Ionization Enthalpies Research Articles

Proton Transfer via Arginine with Suppressed pKa Mediates Catalysis by Gentisate and Salicylate Dioxygenase.

Gentisate and salicylate 1,2-dioxygenases (GDO and SDO) facilitate aerobic degradation of aromatic rings by inserting both atoms of dioxygen into their substrates, thereby participating in global carbon cycling. The role of acid-base catalysts in the reaction cycles of these enzymes is debatable. We present evidence of the participation of a proton shuffler during catalysis by GDO and SDO. The pH dependence of Michaelis-Menten parameters demonstrates that a single proton transfer is mandatory for the catalysis. Measurements at variable temperatures and pHs were used to determine the standard enthalpy of ionization (ΔHion°) of 51 kJ/mol for the proton transfer event. Although the observed apparent pKa in the range of 6.0-7.0 for substrates of both enzymes is highly suggestive of a histidine residue, ΔHion° establishes an arginine residue as the likely proton source, providing phylogenetic relevance for this strictly conserved residue in the GDO family. We propose that the atypical 3-histidine ferrous binding scaffold of GDOs contributes to the suppression of arginine pKa and provides support for this argument by employing a 2-histidine-1-carboxylate variant of the enzyme that exhibits elevated pKa. A reaction mechanism considering the role of the proton source in stabilizing key reaction intermediates is proposed.

Complexation between chitosan and carboxymethyl cellulose in weakly acidic, neutral, and weakly alcaline media

The interaction between carboxymethyl cellulose and partially reacetylated chitosan soluble in acidic and alkaline aqueous media is studied by light scattering and isothermal titration calorimetry in a wide pH range. It is shown that the formation of polyelectrolyte complexes (PEC) can occur in the pH range of 6–8, while this pair of polyelectrolytes loses the ability to complexation upon transition to a more alkaline medium. The revealed dependence of the observed enthalpy of interaction on the ionization enthalpy of the buffer indicates the participation of proton transfer from the buffer substance to chitosan and its additional ionization in the binding process. This phenomenon is first observed in a mixture of a weak polybase chitosan and a weak polyacid. The possibility to obtain soluble nonstoichiometric PEC by a direct mixing of the components in a weakly alkaline medium is shown. The resulting PECs are polymolecular particles in shape close to homogeneous spheres with a radius of about 100 nm. The obtained results are promising for creating of biocompatible and biodegradable drug delivery systems.

Sulfo-phosphoric attack of the phosphate ore: energetic contribution of each phenomenon intervening during the attack and effect of the time of addition of the sulfuric acid

ABSTRACT The energetic contribution of various phenomena involved in the reaction between the Tunisia phosphate ore and a mixture of phosphoric and sulfuric acids was studied calorimetrically. The enthalpies of the sulfuric acid ionization (), the H2PO4 − neutralization (), and the phosphate ore dissolution in phosphoric acid alone () or in the mixture of the latter with sulfuric acid () were determined and equal to −58.7 ± 0.7 kJ mol−1, 12.7 ± 1.3 kJ mol−1H2SO4, −186.2 ± 2.2 J g−1 and – 425.6 ± 3.3 J g−1, respectively. The addition time of the sulfuric acid was systematically investigated to reduce the loss of P2O5 in gypsum and the optimal condition for a better yield was deduced.

