of methane hydration are in excellent agreement with experiment, the chemical potential and enthalpy are systematically shifted upwards. We subsequently reoptimize the methane interaction to accurately reproduce the experimental solubilities as a function of temperature by accounting for missing attractive interactions.

Molecular origin of the negative heat capacity of hydrophilic hydration

The hydrophobic and hydrophilic hydrations are analyzed with the emphasis on the sign of the heat capacity of hydration (HCH). The angle-dependent integral equation theory combined with a multipolar water model is employed in the analysis. The hydration entropy (HE) is decomposed into the translational and orientational parts. It is found that the orientational part governs the temperature dependence of the HE. The orientational part is further decomposed into the solute-water pair correlation component (component 1) and the water reorganization component (component 2). For hydrophilic solutes, components 1 and 2 are negative and positive, respectively. As the temperature becomes higher, component 1 increases while component 2 decreases: They make positive and negative contributions to the HCH, respectively. The strong solute-water electrostatic attractive interactions induce the distortion of water structure near the solute and the break of hydrogen bonds. As the temperature increases, the effect of the attractive interactions becomes smaller and the distortion of water structure is reduced (i.e., more hydrogen bonds are recovered with increasing temperature). The latter effect dominates, leading to negative HCH. During the heat addition the formation of hydrogen bonds, which accompanies heat generation, occurs near the solute. Consequently, the addition of the same amount of heat leads to a larger increase in the thermal energy (or equivalently, in the temperature) than in the case of pure water. The hydrophobic hydration, which is opposite to the hydrophilic hydration in many respects, is also discussed in detail.

Water Molecule Coupling to MeCl+ and MeClO4 + Ions Studied by the Electrospray Ionisation Mass Spectrometry

Abstract The binding stability of water molecules with MeCl+ and MeClO4 + ions has been studied by the electrospray ionisation mass spectrometry (ESI-MS). The Me ions studied were the typical transition metals (Me = Fe, Co, Ni, Cu, Zn, or Cd) and the typical alkaline-earth metals (Me = Mg, Ca, Sr, or Ba). The increasing voltage between the skimmer cones (referred to as the cone voltage) induced the abstraction of water molecules from MeCl+ and MeClO4 + ions. The binding stability of water molecules with MeCl+ and MeClO4 + ions has been correlated with the Me cation mass and with the heat capacity of hydration of Me cations. Moreover, the physicochemical properties of the ions studied have been compared by the cluster analysis and the principal component analysis of the experimental curves obtained for the [MeCl-H2O]+ and [MeClO4-nH2O]+ ions, where n is 1 or 2. Our results can be useful in examination of the ion dehydration process.

Thermal stability of proteins does not correlate with the energy of intramolecular interactions

Small monomeric proteins from mesophilic and thermophilic organisms were studied. They have close structural and physical and chemical properties but vary in thermal stability. A thermodynamic analysis of heat unfolding was made and integral enthalpy of unfolding (Δ H unf), heat capacity of hydration (Δ C p hyd) and enthalpy of hydration (Δ H hyd) and of the buried surface area (ΔASA) of nonpolar and polar groups as well as the enthalpy of disruption of intramolecular interaction (Δ H int in gas phase) at 298 K were determined. The absence of correlation between protein thermostability and energetic components suggests that regulatory mechanism of protein thermal stabilization has entropic nature.

Anomalous Temperature Dependence of the Heat of Hydration of Natrolite

The temperature dependence of the heat of hydration of natrolite was studied by isothermal adsorption calo- rimetry from 412 to 472 K. The heat capacity of hydration implied by these results is about 5 times greater than the calo- rimetric heat capacity of reaction, suggesting atypical behavior across this solid solution.

