Dehydration Energy Research Articles

Dehydration energies of alkaline earth metal halides – a novel simulation methodology

Abstract The apparent dehydration energies and surface potentials of diverse alkaline earth halides are computed employing a novel simulation methodology incorporating hydration numbers, ion pairing effects and random distribution of molecules. The progressive variation of the hydration numbers and hydrated radii are estimated. The significance of the formalism in rationalizing the trend in the variation of the solubility, lattice energy, etc. is indicated.

Anchor profiles of HLA‐specific peptides: Analysis by a novel affinity scoring method and experimental validation

The study of intermolecular interactions is a fundamental research subject in biology. Here we report on the development of a quantitative structure-based affinity scoring method for peptide-protein complexes, named PepScope. The method operates on the basis of a highly specific force field function (CHARMM) that is applied to all-atom structural representations of peptide-receptor complexes. Peptide side-chain contributions to total affinity are scored after detailed rotameric sampling followed by controlled energy refinement. A de novo approach to estimate dehydration energies was developed, based on the simulation of individual amino acids in a solvent box filled with explicit water molecules. Transferability of the method was demonstrated by its application to the hydrophobic HLA-A2 and -A24 receptors, the polar HLA-A1, and the sterically ruled HLA-B7 receptor. A combined theoretical and experimental study on 39 anchor substitutions in FxSKQYMTx/HLA-A2 and -A24 complexes indicated a prediction accuracy of about two thirds of a log-unit in Kd. Analysis of free energy contributions identified a great role of desolvation and conformational strain effects in establishing a given specificity profile. Interestingly, the method rightly predicted that most anchor profiles are less specific than so far assumed. This suggests that many potential T-cell epitopes could be missed with current prediction methods. The results presented in this work may therefore significantly affect T-cell epitope discovery programs applied in the field of peptide vaccine development.

A simple simulation methodology for estimation of dehydration energies and surface potentials of concentrated NaCl solutions.

A novel simulation procedure for estimating the dehydration energies of NaCl solutions spanning a wide range of concentrations, which incorporates ionic and molecular sizes, ion-pair formation, etc., is proposed on geometric and phenomenological considerations. The extent of dehydration during each movement of the hydrated molecule is evaluated using the expected and actual displacement of the species and mean nearest-neighbor distances. The interdependence between the size of the simulation box, number of molecules, and electrolyte concentration is pointed out.

Dehydration kinetics of calcium aluminate cement hydrate under non-isothermal conditions

Calcium aluminate cement (containing 75% Al 2O 3), which finds extensive high temperature applications, was synthesized by sintering a mixture of calcined alumina and synthetic calcium carbonate. The cement was hydrated and its kinetics of thermal dehydration was studied under non-isothermal conditions by thermo-gravimetry using the integral approximation model. It was observed that on heating the cement hydrate followed multistage dehydration with different reaction orders at different stages. The activation energies of dehydration for these different stages were compared.

