Thermodynamic Functions Of Activation Research Articles

Thermodynamic functions of activation for viscous flow of cholesterol in some non-aqueous solutions

The viscosities of solutions of cholesterol in benzene, toluene, carbon tetrachloride, chloroform and 1,2-dichloroethane up to 0.4 mol kg −1 at 293, 298, 303, 313, 323 and 333 K have been measured. On the basis of Eyring's theory of rate processes, the molar thermodynamic functions of activation for viscous flow of solution, i.e. the molar Gibbs free energy Δ# Ḡ 1,2, molar enthalpy, Δ# H̄ H̄ 1,2 and molar entropy, Δ# S̄ 1,2, and corresponding partial molar quantities of activation for viscous flow of solute and solvent have been determined. Additionally, the viscosity coefficients B and D and their temperature derivatives are calculated and correlated with solvent dielectric parameter β. The influence of the “solvent effect” on the above parameters is discussed on the basis of Eyring's theory of the transition state and the structure of solution.

Solvent and temperature effects on the rate constants of solvolysis of tert-butyl bromide in mono- and di-alcohols

Rate constants, k, for the solvolytic reactions of 2-bromo-2-methylpropane (ButBr) in the monoalcohols propan-1-ol and butan-1-ol and in the dialcohols ethane-1,2-diol, propane-1,3-diol and butane-1,4-diol are reported, at different temperatures. These values, the Arrhenius activation energies, Ea, and the thermodynamic functions of activation, Gibbs energy (Δ‡G⊖), enthalpy (Δ‡H⊖) and entropy (Δ‡S⊖) are interpreted and compared with previous results in methanol and ethanol. According to the Intersecting-state model, the reaction-energy profile is shaped and the solvent effect on the Gibbs energy of activation, Δ‡G⊖, is analysed.

Thermodynamic functions of activation for viscous flow of some monosaccharides in aqueous solutions

The viscosities of aqueous solutions of some monosaccharides D-pentoses and (D-hexoses) were measured up to 2.5 mol kg −1 in the temperature range from 293.15 to 318.15 K. On the basis of Eyring's theory of the transition state, the relations for the thermodynamic functions of activation for viscous flow of binary liquid mixtures, i.e. molar Gibbs free energy, Δ #Ḡ 1,2, entropy, Δ #S̄ 1,2, and enthalpy, Δ #H̄ 1,2, and for the partial molar quantities of activation for viscous flow of solute and solvent, respectively, were obtained. Thereby, the dependence of the molar thermodynamic functions of activation for a viscous flow of solution against the solute mole fraction was given as a second-degree polynomial. It was shown that the linear correlation coefficient β Y, ( Y denotes G, H or S), is given by the difference between the partial molar activation parameter of the solute at infinite dilution and the respective molar quantity of the pure solvent. In addition, correlations between the coefficient β Y and the viscosity coefficient B and/or temperature derivative of the viscosity coefficient B were found. For the solutions investigated, the molar thermodynamic activation parameters for viscous flow were found to be linearly dependent on the solute mole fraction, which means that the partial molar quantities of activation of the solutes are concentration independent.

Thermodynamic activation functions of viscous flow of non-polar liquids

The thermodynamic activation function of viscous flow may be determined from the expression for the pre-exponential factor in the Eyring relationship (the viscosity coefficient), which is a function of density and relative permittivity, together with the thermal dependence of the viscosity coefficient. This method of determination is demonstrated for a series of n-alkanes C6–C20.

Vapour and liquid diffusion of model penetrants through human skin; correlation with thermodynamic activity.

This work investigates vapour and liquid permeation through human skin of model penetrants benzyl alcohol, benzaldehyde, aniline, anisole and 2-phenylethanol applied in model vehicles butanol, butyl acetate, isophorone, isopropyl myristate, propylene carbonate, toluene, n-heptane and water. Vapour permeation was a linear function of thermodynamic activity as measured by headspace gas chromatography, except when the vehicle was n-heptane. Liquid permeation did not always follow simple thermodynamic predictions, e.g. for the penetrant, benzyl alcohol, when the vehicle damaged the skin (toluene, n-heptane) or when propylene carbonate produced low fluxes and isopropyl myristate, high values. At comparable thermodynamic activities, liquid fluxes were often ten-fold higher than vapour fluxes, and these differences were reflected by the partition coefficients and the amount of penetrant entering the stratum corneum membrane. The conclusion was that liquid fluxes were membrane controlled, whereas an interfacial effect probably contributed to low vapour permeation.

Kinetic studies on the chymotrypsin A α-catalyzed hydrolysis of a series of N-acetyl- l-phenylalanyl peptides

A series of N-acetyl- l-phenylalanyl peptides of general formula Ac-Phe-(Gly) n -NH 2 ( n = 0–2) has been synthesized to study the effect of leaving group chain length on the efficiency of chymotrypsin A α amidase and peptidase activities. The effect upon catalysis of hydrophobic side chains on the leaving group was investigated using similar substrates with one of the glycine residues selectively substituted by an alanine residue as in AcPheAlaNH 2, AcPheAlaGlyNH 2, and AcPheGlyAlaNH 2. Values of k cat and K m have been obtained from kinetic measurements at pH 8.00 and 25 °C. The results are shown to be consistent with binding schemes postulated from published model building studies. The catalytic reactions were studied over a range of temperature (15–35 °C) and in each case the Arrhenius law was obeyed. It was thus possible to obtain meaningful values for the thermodynamic functions of activation for the acylation step of the catalytic reaction. The results are shown to confirm the findings of postulated binding schemes but indicate that conclusions drawn from kinetic measurements at a single temperature may sometimes be misleading.

