Enthalpies Of Transfer Research Articles

Effects of water-alcohol binary solvents on the thermochemical characteristics of L-tryptophane dissolution at 298.15 K

The enthalpies of L-tryptophane solution in water-methanol, water-ethanol, water-1-propanol, and water-2-propanol mixtures at alcohol concentrations of x2 = 0–0.4 mole fractions were measured by calorimetry. The standard enthalpies of L-tryptophane solution (ΔsolH∘) and transfer (ΔtrH∘) from water to the binary solvent were calculated. The influence of the composition of the water-alcohol mixture and the structure and properties of L-tryptophane on the enthalpy characteristics of the latter was considered. The enthalpy coefficients of pair interactions (hxy) of L-tryptophane with alcohol molecules were calculated. The coefficients were positive and increased in the series: methanol (MeOH), ethanol (EtOH), 1-propanol (1-PrOH), and 2-propanol (2-PrOH). The solution and transfer enthalpies of L-tryptophane were compared with those of aliphatic amino acids (glycine, L-threonine, DL-alanine, L-valine, and L-phenylalanine) in similar binary solvents.

Enthalpies of solution of L-phenylalanine in aqueous solution of amides at 298.15 K

The enthalpies of solution ΔsolHm have been determined for L-phenylalanine in aqueous solutions of formamide (FA), N-methylformamide (MFA), N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA). The measurements have been made at 298.15K and mole fraction of amide x2=(0–0.4) on the precise isothermic calorimeter. The results obtained have been used to calculate the standard enthalpies of solution (ΔsolH°) and transfer (ΔtrH°) of L-phenylalanine from water into mixtures as well as the enthalpy coefficients of pairwise interaction (hxy) of the solute with amide in aqueous media. The interrelation between enthalpy characteristics of dissolution (transfer) of L-phenylalanine and the composition of (water+amide) binary mixtures has been established. Contributions of the amides properties (molar volume, dipolarity/polarizability, acidity and basicity) to the energy of pairwise interactions have been estimated quantitatively by means of a modified Kamlet–Taft equation.

Relative Enzymatic Activity Levels from In Silico Mutagenesis.

We propose a new computational approach for predicting the impact of point mutations on residual enzymatic activities. We build on the general linear trends existing between free energy and enthalpy of transfer of substrates from cytosol to enzyme active sites (protein–ligand binding), therefore linking the docking energies to the binding free energies. In this very first step, we rationalize these trends in terms of a compensation effect decomposed into explicit thermodynamics contributions. In a second step, we combine the latter with the assumption that free energies of transfer, estimated from docking, and free energies of activation are linearly related through a Brönsted–Evans–Polanyi (BEP) relationship, allowing us in fine to predict enzyme activity. As a result, we propose generic Langmuir–Hinshelwood kinetic equations “trained” on the wild type, which provide excellent predictions of rates of catalytic transformations for mutated enzymes from the combination of in silico docking energies to a set of system-specific experimental data. This generalized approach is validated against clinical data on the particular case of human fumarase with major implications for the understanding of hereditary fumarase deficiency.

DFT/B3LYP Study of the Substituent Effects on the Reaction Enthalpies of the Antioxidant Mechanisms of Sesamol Derivatives in the Gas phase and water

In this paper, the study of various ortho and meta–substituted Sesamol derivatives is presented. The reaction enthalpies related to three antioxidant action mechanisms HAT, SET–PT, and SPLET for substituted Sesamols, have been calculated using the DFT/B3LYP method in gas phase and water. Calculated results show that electron-withdrawing substituents increase the bond dissociation enthalpy (BDE), ionization potential (IP), and electron transfer enthalpy (ETE), while electron-donating ones cause a rise in the proton dissociation enthalpy (PDE) and proton affinity (PA). In the ortho position, substituents show a larger effect on reaction enthalpies than in the meta position. In comparison with the gas phase, water attenuates the substituent effect on all reaction enthalpies. In the gas phase, BDEs are lower than PAs and IPs, i.e., HAT represents the thermodynamically preferred pathway. On the other hand, the SPLET mechanism represents the thermodynamically favored process in water. Results show that calculated enthalpies can be successfully correlated with Hammett constants (σm) of the substituted Sesamols. Furthermore, calculated IP and PA values for substituted Sesamols show linear dependence on the energy of the highest occupied molecular orbital (EHOMO).