Comprehensive Quantum Chemical Characterization of the Astrochemically Relevant HCnH Chain Family: An Attempt to Aid Astronomical Observations

AbstractIn this paper, the results of a comprehensive theoretical investigation of neutral HCnH0 and charged HCnH± chains are reported using state‐of‐the‐art compound model chemistries (W1BD, G4, CBS‐QB3, CBS‐APNO, CBS‐4M) and single point methods (including “gold standard” CCSD(T)). The properties envisaged include electronic and chemical bonding structure data, enthalpies of formation, adiabatic and vertical ionization (IP), and electron attachment (EA) energies, gas phase acidities, C−H bond strengths, and chain “deprotecting” (HCnH → H + Cn + H) energies. The best theoretical estimates based on composite models reported here for the various properties investigated agree with the available experimental data and are better than all previous theoretical estimates. To exemplify, for the enthalpies of formation, the G4 composite model achieves a RSMD (root square mean deviation) of 0.28 kcal mol−1 and a MUD (maximum unsigned deviation) of 0.45 kcal mol−1. Most importantly from the astrochemical perspective, it is predicted that the stable HCnH− anions with are nonlinear. Having sufficiently large dipole moments, the anion HCnH− cis conformers are potential candidates for detection in space via rovibrational spectroscopy.

Peculiarities of the interaction of sodium dodecyl sulfate with chitosan in acidic and alkaline media

In this work, the interaction between the negatively charged surfactant sodium dodecyl sulfate (SDS) and partially N-reacetylated chitosan (RA-CHI), which is soluble at pH range up to pH 12, is studied in a wide pH range including alkaline media by light scattering (LS) and isothermic titration calorimetry (ITC). It is shown that in the weakly alkaline medium (pH 7.4), RA-CHI/SDS interaction is exothermic and cooperative. This interaction is found to be coupled with proton transfer from the buffer substance to chitosan as it is revealed by the dependence of the measured heat release on the ionization enthalpy of the buffer. At higher pH values (pH > 8), another mechanism of interaction is observed that include SDS micellization induced by hydrophobic interactions with polymer segments, so that no phase separation occurred in these mixtures. The results obtained can contribute to expand the knowledge about application of chitosan for preparation of pharmaceutical and cosmetic compositions containing anionic surfactants.

Study of alkali metals and alkaline earth metals in chlorobutyl rubber-based model truck inner tube compound

This paper describes the effects of various alkali metal and alkaline earth metal compounds, as well as walnut shell ash (WNS ash), on the cure characteristics (ts2, tc90) of a chlorobutyl rubber (CIIR)-based heat resistance truck inner tube compound. We observed that CIIR-F (KOH) has a very high scorch safety and a very slow cure. As it is a strong base with a high pH (13.84), it cures very slowly. Despite having a pH of 10.6, a ts2 of 6.36, and a tc90 of 18.84 min, CIIR-D (CaCO3) cures faster than CIIR-A (blank). The ts2 and tc90 values for CIIR-A (blank) were 6.55 and 18.88 min, respectively. Except for CIIR-D (CaCO3), all of the materials used to have higher scorch safety (ts2) and optimum cure time (tc90) in rheometer than CIIR-A (blank). CIIR-C (BaCO3) performed nearly as well as a blank. This study revealed that the trend of cure characteristics (ts2, tc90) in rheometer and Mooney scorch (t5) in Mooney viscometer not only depends on pH but also depends on ionic radius, ionization enthalpy and oxidation number of alkali metal and alkaline earth metal and the type of compound of alkali metal and alkaline earth metal.

Enthalpy profile of pH-induced flocculation and redispersion of polyacrylic acid-coated nanoparticles in protic ionic liquid, N,N-diethylethanolammonium trifluoromethanesulfonate

