Hydration Energetics Research Articles

A quantum chemical study on the hydration energetics of trimethylamine N-oxide, its protonated form and tert-butanol

Trimethylamine N-oxide (TMAO) stabilizes the native state of globular proteins and counteracts the destabilizing action of denaturants. However, at pH values lower than its pKa = 4.7, TMAO destabilizes the native state. We studied, via DFT calculations, the interactions of TMAO, its protonated form and isosteric tert-butanol with one, two and three water molecules in vacuo, PCM and SMD water. The energetic strength of the H-bonds the considered solutes make with three water molecules, coupled to a model of globular protein stability, allows us to propose a rationalization of the pH-dependent TMAO behavior toward globular proteins.

Determining the hydration energetics on carbon-supported Ru catalysts: An adsorption calorimetry and density functional theory study

Fundamental knowledge on the energetics at the interface between a water layer and a metal catalyst is essential so as to understand the roles that water can play in the synthesis, activation, and regeneration of noble metal-based catalysts. Here, we report the direct measurement of the enthalpy of water adsorption (Δhads) on activated carbon (C) and activated C-supported ruthenium (Ru) nanoparticles, which is a promising catalyst as applied to the hydrogenation/hydrodeoxygenation (HDO) of oxygenates (phenolics, aldehydes, etc.). Specifically, the near-zero coverage enthalpy of water adsorption on a C-supported Ru catalyst is −75.3 ± 0.4 kJ/(mol water), suggesting favorable water–metal binding. This is much more exothermic than that on C, which has an enthalpy of adsorption of −50.3 ± 1.3 kJ/(mol water). Despite the favorable initial binding, the magnitudes of enthalpies of water condensation on C and Ru-C indicate that overall, their surfaces are both hydrophobic. Moreover, the experimentally-measured near-zero coverage water adsorption enthalpy at the Ru sites is in very good agreement with our density functional theory based calculations. At low coverages, we obtain a water binding energy of −61.7 kJ/(mol water), which increases to −78.1 kJ/(mol water) at saturation. Complementary results are also obtained from a thermal analysis, which employed a thermogravimetric analysis–mass spectrometry (TG-MS), a spectroscopic investigation using ex situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and a morphological evaluation with transmission electron microscopy (TEM). We point out that in carbon-supported metal catalysts, such as Ru-C, a strong hydration at near-zero coverage and relative weak water-surface interactions occur upon saturation. Such heterogeneity is essential and crucial for catalytic hydrogenation/HDO reactions that involve balanced interactions among the water-rich reactant mixture and nonpolar organic products.

Energetics of hydration on uranium oxide and peroxide surfaces

Abstract

Electric field modulated redox-driven protonation and hydration energetics in energy converting enzymes.

Biological energy conversion is catalysed by proton-coupled electron transfer (PCET) reactions that form the chemical basis of respiratory and photosynthetic enzymes. Despite recent advances in structural, biophysical, and computational experiments, the mechanistic principles of these reactions still remain elusive. Based on common functional features observed in redox enzymes, we study here generic mechanistic models for water-mediated long-range PCET reactions. We show how a redox reaction within a buried protein environment creates an electric field that induces hydration changes between the proton acceptor and donor groups, and in turn, lowers the reaction barrier and increases the thermodynamic driving forces for the water-mediated PCET process. We predict linear free energy relationships, and discuss the proposed mechanism in context of PCET in cytochrome c oxidase.

When Is Ligand p Ka a Good Descriptor for Catalyst Energetics? In Search of Optimal CO2 Hydration Catalysts.