Quantum-chemical Calculations of the Antioxidant Properties of trans-p-coumaric Acid and trans-sinapinic Acid

Trans-p-coumaric and trans-sinapinic acids are cinnamic acids derivatives which show strong antioxidant properties. In this work full optimization of both chemical structures and their radical, cation radical and anionic forms in vacuum and water medium has been performed, and antioxidant descriptors: Bond Dissociation Enthalpy, Adiabatic Ionization Potential, Proton Dissociation Enthalpy, Proton Affinity, Electron Transfer Enthalpy, Gas Phase Acidity, Free Gibbs Energy have been calculated. The Highest Occupied and Lowest Unoccupied Molecular Orbital energies have been employed to determine groups in compounds studied with the highest electron density. All calculations were performed using Gaussian 03W software package at the DFT level of theory (B3LYP hybrid functional) together with 6-311+G(2d,2p) basis set. Strong antioxidant properties of both investigated compounds were determined in this study. Based on the results it may be suggested that trans-p-coumaric and trans- sinapinic acids react according to the Hydrogen Atom Transfer mechanism in vacuum and according to Single- Electron Transfer followed by the Proton Transfer mechanism in water medium. Moreover, in both compounds studied the O-H bond is a preferred place of free radical attack.

Enthalpies of solvation for dopamine hydrochloride in water-ethanol solutions

The enthalpies of dissolution of dopamine hydrochloride (H2Dop · HCl) in water-ethanol solvents containing from 0 to 0.8 mole fraction of ethanol are measured by calorimetry at 298.15 K. Standard enthalpies of transfer (Δtr H ∘) for the molecular (H2Dop) and cationic (H3Dop+) forms of dopamine from water into binary solvents are calculated from the obtained data. The enthalpies of transfer of H3Dop+ cation are determined from the enthalpies of dissolution of H2Dop · HCl using the familiar method of separating the molar quantities into ionic contributions (Ph4P+ = BPh 4 − ), and by an original alternative procedure. The effect of the composition of the binary solvent on the solvation of dopamine is considered.

DFT/B3LYP study of the substituent effects on the reaction enthalpies of the antioxidant mechanisms of Indole-3-Carbinol derivatives in the gas-phase and water

In this paper the reaction enthalpies of three antioxidant action mechanisms, HAT, SET-PT and SPLET, for mono-substituted Indole-3-Carbinols were calculated in gas-phase and water. Results show that electron-withdrawing substituents increase the bond dissociation enthalpy (BDE), ionization potential (IP) and electron transfer enthalpy (ETE), while electron-donating ones cause a rise in the proton dissociation enthalpy (PDE) and proton affinity (PA). In comparison to gas-phase, water attenuates the substituent effect on all reaction enthalpies. Results show that IP and BDE values can be successfully correlated with the indolic NH bond length after electron abstraction, R(NH+), and the partial charge on the indolyl radical nitrogen atom, q(N). Furthermore, calculated IP and PA values for mono-substituted Indole-3-Carbinols show linear dependence on the energy of the highest occupied molecular orbital (EHOMO) of studied molecules in the two environments. SPLET represents the thermodynamically preferred mechanism in water.

A DFT STUDY ON THE ROLE OF DIFFERENT OH GROUPS IN THE RADICAL SCAVENGING PROCESS

The molecular properties of robinetin and melanoxetin which are the two naturally occurring flavonoid compounds have been studied theoretically by means of density functional theory approach (DFT) at the level of B3LYP/6-311G(d,p). The analysis of computed bond dissociation enthalpy (BDE), proton dissociation enthalpy (PDE), proton affinity (PA), electron transfer enthalpy (ETE) values for both the flavonoid compounds indicate the role of B-ring for the significant antioxidant characteristics and the instability of the A-ring. It also concerns the dominant role of BDE mechanism for antioxidant activity than PDE, PA and ETE mechanisms. Ionization potential (IP) is also found to be trustworthy in the study of antioxidant activity and the computed IP magnitudes are in agreement with the values of synthetic food additives. Further, the various molecular descriptors along with the plot of frontier molecular orbitals and Mulliken spin population analysis have been obtained and the validity of Koopmans' theorem is also verified with reference to antioxidant behavior.