Maghemite nanoparticles coated with polyacrylic acid (pAA) were dispersed in the protic ionic liquid, diethylethanolammonium trifluoromethanesulfonate (DEEAH+⋅TfO−), and the thermodynamics of particle dispersion was studied by means of potentiometric and calorimetric titrations over the entire accessible pH range. As previously reported, the domain of colloidal stability is divided into two pH regions, mildly acidic and basic, separated by a flocculation domain at an intermediate pH. In this study, solvent DEEAH+⋅TfO− was first characterized in terms of the thermodynamic parameters of two reference reactions: autoprotolysis and ionization of acetic acid. A negative autoprotolysis entropy is obtained due to hydrogen-bond formation between the neutralized solvent cation, DEEA, and the neighboring cation DEEAH+. This suggests that the solvent structure is reinforced by the formation of neutral species, being an opposite trend to both ethylammonium nitrate and water. Second, the ionization and flocculation of pAA-coated nanoparticles (CNps) in the ionic liquid were examined. The potentiometric results could be modeled using a simple pAA ionization, independent of the aggregation state over the entire pH range. However, the calorimetric titration detected extra heat generation in an acidic condition prior to flocculation, in addition to an ionization enthalpy of pAA of 30 kJ/mol. This exothermic contribution is attributed to a change in the solvation process of CNps in the ionic liquid. Herein, we propose a model in which different types of solvation in the acidic and basic domains are sufficient to ensure colloidal stability, while aggregation unfolds between the switching pH.

Thermophysical Properties of Electric Arc Plasma and the Wire Melting Effect with Lanthanum and Sulfur Fluorides Addition in Wire Arc Additive Manufacturing

Achieving a higher quality in wire arc additive manufacturing (WAAM) is a result of the development of welding metallurgy, the development of filler wires, and the control of the thermophysical properties of the electric arc. In this paper, the authors developed composite wires for WAAM with a Ni-LaF3, Ni-LaB6 coating. The addition of LaF3, LaB6, and SF6 increases specific heat, thermal conductivity, enthalpy, and degree of plasma ionization, which leads to the increase in the transfer of heat from the arc plasma to the wire and to the change in the balance of forces during wire melting. The increase in the Lorentz electromagnetic force and the decrease in the surface tension force made it possible to reduce the droplet diameter and the number of short circuits during wire melting. The change in the thermophysical properties of the plasma and droplet transfer with the addition of LaF3, LaB6, and SF6 made it possible to increase the welding current, penetration depth, accuracy of the geometric dimensions of products in WAAM, reduce the wall thickness of products, and refine the microstructure of the weld metal using G3Si1, 316L, AlMg5Mn1Ti, and CuCr0.7 wires.

Expansion Tube Test Flow Design for Magnetohydrodynamic Aerobraking

This paper explores the operating envelope for the University of Queensland’s free-piston driven X2 expansion tube to determine if flow conditions can be produced that generate a significant magnetohydrodynamic interaction. Numerical calculations are presented to demonstrate the existence of viable operating conditions to study magnetohydrodynamics. Argon is selected as the test gas due to its simple chemistry and low ionization enthalpy. A candidate flow condition is selected for detailed analysis and experimental validation. Finite-rate reacting argon numerical simulations indicate that relatively short shock-layer length scales for the models tested in X2 limit the degree of argon ionization that is generated. However, a strong interaction remains available, and use of a larger facility can fully mitigate this issue. It is shown that expansion tubes are capable of generating the required flowfield for these experiments, and important considerations for design of suitable test flows are identified.

WITHDRAWN: Is antioxidant activity of flavonoids mainly through the hydrogen-atom transfer mechanism?

Abstract Thirteen flavonoids (quercetin, morin, myricetin, luteolin, fisetin, kaempferol, galangin, apigenin, chrysin, EC, ECG, EGC and EGCG) were used to investigate their main antioxidant mechanism as well as structure-activity relationship by quantum chemical calculation and verifying by the in vitro typical antioxidant assays (DPPH, ABTS and FRAP). The reaction enthalpies related to three possible radical-scavenging mechanisms were calculated by the density functional theory (DFT) M06-2X method. Both in ethanol and acetone, the bond dissociation enthalpy (BDE) was much lower than ionization enthalpy and proton affinity values, and the BDE rank of 13 flavonoids was similar to that of their antioxidant activity determined by the in vitro trials. It was revealed that flavonoids thermodynamically preferred HAT mechanism in polar media. The number and position of phenolic hydroxyl showed great influences on the antioxidant activity of flavonoids, and the flavonoids possessing both of C3 hydroxyl group and catechol moiety exhibited stronger antioxidant activity than which with one or none of them.