We present a detailed study of nearly 70 Zn molecular catalysts for CO2 hydration from four diverse ligand classes ranging from well-studied carbonic anhydrase mimics (e.g., cyclen) to new structures we obtain by leveraging diverse hits from large organic libraries. Using microkinetic analysis and establishing linear free energy relationships, we confirm that turnover is sensitive to the relative thermodynamic stability of reactive hydroxyl and bound bicarbonate moieties. We observe a wide range of thermodynamic stabilities for these intermediates, showing up to 6 kcal/mol improvement over well-studied cyclen catalysts. We observe a good correlation between the p Ka of the Zn-OH2 moiety and the resulting relative stability of hydroxyl moieties over bicarbonate, which may be rationalized by the dominant effect of the difference in higher Zn-OH bond order in comparison to weaker bonding in bicarbonate and water. A direct relationship is identified between isolated organic ligand p Ka and the p Ka of a bound water molecule on the catalyst. Thus, organic ligand p Ka, which is intuitive, easy to compute or tabulate, and much less sensitive to electronic structure method choice than whole-catalyst properties, is a good quantitative descriptor for predicting the effect of through-bond electronic effects on relative CO2 hydration energetics. We expect this to be applicable to other reactions where is it essential to stabilize turnover-determining hydroxyl species with respect to more weakly bound moieties. Finally, we note exceptions for rigid ligands (e.g., porphyrins) that are observed to preferentially stabilize hydroxyl over bicarbonate without reducing p Ka values as substantially. We expect the strategy outlined here, to (i) curate diverse ligands from large organic libraries and (ii) identify when ligand-only properties can determine catalyst energetics, to be broadly useful for both experimental and computational catalyst design.

Salt-Induced Thermoresponsivity of Cross-Linked Polymethoxyethylaminophosphazene Hydrogels: Energetics of the Volume Phase Transition.

Biodegradable hydrogels of cross-linked polymethoxyethylaminophosphazenes (PMOEAPs) of various cross-linking density and apparent subchain hydrophobicity were investigated by high-sensitivity differential scanning calorimetry and equilibrium swelling measurements. The volume phase transition of the hydrogels was found to be induced by salts of weak polybasic acids. The transition parameters were determined depending on the pH, phosphate concentration, cross-linking density, and apparent hydrophobicity of the gels. The transition enthalpy increased three times and reached 60 J g-1 at the phosphate concentrations 5-100 mM. The transition temperature decreased by 60 °C when the pH changed from 6 to 8. A decrease in the transition temperature (by ∼20 °C) was achieved due to incorporation of 9.4 mol % of some alkyl groups into the gel subchains. The classic theory of the collapse of polymer gels coupled with the data of protein science on hydration energetics for various molecular surfaces reproduces correctly thermodynamics of the collapse of PMOEAP hydrogels.

Ab Initio Study of Stability of Na2Fe2(SO4)3, a High Potential Na-Ion Battery Cathode Material

The free energies of thermal decomposition and hydration of Na2Fe2(SO4)3, used as a high operating potential Na-ion battery cathode material (3.8 V), are calculated with the help of the DFT+U method, augmented by magnetic exchange term. In addition, the energetics of hydration of low potential Na2Fe(SO4)·2H2O cathode (3.25 V) in humidified atmosphere is also analyzed. We find that DFT+U/magnetic exchange method can provide a reasonable agreement between calculated ground state properties of the studied materials (i.e., energetics and magnetic moments) and experiment. Using the evaluated total energies of solid materials and the free energies of the molecular species, we determined that Na2Fe2(SO4)3 is indeed thermally unstable at temperatures T ∼ 450 °C in agreement with experimental observations. Moreover, our calculations predict that Na2Fe2(SO4)3 can be hydrated in room temperature environment (T = 20 °C) also in a reasonable agreement with experiment. Overall, our work shows that the DFT+U method, aug...

How the hydrophobic factor drives protein folding

How hydrophobicity (HY) drives protein folding is studied. The 1971 Nozaki-Tanford method of measuring HY is modified to use gases as solutes, not crystals, and this makes the method easy to use. Alkanes are found to be much more hydrophobic than rare gases, and the two different kinds of HY are termed intrinsic (rare gases) and extrinsic (alkanes). The HY values of rare gases are proportional to solvent-accessible surface area (ASA), whereas the HY values of alkanes depend on special hydration shells. Earlier work showed that hydration shells produce the hydration energetics of alkanes. Evidence is given here that the transfer energetics of alkanes to cyclohexane [Wolfenden R, Lewis CA, Jr, Yuan Y, Carter CW, Jr (2015) Proc Natl Acad Sci USA 112(24):7484-7488] measure the release of these shells. Alkane shells are stabilized importantly by van der Waals interactions between alkane carbon and water oxygen atoms. Thus, rare gases cannot form this type of shell. The very short (approximately picoseconds) lifetime of the van der Waals interaction probably explains why NMR efforts to detect alkane hydration shells have failed. The close similarity between the sizes of the opposing energetics for forming or releasing alkane shells confirms the presence of these shells on alkanes and supports Kauzmann's 1959 mechanism of protein folding. A space-filling model is given for the hydration shells on linear alkanes. The model reproduces the n values of Jorgensen et al. [Jorgensen WL, Gao J, Ravimohan C (1985) J Phys Chem 89:3470-3473] for the number of waters in alkane hydration shells.