Theorization on ion-exchange equilibria: activity of species in 2-D phases

Ion-exchange reactions are naturally occurring at soil and sediment/water interphases, determining soil fertility and water quality. These ion-exchange reactions with inorganic and organic exchangers are applied to chemical analysis, recovery of useful ions from low-grade ores (potentially from sea water), water purification including the preparation of "ultrapure" water, production of foods and medicines, therapy, and other uses. It is important to theorize about or to model ion-exchange reactions for quantitative explanations of ion-exchange phenomena and for efficient operation of ion-exchange processes. This paper describes the modeling of ion-exchange equilibria for hydroxyl sites on metal oxides and carboxyl sites in resins with monovalent cations (alkali metal ions), a monovalent anion (nitrate ion), and divalent heavy metal ions. The procedure of modeling is as follows: the stoichiometry and material balance equations of the respective ion-exchange reactions were established based on findings here and by others. The equilibrium conditions were given by the Frumkin equation, where the mass-action relation is modified with lateral interactions between species at the interphase. The model equations were fitted to the measured data and model parameter values were determined by nonlinear regression analysis. The formation of bonds between ions and exchanger sites was evaluated by the equilibrium constant and the suppression of bond formation by electrostatic, geometric, and other lateral interactions was evaluated by the interaction constant. It was established that the properties of ions are determined by the valence, size, and hydration state of the ions. Monovalent ions (anions and cations) react with oxide surface hydroxyl and resin carboxyl sites as hydrated ions and form loose ion-site pairs by a weak electrostatic bond (nonspecific adsorption). However, the lateral interactions are large because of a large polarization of the ion-site pairs. When the monovalent cations are dehydrated to react with carboxyl sites in narrow resin nanopores, the bond formation is difficult because energy for dehydration is necessary. The suppressive lateral interactions here are small because of a small polarization of the dehydrated ion-site pairs that are in direct contact. Divalent heavy metal ions react with oxide hydroxyl sites by replacing their hydrated water molecules and form ion-site pairs in direct strong contact (specific adsorption). The bond formation becomes easier with increasing charge density of the ions evaluated by the charge/radius ratio, agreeing with the order of these ions to form hydroxo complexes in solution. The suppressive lateral interaction is, however, small for ions with large charge densities, because a strong contact bond reduces the polarization of ion-site pairs by neutralization. The properties of exchangers are functions of the molecular and pore environments around the functional groups. The acid-base nature of oxide surface-hydroxyl groups is determined by the electronegativity of surrounding lattice metal ions, and that of resin carboxyl groups by the electron-repelling effect of adjacent methyl groups. Pores in oxides have diameters sufficient to accommodate hydrated ions, and the suppression is large because of repulsion from ions adsorbed on opposite pore walls (across-pore interaction). Pores in resins differentiate ions that can access or not access sites on the internal surfaces of the pores. Narrow nanopores with diameters less than those of the hydrated ions require ions to dehydrate before they can enter. The ion-exchange reactivity here is small, as described above for dehydrated monovalent ions. In wide nanopores where hydrated ions can enter, bond formation is easier, but suppression is greater because of a larger polarization of hydrated ion-site pairs and also of the across-pore interaction. Macropores have diameters much larger than those of the hydrated ions and the bond formation is the same as that in wide nanopores, but the suppression is smaller because of the absence oe of the absence of the across-pore interaction. Finally, this paper attempts a formulation of activity coefficients of exchanging sites and adsorbed ion-site pairs and compares the proposed activity coefficients of interphase species with that of solution species given by the Debye-Hückel equation.

Characterization of dehydration behavior of untreated and pulverized creatine monohydrate powders

Creatine, which is well known as an important substance for muscular activity, is synthesized from amino acids such as glycine, arginine and ornithine in liver and kidney. It then accumulates in skeletal muscle as creatine phosphoric acid. The aim of this study was to understand the dehydration behavior of untreated and pulverized creatine monohydrate at various temperatures. The removal of crystal water was investigated by using differential scanning calorimetry (DSC), X-ray powder diffraction and scanning electron microscopy (SEM). The X-ray diffraction pattern of untreated and pulverized creatine monohydrate agreed with reported data for creatine monohydrate. However, the diffraction peaks of the (1 0 0), (2 0 0) and (3 0 0) planes of pulverized creatine monohydrate were much stronger than those of untreated creatine monohydrate. On the other hand, the diffraction peaks of the (0 1 2) and (0 1 3) planes of untreated creatine monohydrate were much stronger than those of pulverized creatine monohydrate. The dehydration of untreated and pulverized creatine monohydrate was investigated at various storage temperatures, and the results indicated that untreated and pulverized creatine monohydrate were transformed into the anhydrate at more than 30 °C. After dehydration, the particles of untreated and pulverized creatine anhydrate had many cracks. The dehydration kinetics of untreated and pulverized creatine monohydrate were analyzed by the Hancock–Sharp equation on the basis of the isothermal DSC data. The dehydrations of untreated and pulverized creatine monohydrate both followed a zero-order mechanism (Polany–Winger equation). However, the transition rate constant, calculated from the slope of the straight line, was about 2.2–7.7 times higher for pulverized creatine monohydrate than for untreated creatine monohydrate. The Arrhenius plots (natural logarithm of the dehydration rate constant versus the reciprocal of absolute temperature) of the isothermal DSC data for untreated and pulverized creatine monohydrate were linear. The activation energies of dehydration in the 40–60 °C range for untreated and pulverized creatine monohydrate were 15.02 and 10.1 kJ/mol, respectively. Dehydration of untreated creatine monohydrate had a pronounced effect on the particle size of the powder. Compared with pulverized creatine monohydrate, the particle size of untreated creatine monohydrate was significantly decreased by dehydration.

Dehydration Energies of Alkali Metal Halides. A New Simulation Methodology Involving Mean Nearest Neighbor Distances and Thermodynamic Forces

Dehydration energies of various alkali metal halides are estimated by employing a novel simulation methodology with the help of hydrated molecular radii. The hydration numbers at (i) infinite dilution and (ii) each movement of the molecules are calculated by employing random distribution of species in conjunction with ionic and molecular sizes. A satisfactory agreement with the reported estimates is noticed.