Conductometric Studies of Sodium Tetraphenylborate, Tetrabutylammonium Bromide, and Sodium Tetrafluoroborate in N,N-Dimethylformamide at Temperatures from (283.15 to 318.15) K

The electrical conductances of dilute solutions of sodium tetraphenylborate (NaBPh4), tetrabutylammonium bromide (Bu4NBr), and sodium tetrafluoroborate (NaBF4) in N,N-dimethylformamide have been measured over the temperature range from (283.15 to 318.15) K. The ionic association constant, KA, limiting molar conductances, Λo, and distance parameters, R, were obtained using the Fuoss–Justice equation for the investigated systems. The examined electrolytes are weakly associated in DMF at all experimental temperatures. From the temperature dependence of the limiting molar conductances the Eyring's activation enthalpy of charge transport was estimated. The thermodynamic functions such as Gibbs free energy, entropy, and enthalpy for the process of ion pair formation were calculated from the temperature dependence of the ion association constants. The limiting ionic conductivities and the activation enthalpy of charge transfer for these ions were determined.

On the energetics of homolytic and heterolytic O[sbnd]H bond cleavage in flavonoids

Comprehensive study of three mechanisms of phenolic chain-breaking antioxidant action, i.e. hydrogen atom transfer (HAT), Single Electron Transfer–Proton Transfer (SET–PT) and Sequential Proton-Loss Electron-Transfer (SPLET), for eight naturally occurring flavonoids (polyphenols): apigenin, luteolin, fisetin, kaempferol, quercetin, epicatechin, taxifolin and cyanidin, is presented. Gas-phase OH bond dissociation enthalpies, ionization potentials, proton dissociation enthalpies, proton affinities and electron transfer enthalpies related to these mechanisms were investigated using B3LYP/6-311++G** method. Selection of flavonoids enables to evaluate the effects of various structural features, such as hydroxy groups (3′-OH, 3-OH, 5-OH), C2C3 double bond and C4O keto group, present in the molecules on studied reaction enthalpies. Lowest OH bond dissociation enthalpies (HAT) and proton affinities (SPLET) have been found mostly for 4′-OH groups at ring B. In the second step of SET–PT mechanism, formation of radicals at 4′-OH positions is also thermodynamically favored. However, for five flavonoids, lowest values of electron transfer enthalpies were found in rings A or C.

Enthalpy and entropy of transfer of alkanes from water phase to the micelle core

Enthalpy and entropy of transfer from water phase to a sodium dodecyl sulphate (SDS) micelle core and , respectively, have been investigated by molecular dynamics calculation for a series of hydrophobic solutes originally immiscible with water. It is found that both thermodynamic quantities decrease as a function of number of carbon atoms of the alkanes. Structural behaviour of the solubilised alkane molecules in the micelle has been analysed too. The calculated distribution functions show that methane moves around the whole core, whereas the alkane molecules larger than methane are localised in the centre of the core, i.e. the low-density region of the alkyl chains of SDS.

Theoretical investigation of the effect of sugar substitution on the antioxidant properties of flavonoids

Natural flavonoids are secondary phenolic plant metabolites known for their bioactivity as antioxidants. The evaluation of this property is generally done by the estimation of their direct free radical-scavenging activity as hydrogen or electron donating compounds. This paper reviews experimental results available in the literature for a selection of flavonoids and compares them with calculated quantities characteristic of the hydrogen or electron donation. For that purpose, bond dissociation energies, ionization potentials and electron transfer enthalpies are computed by using DFT methods and the ONIOM procedure implemented in the ab initio program Gaussian. This process has been chosen because it can be extended to the study of large molecules. When acid dissociation and interaction with the solvent are taken into account, the results present very good concordance with experimental results, enlightening the complexity of the processes involved in the classical assays which measure the ability of compounds to scavenge the (2,2′-azinobis-(3-ethylbenzthiazoline-6-sulfonic acid) diammonium salt) radical cation (ABTS +) or the 2,2-diphenyl-1-picryl-hydrazyl radical (DPPH·). This study demonstrates the good accuracy of theoretical calculations in obtaining the relative energies involved in free radical scavenging abilities and its capacity for predictive behaviour. It also highlights the necessity to take into account the pKa of the compounds and the solvent interaction. The ability of the method to calculate the antioxidant properties of larger molecules are tested on glycosylated flavonoids and the effects of sugar substitution on the antioxidant properties of flavonoids are investigated, pointing out the importance of the charges on the oxygen atoms.