Energetics of Alkali and Alkaline Earth Ion-Exchanged Zeolite A

Alkali and alkaline earth ion-exchanged zeolite A samples were synthesized in aqueous exchange media. They were thoroughly studied by powder X-ray diffraction (XRD), electron microprobe (EMPA), thermogravimetric analysis and differential scanning calorimetry (TG-DSC), and high temperature oxide melt solution calorimetry. The hydration energetics and enthalpies of formation of these zeolite A materials from constituent oxides were determined. Specifically, the hydration level of zeolite A has a linear dependence on the average ionic potential (Z/r) of the cation, from 0.894 (Rb-A) to 1.317 per TO2 (Mg-A). The formation enthalpies from oxides (25 °C) range from −93.71 ± 1.77 (K-A) to −48.02 ± 1.85 kJ/mol per TO2 (Li-A) for hydrated alkali ion-exchanged zeolite A, and from −47.99 ± 1.20 (Ba-A) to −26.41 ± 1.71 kJ/mol per TO2 (Mg-A) for hydrated alkaline earth ion-exchanged zeolite A. The formation enthalpy from oxides generally becomes less exothermic as Z/r increases, but a distinct difference in slope is o...

Spatial-Decomposition Analysis of Energetics of Ionic Hydration

Hydration energetics is analyzed for a set of ions. The analysis is conducted on the basis of a spatial-decomposition formula for the excess partial molar energy of the solute that expresses the thermodynamic quantity as an integral over the whole space of the solute-solvent and solvent-solvent interactions conditioned by the solute-solvent distance. It is observed for all the ionic solutes treated in the present work that the ion-water interaction is favorable at the expense of the water-water interaction and that the variations of the ion-water and water-water interactions with the ion-water distance compensate against each other beyond the contact distance. The extent of spatial localization of the excess partial molar energy is then assessed by introducing a cutoff into the integral expression and examining the convergence with respect to the change in the cutoff. It is found that the excess energy is not quantitatively localized within the first and second hydration layers, while its correlations over the variation of ions are good against the first-layer contribution.

The Relationship Between Segmental Motions, Hydration and Overall Mass Transport in Pfsas

A great many membranes have been studied for use in fuel cells and other electrochemical devices. Prominent amongst these are the perfluorosulfonic acids, including Nafion as an archetype and materials prepared by Dow (later others) and 3M presented over time. Generally, similar mass transport (conductivity and water transport) phenomena have been observed, with high transport rates closely tied to high levels of hydration. However, maintaining high hydration of the membrane by maintaining an environment of high relative humidity requires system level measures that represent parasitic losses. Achieving high conductivity with less hydration requires more substantial understanding and then control of the transport mechanisms involved at low water content. There are clear differences in the behavior of materials at low and high hydration, with a line of demarcation at hydration levels of ca. 5 or 6 waters per sulfonate. Above this level, the water in the membrane is more continuous across multiple pores and protons move in an environment that increasing resembles that in aqueous solutions as hydration levels increase. Below this level, such factors as the acidity of the sulfonic acid, the mobility of water within the first or second hydration spheres of the sulfonate and proton and possibly sidechain movements in the polymer all have been suggested to play roles in the transport processes. In this contribution, we will explore various processes in the low water content regime. For a series of PFSAs, we will gather experimental information to reveal the approximate rate of side chain motion, use vibrational spectroscopy and related methods to provide an in-depth view into the hydration of ions and water interactions in the membrane and present information on the energetics of hydration. These will be synthesized into an experimentally supported picture allowing us to assess the relative importance of the motions of anions, cations and water molecules in determining transport in PFSAs at RH < 50% under different conditions.