Partial molar volumes, partial molar expansibilities and viscosity of benzene solutions of tri-n-octylammonium halides

The densities of benzene solutions of tri-n-octylammonium chloride, tri-n-octylammonium bromide and tri-n-octylammonium iodide up to 0.25 mol kg–1 at 293.15, 298.15, 303.15, 313.15 and 323.15 K were measured. The partial molar volumes and partial molar expansibilities of the solutes were found to be independent of concentration, and the partial molar expansibilities were found to be equal for all the solutes investigated. For these systems viscosity measurements were also made and the viscosity coefficients B and D determined. In addition, the relative viscosity was interpreted on the basis of the theory of rate processes and regular solution theory, and thermodynamic functions of activation for viscous flow and the solubility parameters of the solutes investigated were calculated at 298.15 K.

The kinetics of hydrolysis of some extended N-aminoacyl- l-phenylalanine methyl esters by bovine chymotrypsin A α Evidence for enzyme subsite S 5

A series of N-acetylated peptide methyl esters of general formula N-acetyl-(glycyl) n - l-phenylalanine methyl ester ( n = 0–3) has been synthesized to study the effect of varying aminoacyl chain length of the efficiency of chymotrypsin A α (EC 3.4.21.1) catalysed ester hydrolysis. Values of k cat and K m for each substrate have been obtained from kinetic measurements at pH 8.00 and 25.0°C. It has been found that for the first three members of the series ( n = 0–2) there is an increase in k cat value as the aminoacyl chain length is increased. However, the kinetic constants ( k cat and K m) for the third ( n = 2) and fourth ( n = 3) members of the series were found to be very similar. These results are shown to be consistent with a substrate binding scheme proposed for the isomeric enzyme chymotrypsin A γ. The enzyme-catalysed reactions were also investigated over a range of temperature (15–35°C). In each case the Arrhenius law was obeyed, within experimental error, and evaluation of meaningful values for the thermodynamic functions of activation ( ΔH ‡ and ΔS ‡ ) was possible with certain assumptions. In contrast to the similarity of kinetic constants found for the third and fourth members of the substrate series, the corresponding values of ΔH ‡ and ΔS ‡ were markedly different. These results, together with those for the first two members of the series are interpreted in terms of a model binding system which is consistent with the existence of further enzyme subsites in the S 4-S 5 region.

Thermodynamic functions of activation of ion-molecule reactions in alcohol-water mixtures over composition and temperature ranges — I. A study of the (un-monitored) conductimetric technique used by Iskander and others for evaluating the second-order rate constant of alkaline hydrolysis reactions

The unmonitored conductimetric technique, which had been used by Iskander, Wasif and Tewfik[3] for determining the second-order rate constant (k 2) of the alkaline hydrolysis of ethyl benzoate and of ethyl p-nitrobenzoate in ethanol-water mixtures, has been initially used in this study to extend those measurements to other solvent compositions and temperatutes. Unfortunately, it has been found that the technique, as applied by those workers, can lead to ambiguous or inaccurate k 2 values. Consequently, their results for the two esters are found to deviate markedly from those obtained in other laboratories using more direct titrimetric methods. Some of the possible causes of failure of the former technique have been examined. The k 2 values for the two esters have been re-evaluated using our pH-titration technique described elsewhere[4].

Effects of anions on the activation thermodynamics and fluorescence emission spectrum of alkaline phosphatase: evidence for enzyme hydration changes during catalysis.

The effect of anions on the thermodynamic activation functions for a model enzyme, calf intestinal alkaline phosphatase (EC 3.1.3.1), have been studied in order to examine the role of protein hydration changes in establishing the energetics of enzyme catalysis. The influences of these anions on the activation volume (delta V) and activation free energy (delta G) reflected clear Hofmeister (lyotropic) series effects, in the order F- greater than Cl- greater than Br- greater than I- (order of increasing salting-out potential). A pronounced covariation was observed between the influences of these anions on Vmax, which is proportional to delta G, and on the negative activation volume of the reaction. Fluoride was able to counteract the influences of Br- and I- on both Vmax and delta V when combinations of these anions were employed. The effects of Br- and I- on Vmax and delta V were more pronounced at lower temperatures. The control delta V was increasingly negative at reduced temperatures. The effects of the neutral salts and propanol on delta V and delta G, as well as the effects of salting-in anions on the activation enthalpy and the negative activation entropy of the reaction, are consistent with a model which proposes that peptide groups or polar side chains on the native enzyme exergonically increase their exposure to solvent during the catalytic activation event. These conclusions are in accord with the known free energy, enthalpy, entropy, and volume changes which occur when model peptide groups are transferred between water and concentrated salt solutions. Consistent with the kinetic results, the fluorescence emission wavelength maximum of alkaline phosphatase increased in the presence of anions in the order F- greater than Cl- greater than Br- greater than I-. The salting-out ion (F-) and the salting-in ions (Br- and I-) shifted lambda max in different directions, and these lambda max shifts could be counterbalanced by using equimolar combinations of salting-in and salting-out anions. Control experiments with a model compound, N-acetyltryptophanamide, showed that the spectra shifts caused by the salts did not result solely from differential quenching by the anions of the solvent-exposed tryptophan(s) on the enzyme. Hofmeister additivity phenomena indicated that the solvent is at the basis of these salt-induced enzyme structural changes. It is concluded that changes in protein solvation during enzymic reactions contribute significantly to the thermodynamic activation parameters in both the native and the salt-perturbed enzyme.