Fermi-Löwdin orbital self-interaction corrected density functional theory: Ionization potentials and enthalpies of formation.

The Fermi-Löwdin orbital self-interaction correction (FLO-SIC) methodology is applied to atoms and molecules from the standard G2-1 test set. For the first time FLO-SIC results for the GGA-type PBE functional are presented. In addition, examples where FLO-SIC like any proper SIC provides qualitative improvements compared to standard DFT functionals are discussed in detail: the dissociation limit for , the step-wise linearity behavior for fractional occupation, as well as the significant reduction of the error of static polarizabilities. Further, ionization potentials and enthalpies of formation obtained by means of the FLO-SIC DFT method are compared to other SIC variants and experimental values. The self-interaction correction gives significant improvements if used with the LDA functional but shows worse performance in case of enthalpies of formation if the PBE-GGA functional is used. The errors are analyzed and the importance of the overbinding of hydrogen is discussed. © 2018 Wiley Periodicals, Inc.

Thermodynamics of the Binding of Lysozyme to a Dendritic Polyelectrolyte: Electrostatics Versus Hydration.

The interaction between dendritic polyglycerol sulfate (dPGS) of the second generation and lysozyme was studied by isothermal titration calorimetry (ITC) at different temperatures and salt concentrations. Analysis by ITC showed that 2–3 lysozyme molecules were bound to each dPGS. The resulting binding constant Kb and the Gibbs free energy ΔGo decreased markedly with increasing salt concentration but were nearly independent of temperature. The salt dependence of Kb led to the conclusion that ca. 3 counterions bound to dPGS were released upon complex formation. The gain in entropy ΔGci by this counterion-release scales logarithmically with salt concentration and is the main driving force for binding. The temperature dependence of ΔGo was analyzed by the nonlinear van’t Hoff plot, taking into account a finite heat capacity change ΔCp,vH. This evaluation led to the binding enthalpy ΔHvH and the binding entropy ΔSvH. Both quantities varied strongly with temperature and even changed sign, but they compensated each other throughout the entire range of temperature. Coarse-grained computer simulations with explicit salt and implicit water were used to obtain the binding free energies that agreed with ITC results. Thus, electrostatic factors were the driving forces for binding whereas all hydration contributions leading to the strongly varying ΔHvH and ΔSvH canceled out. The calorimetric enthalpy ΔHITC measured directly by ITC differed largely from ΔHvH. ITC measurements done in two buffer systems with different ionization enthalpies revealed that binding was linked to buffer ionization and a partial protonation of the protein.

Practical Tight-Binding Hückel-Like Modeling of Electronic Properties of Saturated Hydrocarbons: On σ-Aromaticity of Cyclopropane and Correlations of Alkane Ionization Energies and Enthalpies of Formation Corrected for Protobranching.

This testing of σ-aromaticity in cyclopropane and protobranching in alkanes via a Hückel-like model proposed by Klein and Larson is expedited by mirror-plane fragmentation per the method of McClelland originally developed for conjugated polyenes. Alkane carbon sp3 orbitals are represented by edge weighted tetrahedral stellate graphs connected to hydrogen orbitals.

Purification characterization and inhibition studies on Phospholipase C from <i>Opisthacanthus capensis</i> (Black Scorpion) venom