First-Principles Insight into the Hydration Ability and Proton Conduction of the Solid State Proton Conductor, Y and Sn Co-Doped BaZrO3

Y and Sn co-doped BaZrO3 (BZSY) has recently been shown to exhibit superior hydration ability and improved power output performance compared to those of the traditional solid oxide fuel cell (SOFC) electrolyte, Y-doped BaZrO3. The fact that BZSY is also chemically stable in both H2O and CO2 atmospheres further illustrates the great potential for BZSY as a future electrolyte material in proton-conducting SOFCs. In this work, we conducted comprehensive density functional theory calculations to analyze the energetics of hydration and proton migration in BZSY. The energy of hydration is calculated at four different locations in the cell to better assess the specific contributions of each defect type. For all locations tested, hydration is found to be strongly exothermic and is most favorable when Y ions are in the vicinity. The calculated energies of hydration (−1.72 to −1.11 eV) are significantly lower than any previously calculated for acceptor-doped BaZrO3-based electrolytes. Nudged elastic band calculations confirm low proton reorientation energies (0.08–0.24 eV) and low intraoctahedral hopping energies (0.21–0.41 eV) and that these diffusion barriers are at minimum when the proton is migrating to an oxygen ion in a YO6 octahedron. Our results show how the synergy of the Sn and Y dopant ions produces the excellent hydration and conduction performance of BZSY and fully support its potential application in next-generation SOFCs.

Dynamic hydration shell restores Kauzmann's 1959 explanation of how the hydrophobic factor drives protein folding.

Kauzmann's explanation of how the hydrophobic factor drives protein folding is reexamined. His explanation said that hydrocarbon hydration shells are formed, possibly of clathrate water, and they explain why hydrocarbons have uniquely low solubilities in water. His explanation was not universally accepted because of skepticism about the clathrate hydration shell. A revised version is given here in which a dynamic hydration shell is formed by van der Waals (vdw) attraction, as proposed in 1985 by Jorgensen et al. [Jorgensen WL, Gao J, Ravimohan C (1985) J Phys Chem 89:3470-3473]. The vdw hydration shell is implicit in theories of hydrophobicity that contain the vdw interaction between hydrocarbon C and water O atoms. To test the vdw shell model against the known hydration energetics of alkanes, the energetics should be based on the Ben-Naim standard state (solute transfer between fixed positions in the gas and liquid phases). Then the energetics are proportional to n, the number of water molecules correlated with an alkane by vdw attraction, given by the simulations of Jorgensen et al. The energetics show that the decrease in entropy upon hydration is the root cause of hydrophobicity; it probably results from extensive ordering of water molecules in the vdw shell. The puzzle of how hydrophobic free energy can be proportional to nonpolar surface area when the free energy is unfavorable and the only known interaction (the vdw attraction) is favorable, is resolved by finding that the unfavorable free energy is produced by the vdw shell.

Predicting hydration free energies of polychlorinated aromatic compounds from the SAMPL-3 data set with FiSH and LIE models

Next-generation solvation models are devised to mimic the accuracy and generality of explicit solvation models at the speed of current popular implicit solvation models. One such method is the first-shell of hydration (FiSH) continuum model that was trained on hydration energetics from LIE calculations and molecular dynamics simulations in explicit solvent. Here we tested prospectively the FiSH model on the SAMPL-3 hydration data set that zooms in the effect of chlorination on solvation. We compare these FiSH predictions with those from retrospective LIE calculations. We find that neither FiSH nor LIE can reproduce well the absolute values and the trend of hydration free energies in the biphenyl and dioxin aromatic chlorination series. Some of the hypotheses behind this performance are discussed and tested. The LIE explicit-solvent model shows some improvement relative to the FiSH continuum model, and we correct a systematic deviation in the continuum van der Waals term of FiSH associated with aromatic Cl atom type.

Molecular Modeling of Hydrotalcite Structure Intercalated with Transition Metal Oxide Anions: CrO42– and VO43–

Molecular dynamics (MD) simulations are used to study the interlayer structure, hydrogen bonding, and energetics of hydration of Mg/Al (2:1 and 4:1) layered double hydroxide (LDH) or hydrotalcite (HT) intercalated with oxymetal anions, CrO(4)(2-), and VO(4)(3-). The ab initio forcefield COMPASS is employed for the simulations. The charge on the oxymetal anions is determined by quantum mechanical density functional theory. The structural behavior of the oxymetal anions in LDH directly relates to the energetic relationships, with electrostatic and H-bonding interactions between the anions, hydroxide sites of the metal hydroxide layers, and the interlayer water molecules. Distinct minima in the hydration energy indicate the presence of energetically well-defined structural states with specific water content. The experimentally identified variability in the retention of the CrO(4)(2-) and VO(4)(3-) is well reflected in the calculations and self-diffusion coefficients obtained from the simulations give insight into the mobility of the intercalated species.