Thermochemistry of solvation and complexation in the Cu(NO3)2-water-ethanol-L-α-alanine system at 298.15 K

The heats of mixing of aqueous solutions of copper(II) nitrate and sodium alaninate and the heats of dilution of an aqueous solution of sodium alaninate in water-ethanol mixtures were calorimetrically measured at 298.15 K. The enthalpies of transfer of α-alaninate from water into water-alcohol mixtures and the standard molar enthalpies of the reaction of copper(II) complexation with α-alaninate ion in the mixtures under study are calculated. The resulting enthalpy parameters are compared with those of the similar system containing synthetic β-alanine, which was studied earlier.

A DFT study on the structural, electronic properties and radical scavenging mechanisms of calycosin, glycitein, pratensein and prunetin

Antioxidative properties of naturally occurring isoflavonoids calycosin, glycitein, pratensein, prunetin and their radicals were investigated through density functional theory (DFT) method employing B3LYP/6-311G(d,p) and B3LYP/6-311+G(d,p) methods.The reaction enthalpies related to the individual steps of three phenolic antioxidant action mechanisms–hydrogen atom transfer (HAT), single-electron transfer–proton transfer (SET–PT) and sequential proton loss electron transfer (SPLET) have been computed. The bond dissociation enthalpy (BDE), ionization potential (IP), proton dissociation enthalpy (PDE), proton affinity (PA), electron transfer enthalpy (ETE) were determined in gas phase and in two solvents. The relative energies of each OH groups, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and spin density distribution for radicals were computed and discussed. From the molecular structure and thermodynamic point of view, it was found that B-ring of isoflavonoids to be the active center and the hydrogen atom transfer (HAT) appeared as a major mechanism which contributes much in antioxidant action in gas phase. The SPLET mechanism will be the thermodynamically preferred pathway in polar solvents. On the basis of the computed IP, BDE and PA values most active system was found to be pratensein followed by calycosin, glycitein and prunetin. The analysis of structure–activity relationships for four isoflavonoids clearly indicates that hydroxyl group at position 7 to be the most acidic site.

Temperature and emulsifier concentration effects on gallic acid distribution in a model food emulsion

We determined the effects of emulsifier concentration and temperature on the distribution of gallic acid (GA) in a food-grade emulsion composed of 1:9 vol:vol stripped corn oil, acidic water and Tween 20. The distribution of GA can be defined by the partition constant between the aqueous and the interfacial regions, PWI, which was determined by using a kinetic method and the pseudophase kinetic model. Once PWI is known, determining the distribution of GA is straightforward. Our results show that at least 40% of the total GA is located in the interfacial region of the emulsion at 0.005 volume fraction of Tween 20, and this percentage increases to ca. 85% of the total GA at 0.04 volume fraction of Tween 20. The variation of PWI with the temperature was used to estimate the thermodynamic parameters for the GA transfer from the aqueous to the interfacial region of the emulsion and the activation parameters for the reaction between 16-ArN2+ and GA in the interfacial region. The free energy of transfer from the aqueous to the interfacial region, ΔGT0,W→I, is negative, the enthalpy of transfer is small and negative, but the entropy of transfer is large and positive. Our results demonstrate that the partitioning of GA in acidic emulsions between aqueous and interfacial regions depends primarily on droplet concentration and is only slightly dependent on temperature.