Phospholipase C from Opisthacanthus capensis venom was partially purified and characterized. The enzyme was purified 105.2 fold with an overall yield of 13% using various steps involving ammonium sulphate precipitation, protamine sulphate treatment, sephadex G-75 fractionation and DE-52 anion exchange chromatography. The purified enzyme was homogeneous by the criteria of SDS polyacrylamide gel electrophoresis with a molecular weight of 29 kDa. The phospholipase C has pH and temperature optima of 7.2 and 600C, respectively with activation energy of 25KJ/mol and t1/2 of 1.50 hr. Initial velocity studies on Opisthacanthus capensis venom phospholipase C revealed a KM of 0.02 mM and Vmax of 0.015 µmol/min respectively. Studies on the effect of pH on KM and Vmax gave PKa1 of 6.9 and PKa2 of 7.4 with enthalpy of ionization of 20 KJ/mol suggesting histidine in the active site. The enzyme was positively modulated by Mg2+, Zn2+ and Ca2+ and negatively by Fe2+. While Hg2+ produced complete inhibition. Various concentrations of leaf aqueous extract of Momordica charantia also inhibited the activity of Opisthacanthus capensis venom phospholipase C in vitro with a display of competitive

A theoretical study of antioxidant activity of some Schiff bases derived from biologically important phenolic aldehydes and phenylenediamines

AbstractBecause of their numerous biological as well as industrial importance, the study of Schiff bases is an emerging field for the researchers, in recent time. In this study, we have designed some Schiff bases derived from biologically important precursors. The antioxidant activities of the designed compounds are thoroughly studied theoretically using density functional theory taking various parameters like bond dissociation enthalpy, ionization enthalpy, proton dissociation enthalpy, and electron transfer enthalpy followed by the study of effects of solvent, spin density, and molecular orbital on antioxidant activity of the compounds. The comparison of antioxidant activity of the compounds with that of phenol and their parent aldehydes reveals the superior antioxidant activity of the designed compounds. This study contributes towards the information of an important bridge between bioorganic and computational chemistry.

Insights into the Cation Selectivity and Cooperativity with Sugar in Salmonella Typhimurium Melibiose Permease

The melibiose permease of Salmonella typhimurium (MelBSt), a member of Major Facilitator Superfamily, utilizes the electrochemical gradient of Na+, Li+ or H+ for accumulation of galactosides. Based on crystal structures and functional studies, it has been proposed that MelBSt has a single galactoside-binding site and all three cations compete for a single cation site. Using purified wild-type or mutants MelBSt, direct measurement of Na+ or melibiose binding were performed with isothermal titration calorimetry. The results show that the binding affinity for Na+ or melibiose is mutually increased by about 6-8 fold, exhibiting a positive cooperativity. To reveal the cation selectivity between Na+ and H+, the dissociation constant (Kd) of Na+ binding was measured as a function of pH. The absolute KD value for Na+ of 600µM and for H+of 0.4 µM (i.e. pKa = 6.4) are determined, showing that MelBSt selects for H+ over Na+. With a pKa value slightly lower than the neutral pH, MelB is able to use either H+ or Na+ as the coupling cation. Furthermore, H+ release during Na+ binding was analyzed by measuring the overall heat changes in three reaction buffers with different ionization enthalpies. While Kd for Na+ is similar, the enthalpy changes obtained in the three buffers fit to a linear function with a slope of −0.3 and −0.03 at pH 7.45 or 8.2, respectively. It is likely that about 30% MelBSt proteins engage H+ release during Na+ binding at pH 7.45 and nearly all MelBSt is deprotonated at pH 8.2. The results also support the notion that Na+ and H+ exclusively bind to a single site. This study provides insights into the molecular mechanisms operated by MelB to utilize either Na+ or H+.

Explicit accounting of electronic effects on the Hugoniot of porous materials

A generalized enthalpy based equation of state, which includes thermal electron excitations explicitly, is formulated from simple considerations. Its application to obtain Hugoniot of materials needs simultaneous evaluation of pressure-volume curve and temperature, the latter requiring solution of a differential equation. The errors involved in two recent papers [Huayun et al., J. Appl. Phys. 92, 5917 (2002); 92, 5924 (2002)], which employed this approach, are brought out and discussed. In addition to developing the correct set of equations, the present work also provides a numerical method to implement this approach. Constant pressure specific heat of ions and electrons and ionic enthalpy parameter, needed for applications, are calculated using a three component equation of state. The method is applied to porous Cu with different initial porosities. Comparison of results with experimental data shows good agreement. It is found that temperatures along the Hugoniot of porous materials are significantly modified due to electronic effects.