Classical Simulations with the POLIR Potential Describe the Vibrational Spectroscopy and Energetics of Hydration: Divalent Cations, from Solvation to Coordination Complex

POLIR, a polarizable water potential optimized for vibrational and intermolecular spectroscopy in pure water but not optimized for solvation, is used to describe solutions of the divalent metal cations Ca(2+), Mg(2+), and Cu(2+). The spectral shifts in the O-H stretch region obtained from classical simulations are in agreement with experiment. The water-ion binding energies are dominated by classical electrostatics, even though the Cu(2+) case might be considered to involve an intermediate-strength chemical bond. Three-body energies of the ion with the first solvation shell are in agreement with ab initio calculations. Our results indicate the importance of polarization in the development of accurate, transferable, force fields and the power of classical methods when it is carefully included.

Hydration of gas-phase ytterbium ion complexes studied by experiment and theory

Hydration of ytterbium (III) halide/hydroxide ions produced by electrospray ionization was studied in a quadrupole ion trap mass spectrometer and by density functional theory (DFT). Gas-phase YbX2 + and YbX(OH)+ (X = OH, Cl, Br, or I) were found to coordinate from one to four water molecules, depending on the ion residence time in the trap. From the time dependence of the hydration steps, relative reaction rates were obtained. It was determined that the second hydration was faster than both the first and third hydrations, and the fourth hydration was the slowest; this ordering reflects a combination of insufficient degrees of freedom for cooling the hot monohydrate ion and decreasing binding energies with increasing hydration number. Hydration energetics and hydrate structures were computed using two approaches of DFT. The relativistic scalar ZORA approach was used with the PBE functional and all-electron TZ2P basis sets; the B3LYP functional was used with the Stuttgart relativistic small-core ANO/ECP basis sets. The parallel experimental and computational results illuminate fundamental aspects of hydration of f-element ion complexes. The experimental observations—kinetics and extent of hydration—are discussed in relationship to the computed structures and energetics of the hydrates. The absence of pentahydrates is in accord with the DFT results, which indicate that the lowest energy structures have the fifth water molecule in the second shell.

Electrostatics of Deformable Lipid Membranes

It was recently demonstrated that significant local deformations of biological membranes take place due to the fields of charged peptides and ions, challenging the standard model of membrane electrostatics. The ability of ions to retain their immediate hydration environment, combined with the lack of sensitivity of permeability to ion type or even ion pairs, led us to question the extent to which hydration energetics and electrostatics control membrane ion permeation. Using the arginine analog methyl-guanidinium as a test case, we find that although hydrocarbon electronic polarizability causes dramatic changes in ion solvation free energy, as well as a significant change (∼0.4 V) in the membrane dipole potential, little change in membrane permeation energetics occurs. We attribute this to compensation of solvation terms from polar and polarizable nonpolar components within the membrane, and explain why the dipole potential is not fully sensed in terms of the locally deformed bilayer interface. Our descriptions provide a deeper understanding of the translocation process and allow predictions for poly-ions, ion pairs, charged lipids, and lipid flip-flop. We also report simulations of large hydrophobic-ion-like membrane defects and the ionophore valinomycin, which exhibit little membrane deformation, as well as hydrophilic defects and the ion channel gramicidin A, to provide parallels to membranes deformed by unassisted ion permeation.

2SB1105 機能性分子およびイオンの水和の構造論とエネルギー論(2SB ATPエネルギーの分子論,第48回日本生物物理学会年会)

2SB1105 機能性分子およびイオンの水和の構造論とエネルギー論(2SB ATPエネルギーの分子論,第48回日本生物物理学会年会)

Cation behavior at an artificial cell interface: binding distinguished by ion hydration energetics and size

The unique molybdenum oxide-based nucleophilic porous capsule/artificial cell [{(MoVI)MoVI5O21(H2O)6}12{MoV2O4(SO4)30}]72-, according to an X-ray crystallographic study, traps [Al(H2O)6]3+ complexes above the pores while interacting with the latter via hydrogen bonds; this is supported by 27Al NMR studies of the interaction of the capsule with hydrated Al3+ cations in aqueous solution.