Ion‐Selective Adsorption/Desorption Processes at Inorganic Materials/Solution Interfaces as a Novel Mode for Ion Sensing†

Several studies on response mechanisms of solid membrane ion‐selective electrodes (ISEs) and developments of ion sensors mainly from the authors' laboratory were reviewed. Face‐specific responses of CdS single crystals showed that ion‐selective response of solid membrane ISEs based on scarcely soluble salts is due to adsorption of component ions, and resultant charge separation at the membrane/solution interfaces. Potentiometric response mechanisms of CuS‐based ISEs in metal buffer solutions were discussed. Crystal surfaces of scarcely soluble inorganic salts were formed to provide remarkably selective molecular recognition sites at which adsorbed component ions were detected by an underlying quartz crystal microbalance (QCM). Not only single layer adsorption of component ions on the respective inorganic salt, but also multilayer crystal formations at QCM surface also showed ion‐selective response to the respective component ions. Ion selectivities of ion sensors based on scarcely soluble salts, which include ISE and QCM, are basically limited by solubility products. Several studies on ISEs based on inorganic materials having a three‐dimensional (3D) network structure with interstitial ions demonstrated that the formation of vacant sites by dissolution of the interstitial ions from the membrane surfaces is prerequisite for ion‐selective responses. For these ISEs, potentiometric selectivities are essentially governed by size and dehydration energies of analyte ions. Taken these together, approaches for ion‐sensor developments based on inorganic materials were critically evaluated and updated. †In memory of Professor Harry B. Mark Jr., who greatly contributed to the progress of modern electrochemistry and electroanalytical chemistry.

Symmetry, Selectivity, and the 2003 Nobel Prize

I thank Rod MacKinnon, Chris Miller, Peter Agre, and Bob Stroud for valuable discussion, Robin Miller for Figure 1Figure 1, Bertil Hille for Figure 2Figure 2, and Andreas Engel for Figure 4BFigure 4B.

Physicochemical Processes in Pyrolysis and Combustion of Fireproof Polyacrylonitrile and Viscose Fibres

The characteristics of thermooxidative degradation of fibres in the presence of flame retardants, manifested by predominance of cross-linking processes which favors formation of CR over degradation processes, are established. Modification with flame retardants decreases the yield and toxicity of combustible volatile products, the rate of thermolysis, the activation energy of dehydration (for VF) and cyclization (for PAN fibres), and the exothermic effects and reverse heat flow to the polymer. The fibres obtained have OI of 32 vol. % and are resistant to wet treatments with a fireproofing effect.

The parallel dehydrative and dehydrogenative catalytic action of γ-Al 2O 3 pure and doped by MgO : Kinetics, selectivity, time dependence of catalytic behaviour, mechanisms and interpretations

The catalytic test reaction of HCOOH decomposition on pure γ-Al 2O 3 and doped (+ MgO (1% w/w)) was studied at temperatures 275–400 °C. A double dehydrative and dehydrogenative, instead the thought single dehydrative, behaviour of γ-Al 2O 3 was disclosed. The dehydration activity is higher for pure alumina and decreases with operation time while the dehydrogenation activity is higher for doped and increases with time at the higher but decreases at the lower temperatures showing an unusual temperature dependent activation and ageing. Doping and increase of temperature reduce dehydration selectivity. It decreases with operation time at higher but increases at lower temperatures. The Arrhenius kinetic parameters of reactions differ strongly. Activation energy of dehydration seems unaffected and pre-exponential factor slightly affected by catalyst kind and operation time. Parameters concerning dehydrogenation are affected slightly by catalyst kind and strongly by operation time. Based on an atomic scale structure analysis and results, two dehydration and one dehydrogenation coexisting mechanisms stemming from identical initial step of dissociative adsorption of HCOOH as H + and HCOO − were suggested interpreting the results. The observed behaviour is due to changes of surface atomic spacing by doping Mg 2+ and of mean distances of adsorptive pair sites with operation time caused by the second mechanism.

Anion Permeation in Human ClC-4 Channels

ClC-4 and ClC-5 are mammalian ClC isoforms with unique ion conduction and gating properties. Macroscopic current recordings in heterologous expression systems revealed very small currents at negative potentials, whereas a substantially larger instantaneous current amplitude and a subsequent activation were observed upon depolarization. Neither the functional basis nor the physiological impact of these channel features are currently understood. Here, we used whole-cell recordings to study pore properties of human ClC-4 channels heterologously expressed in tsA201 or HEK293 cells. Variance analysis demonstrated that the prominent rectification of the instantaneous macroscopic current amplitude is due to a voltage-dependent unitary current conductance. The single channel amplitudes are very small, i.e., 0.10 ± 0.02 pA at +140 mV for external Cl − and internal I −. Conductivity and permeability sequences were determined for various external and internal anions, and both values increase for anions with lower dehydration energies. ClC-4 exhibits pore properties that are distinct from other ClC isoforms. These differences can be explained by assuming differences in the size of the pore narrowing and the electrostatic potentials within the ion conduction pathways.