Calculating the solvation contributions from reagents to the change in enthalpy of silver(I) complexation with 18-crown-6 ether in binary methanol-acetonitrile solvents

The enthalpies of the transfer of silver(I) ions from methanol to mixed methanol-acetonitrile solvents are calculated using the literature data. An estimative calculation of the enthalpies of transfer of the reaction of formation of [Ag18C6]+ upon a change in the solvent composition MeOH → AN is performed on the basis of previously found laws of change in the thermodynamic characteristics of complexation and solvation of reagents. It is shown that the governing factor in the change in the energy of the reaction is the enhancement of the solvation of the central ion.

Ab initio study regarding the evaluation of the antioxidant character of cyanidin, delphinidin and malvidin

Abstract A theoretical study regarding the evaluation of the antioxidant character of three of the most wide-spread anthocyanidins (cyanidin, delphinidin and malvidin) was carried out at ab initio level. Different parameters (bond dissociation enthalpy, ionization potential, proton affinity, and electron transfer enthalpy) were computed for each OH group of the compounds in order to predict their antioxidant capacity. Several molecular descriptors based on frontier molecular orbital theory (hardness, electrophilicity, frontier charge density) were also calculated, as well as the atomic charges corresponding to the O atoms of the hydroxyl groups.

Thermochemistry of electrolyte solutions

The results of calorimetric investigations of electrolyte solutions in the mixtures of water, methanol, N,N-dimethylformamide, and acetonitrile with numerous organic cosolvents are discussed with regard to the intermolecular interactions that occur in the solution. Particular attention is given to answer the questions how and to what extent the properties of the systems examined are modified by the cosolvent added and how much the properties of the cosolvent are revealed in the mixtures with the solvents mentioned above. To this goal, the analysis of the electrolyte dissolution enthalpies, single ionic transfer enthalpies, and enthalpic pair interaction coefficients as well as the preferential solvation (PS) model are applied. The analysis performed shows that in the case of the dissolution enthalpies of simple inorganic electrolytes in water–organic solvent mixtures, the shape of the dependence of the standard dissolution enthalpy on the mixed solvent composition reflects to a large extent the hydrophobic properties of the organic cosolvent. In the mixtures of methanol with organic cosolvents, the ions are preferentially solvated either by methanol molecules or by molecules of the cosolvent, depending on the properties of the mixed solvent components. The behavior of inorganic salts in the mixtures containing N,N-dimethylformamide is mostly influenced by the DMF which is a relatively strongly ion solvating solvent, whereas in acetonitrile mixtures, the thermochemical behavior of electrolyte solutions is influenced to a large extent by the properties of the cosolvent particularly due to the PS of cation by the cosolvent molecules.

The effect of urea on hydrophobic hydration: Preferential interaction and the enthalpy of transfer

Why does urea weaken the hydrophobic effect? A classical theory proposed that urea breaks the water structure without association with the hydrophobic solutes. Yet this view contradicts a recent statistical mechanical approach which confirmed that urea does accumulate around proteins and the hydrophobic groups. The justification of the water structure hypothesis, however, is the positive hydrophobic enthalpy of transfer from water to urea solution. What, then, is the origin of the positive enthalpy of transfer? Here the Kirkwood–Buff theory of solution shows that the positive enthalpy can be attributed to the temperature-dependence of urea–solute interaction. Thus the positive enthalpy of transfer does not mean the lack of urea-hydrophobic association.

Thermochemistry of ethyl acetate solvation in the 1-octanol-N,N-dimethylformamide system

The thermal effects of ethyl acetate (EtOAc), 1-octanol (OctOH), and N,N-dimethylformamide (DMF) solution in a OctOH-DMF model system were measured using a calorimeter of variable temperature with an isothermal shell at 298 K. The standard enthalpies of solution and ester transfer from an alcohol to a binary mixture, and partial mole enthalpies of mixed solvent components were determined. The state of non-electrolyte molecules in OctOH-DMF and OctOH-DMF-EtOAc systems were studied using extended coordination model. It was found that the binary solvent is subjected to microclusterization, since the fraction of the single-type molecules in the solvation sphere of both components of a mixture is significantly higher than in the liquid phase volume. The conclusion was drawn that in the triple system, the ethyl acetate solvation sphere in the whole range of compositions is significantly enriched with amide owing to stronger esteramide dipole-dipole interaction.