Buffer contribution to formation enthalpy of copper(II)–bicine complex determined by isothermal titration calorimetry method

The isothermal titration calorimetry (ITC) technique supported by potentiometric titration data was used to study a buffer contribution to the formation enthalpy of copper(II)–bicine complex obtained directly from the ITC experiments. The calorimetric measurements were carried out at 298.15 K in 100 mM buffer solutions with a pH of 6. In experiments two biologically relevant pH buffers were used, namely Mes and Caco. These buffers do not bind copper(II) ions and thus do not affect the binding constant. However, due to their different ionization enthalpy, they affect the conditional (observed) enthalpy of binding of the metal ion to a ligand owing to the proton transfer during the interactions. To calculate the conditional-independent binding enthalpy ∆H, the equation based on the Hess’s law was used. Furthermore, pH-independent and buffer-independent parameters (K, ∆G and ∆S) of the interactions were calculated as well. The relationship between the ionization enthalpy of the buffer components and the thermodynamic parameters has been discussed.

M2+•EDTA Binding Affinities: A Modern Experiment in Thermodynamics for the Physical Chemistry Laboratory

Isothermal titration calorimetry was used to experimentally determine thermodynamic values for the ethylenediaminetetraacetic acid (EDTA)(aq) + M2+(aq) reactions (M2+ = Ca2+ and Mg2+). Students showed that for reactions in a N-(2-hydroxyethyl)piperazine-N′-ethanesulfonic acid (HEPES) buffer (pH = 7.4), the Mg2+ + EDTA reaction was endothermic, while Ca2+ binding to EDTA was exothermic. EDTA is triply ionized at pH 7.4 and therefore must shed a proton to the buffer prior to chelating the M2+ ion; thus the observed reaction enthalpies are strongly dependent on pH and buffer ionization enthalpy.

Thermodynamics of binding of structurally similar ligands to histone deacetylase 8 sheds light on challenges in the rational design of potent and isozyme-selective inhibitors of the enzyme.

Among the different histone deacetylase (HDAC) isozymes, HDAC8 is the most highly malleable enzyme, and it exhibits the potential to accommodate structurally diverse ligands (albeit with moderate binding affinities) in its active site pocket. To probe the molecular basis of this feature, we performed detailed thermodynamic studies of the binding of structurally similar ligands, which differed with respect to the “cap”, “linker”, and “metal-binding” regions of the suberoylanilide hydroxamic acid (SAHA) pharmacophore, to HDAC8. The experimental data revealed that although the enthalpic (ΔH°) and entropic (ΔS°) changes for the binding of individual SAHA analogues to HDAC8 were substantially different, their binding free energies (ΔG°) were markedly similar, conforming to a strong enthalpy–entropy compensation effect. This effect was further observed in the temperature-dependent thermodynamics of binding of all SAHA analogues to the enzyme. Notably, in contrast to other metalloenzymes, our isothermal titration calorimetry experiments (performed in different buffers of varying ionization enthalpies) suggest that depending on the ligand, its zinc-binding group may or may not be deprotonated upon the binding to HDAC8. Furthermore, the heat capacity changes (ΔCp°) associated with the ligand binding to HDAC8 markedly differed from one SAHA analogue to the other, and such features could primarily be rationalized in light of the dynamic flexibility in the enzyme structure in conjunction with the reorganization of the active site resident water molecules. Arguments are presented that although the binding thermodynamic features described above would facilitate identification of weak to moderately tight-binding HDAC8 inhibitors (by a high-throughput and/or virtual screening of libraries of small molecules), they would pose major challenges for the structure-based rational design of highly potent and isozyme-selective inhibitors of human HDAC8.