Study of segregation coefficient of cationic impurities in ammonium oxalate monohydrate crystals during growth from aqueous solutions

The segregation coefficient k eff of Cu(II), Fe(III) and Cr(III) ions in ammonium oxalate monohydrate single crystals during growth from aqueous solutions at a constant temperature was investigated as a function of solution supersaturation σ and impurity concentration c i. It was observed that: (1) irrespective of the impurity concentration, there is a threshold supersaturation of about 0.03 above which Cr(III) is captured in the crystals but there is no threshold supersaturation in the case of Cu(II) and Fe(III) ions, (2) above the threshold supersaturation, k eff increases with increasing σ and decreasing c i, and (3) at a given σ eff and c i , k eff decreases in the sequence: k eff[Cu(II)] ⪢ k eff[Fe(III)] > k eff[Cr(III)]. The dependence of effective segregation coefficient k eff on supersaturation and impurity concentration is in agreement with the predictions of the model involving surface coverage due to impurity adsorption and dependence of accumulation and depletion of solvated host molecules at kinks of steps on the F faces of crystals on supersaturation [J. Crystal Growth 212 (2000) 522]. Analysis of the effective segregation coefficient k eff of different impurities suggests that the dehydration energies of cations mainly determine the capability of capture of impurity species by the growing crystal.

K + versus Na + Ions in a K Channel Selectivity Filter: A Simulation Study

Molecular dynamics simulations of a bacterial potassium channel (KcsA) embedded in a phospholipid bilayer reveal significant differences in interactions of the selectivity filter with K + compared with Na + ions. K + ions and water molecules within the filter undergo concerted single-file motion in which they translocate between adjacent sites within the filter on a nanosecond timescale. In contrast, Na + ions remain bound to sites within the filter and do not exhibit translocation on a nanosecond timescale. Furthermore, entry of a K + ion into the filter from the extracellular mouth is observed, whereas this does not occur for a Na + ion. Whereas K + ions prefer to sit within a cage of eight oxygen atoms of the filter, Na + ions prefer to interact with a ring of four oxygen atoms plus two water molecules. These differences in interactions in the selectivity filter may contribute to the selectivity of KcsA for K + ions (in addition to the differences in dehydration energy between K + and Na +) and the block of KcsA by internal Na + ions. In our simulations the selectivity filter exhibits significant flexibility in response to changes in ion/protein interactions, with a somewhat greater distortion induced by Na + than by K + ions.

Thermodynamic Consequences of Burial of Polar and Non-polar Amino Acid Residues in the Protein Interior

Effects of amino acid substitutions at four fully buried sites of the ubiquitin molecule on the thermodynamic parameters (enthalpy, Gibbs energy) of unfolding were evaluated experimentally using differential scanning calorimetry. The same set of substitutions has been incorporated at each of four sites. These substitutions have been designed to perturb packing (van der Waals) interactions, hydration, and/or hydrogen bonding. From the analysis of the thermodynamic parameters for these ubiquitin variants we conclude that: (i) packing of non-polar groups in the protein interior is favorable and is largely defined by a favorable enthalpy of van der Waals interactions. The removal of one methylene group from the protein interior will destabilize a protein by ∼5 kJ/mol, and will decrease the enthalpy of a protein by 12 kJ/mol. (ii) Burial of polar groups in the non-polar interior of a protein is highly destabilizing, and the degree of destabilization depends on the relative polarity of this group. For example, burial of Thr side-chain in the non-polar interior will be less destabilizing than burial of Asn side-chain. This decrease in stability is defined by a large enthalpy of dehydration of polar groups upon burial. (iii) The destabilizing effect of dehydration of polar groups upon burial can be compensated if these buried polar groups form hydrogen bonding. The enthalpy of this hydrogen bonding will compensate for the unfavorable dehydration energy and as a result the effect will be energetically neutral or even slightly stabilizing.