Prediction of gas to water partition coefficients from 273 to 373 K using predicted enthalpies and heat capacities of hydration

In previous works, we have developed methods for the prediction of gas to water partition coefficients at 298 K, log K w (298), and for the prediction of enthalpies of transfer from the gas phase to water, that is enthalpies of hydration, Δ H w ° , using the set of Abraham descriptors. In the present work we collect data on heat capacities of hydration, Δ C p w , for 321 varied compounds and obtain equations that can be used to predict this quantity, again in terms of Abraham descriptors. If the three thermodynamic input quantities, log K w (298), Δ H w ° and Δ C p w , are all available it is trivial to predict log K w at any temperature between 273 and 373 K through equations given by Plyasunov and Shock. However, if one or more of these input quantities are unavailable, we show that predictions of these input quantities using our equations based on Abraham descriptors can be used to predict log K w at any temperature between 273 and 373 K, log K w ( T). In the most rigorous test of this method, log K w (373) values were calculated using predicted values of all three input quantities, log K w (298), Δ H w ° and Δ C p w . For 40 varied compounds covering a range of 5.5 log units in log K w (373), values of log K w (373) were predicted with an average error of −0.02, and a standard deviation of 0.16 log unit. Many other examples of predictions are given. A spreadsheet is available for the prediction of log K w ( T) values; all that is required is insertion of Abraham descriptors for a compound and the temperature. We consider polyaromatic hydrocarbons and polychlorobiphenyls separately, and show that it is possible to predict log K w ( T) for these compounds as well.

Heat capacity of hydration in zeolites

Dehydration of zeolites is a critical factor in their stability, effectiveness as storage media for thermal energy, and influence on the thermal evolution of radioactive waste repositories. Prediction of the hydration states of zeolites as a function of temperature, pressure, and water activity is often hampered by limited data on the heat capacity changes associated with (de)hydration reactions. This property is often assumed to be temperature invariant based on statistical-mechanical reasoning. To test this assumption, heat capacities of homologous hydrated and dehydrated zeolites (analcime, natrolite, wairakite) were measured by differential scanning calorimetry from 130 to 600 K that were subsequently used to calculate the heat capacity of hydration. Simultaneous monitoring of sample mass was used to discard data for hydrated phases affected by thermal effects associated with dehydration at elevated temperature. All three phases exhibited marked variations in the heat capacity of hydration with temperature. This behavior appears to be related to second-order phase transitions involving the water molecules. In wairakite, a second-order phase transition associated with a change in framework symmetry from monoclinic to tetragonal only occurs in the hydrated form, and leads to a prominent peak in the temperature dependence of the heat capacity of hydration. Above the transition temperature, the heat capacity of hydration becomes negligible. Neglecting the temperature dependence of the heat capacity of hydration in zeolites can have a significant effect on thermodynamic properties of dehydration reactions regressed from equilibrium observations at elevated temperatures

Origin of heat capacity changes in a "nonclassical" hydrophobic interaction.

Origin of heat capacity changes in a "nonclassical" hydrophobic interaction.

Temperature Dependence of Limiting Activity Coefficients, Henry's Law Constants, and Related Infinite Dilution Properties of Branched (C3 and C4) Alkanols in Water. Measurement, Critical Compilation, Correlation, and Recommended Data

Limiting activity coefficients ( ) of lower branched alkanols (2-propanol, 2-butanol, 2-methyl-1-propanol, and 2-methyl-2-propanol) in water were measured at several temperatures covering the range from the melting to the normal boiling point of water. Four experimental techniques (namely, headspace analysis, inert gas stripping, Rayleigh distillation, and the method of circulation still) were employed for the purpose. A comprehensive review is further presented of experimental data on the limiting activity coefficients ( ), infinite dilution partial molar excess enthalpies ( ), and heat capacities ( ) of these aqueous solutes. For each alkanol, the compiled data were critically evaluated and together with the data measured in this work correlated with a suitable model equation providing adequate simultaneous description of the equilibrium measurements and the calorimetric information. As a result, a recommended thermodynamically consistent temperature dependence of , , and of superior accuracy was established in the range from the melting point to the normal boiling point of water. In addition, by employing literature data on the respective residual properties of the pure alkanols, analogous recommendations were derived also for the temperature dependence of the Henry's law constants, hydration enthalpies, and hydration heat capacities. Variation of these various infinite dilution thermodynamic properties of aqueous branched alkanols with temperature and alkanol branching is briefly discussed.