CaO/Ca(OH) 2 chemical heat pump system

This study is concerned with dynamic simulation of a CaO/Ca(OH) 2 chemical heat pump system. The system consists of a hydration/dehydration reactor connected to a condenser/evaporator with a control valve between. During the dehydration process, heat is supplied at 673 K for dehydration of Ca(OH) 2, and steam is condensed at 293 K. During the hydration process, heat is supplied at 290 K for evaporation of water, and the heat of hydration is supplied to a load at 353 K. A mathematical model has been developed using time dependent mass and energy conservation equations and kinetic equations. Thermodynamic functions are used to calculate the thermodynamic parameters, and heat transfer in both reactors is considered. Using the dynamic model, simulation of an experimental prototype system has been performed. Various time dependent parameters, such as CaO/Ca(OH) 2 temperature, dehydration energy, condensation energy, evaporation energy, hydration energy etc. have been compared with experimental data. The results showed that the model and the computer code was satisfactory and could be used for dynamic simulation of CaO/Ca(OH) 2 heat pump systems in various applications.

Reaction Kinetics of 2-Propanol Dehydration in Supercritical Water

A study of the kinetics and mechanism of chemical reactions in supercritical fluids is considered. An experimental procedure was proposed for examining reversible chemical reactions in supercritical water. The reaction kinetics of 2-propanol dehydration in supercritical water was studied. It was found that the uncatalyzed reactions of olefin hydrogenation by hydrogen dissolved in supercritical water occur at high rates near the critical point of water. The experimental data on the dehydration of 2-propanol in supercritical water are adequately described by first-order reaction rate equations. The rate constants and activation energies of 2-propanol dehydration near the critical point of supercritical water were found.

Bicarbonate and thiocyanate ions affect the gating of γ-aminobutyric acidA receptors in cultured rat cortical cells

The ionophore function of γ-aminobutyric acidA (GABAA) receptors was studied by whole-cell patch clamp electrophysiology in primary cultures of rat cerebral cortex. Chloride ions were replaced for SCN− (thiocyanate) and HCO3− (bicarbonate) ions. The EC50 values of the GABAA agonist muscimol (HCO3−>Cl−>SCN−) varied parallel with the free energies of dehydration of the anions, while the inhibition constants of the GABAA antagonist bicuculline methiodide were not affected. These findings might be relevant in the interpretation of the contribution of HCO3− versus Cl− currents to the pharmacological differences between depolarizing and hyperpolarizing GABA responses.

Chain dynamics and stability of the poly(3-methacryloxypropyltrimethoxysilane-co-vinylimidazole)

The chain dynamics and the apparent activation energy of dehydration and decomposition of the ester bond for poly(3-methacryloxypropyl-trimethoxysilane-co-vinylimidazole) copolymer (PMPS–PVI), were examined by high-resolution solid-state 29Si and 13C nuclear magnetic resonance spectroscopy and thermogravimetric analysis (TGA). The 29Si–1H cross-polarization process revealed that the 3-methacryloxypropyltrimethoxysilane (MPS) components species had similar CP efficiency. The spin–lattice relaxation time in rotating frame (T1ρH) of the copolymers indicated that the molecular motion of the imidazole ring was independent of the MPS content. The apparent activation energy of dehydration under nitrogen was dependent on the MPS content, while that of decomposition ester bond was independent of the MPS content.

Dehydration, hydration behavior, and structural analysis of fenoprofen calcium

Fenoprofen calcium (FC) is a nonsteroidal, anti-inflammatory, analgesic, and antipyretic agent. The dehydration behavior of FC dihydrate and the rehydration of the dried FC were investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and powder X-ray diffractometry (PXRD). The stoichiometry, the crystal packing arrangement, and water environments in FC dihydrate were determined using single-crystal X-ray diffraction (XRD) analysis. The Arrhenius plot (natural logarithm of the dehydration rate constant versus the reciprocal of absolute temperature) for FC dihydrate from isothermal TGA is not linear. The activation energy of dehydration was 309 kJ/mol in the 50–60°C range and 123 kJ/mol in the 60–80°C range. The difference in activation energy can be explained from the crystal structure data where one water molecule is sandwiched between repeating polar carboxylate groups and the other water is in a slightly less polar region of the crystal. Single-crystal XRD analysis also indicated each calcium ion is coordinated to six oxygens. Two coordinating oxygens are provided by two water molecules and the other four oxygens are provided by the carboxylate group of four separate fenoprofen anions. Each fenoprofen anion, which can provide two oxygens for coordination, is associated with two different calcium ions. Hot-stage PXRD suggested that only a loss of 1 mole of water per mole of FC dihydrate (forming a monohydrate) was required to convert the material to a partially crystalline state. The monohydrate is not completely disordered as evidenced by a strong diffraction peak as well as some weaker peaks in the PXRD pattern. The rehydration of the anhydrous form of FC follows a solution-mediated transformation, prior to crystallizing as the dihydrate. © 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 90:845–859, 2001