Showing papers on "Equilibrium constant published in 1994"

Enthalpy-entropy compensation in drug-receptor binding

Abstract: The thermodynamic parameters (ΔG o , ΔH o , ΔS o ) of the drug-receptor binding equilibrium derived from equilibrium constant measurements at different temperatures and van't Hoff plots are reviewed. The analysis involves 186 independent experiments performed on 136 ligands binding to 10 biological receptors and, for comparison, to DNA and to two different enzymes. ΔH o and ΔS o values correlate according to the regression equation ΔH o (kcal mol -1 )=-9.5+27845 o (kcal K -1 mol -1 ) with a correlation coefficient of 0.981. The correlating equation is of the form ΔH o =βΔS o and is expected for a case of enthalpy-entropy compensation with a compensation temperature β=278 K

An evaluation of the mechanism of nitrous acid formation in the urban atmosphere

Abstract: Nitrous acid (HONO) has been observed to build in the atmosphere of cities during the nighttime hours and it is suspected that photolysis of HONO may be a significant source of HO radicals early in the day. The sources of HONO are poorly understood, making it difficult to account for nighttime HONO formation in photochemical modeling studies of urban atmospheres, such as modeling of urban O3 formation. This paper reviews the available information on measurements of HONO in the atmosphere and suggest mechanisms of HONO formation. The most extensive atmospheric measurement databases are used to investigate the relations between HONO and potential precursors. Based on these analyses, the nighttime HONO concentrations are found to correlate best with the product of NO, NO2 and H2O concentrations, or possibly the NO, NO2, H2O, and aerosol concentrations. A new mechanism for nighttime HONO formation is proposed that is consistent with this precursor relationship, namely, reaction of N2O3 with moist aerosols (or other surfaces) to form two HONO molecules. Theoretical considerations of the equilibrium constant for N2O3 formation and the theory of gas-particle reactions show that the proposed reaction is a plausible candidate for HONO formation in urban atmospheres. For photochemical modeling purposes, a relation is derived in terms of gas phase species only (i.e., excluding the aerosol concentration): NO + NO2 + H2O → 2 HONO with a rate constant of 1.68 x 10-17 e6348/T (ppm-2 min-1). This rate constant is based on an analysis of ambient measurements of HONO, NO, NO2 and H2O, with a temperature dependence from the equilibrium constant for formation of N2O3. Photochemical grid modeling is used to investigate the effects of this relation on simulated HONO and O3 concentrations in Los Angeles, and the results are compared to two alternative sources of nighttime HONO that have been used by modelers. Modeling results show that the proposed relation results in HONO concentrations consistent with ambient measurements. Furthermore, the relation represents a conservative modeling approach because HONO production is effectively confined to the model surface layers in the nighttime hours, the time and place for which ambient data exist to show that HONO formation occurs. The empirical relation derived here should provide a useful tool for modelers until such time as knowledge of the HONO forming mechanisms has improved and more quantitative relations can be derived.

Silicate melts at magmatic temperatures: in-situ structure determination to 1651°C and effect of temperature and bulk composition on the mixing behavior of structural units

Abstract: The abundance of coexisting structural units in K-, Na-, and Li-silicate melts and glasses from 25° to 1654°C has been determined with in-situ micro-Raman spectroscopy. From these data an equilibrium constant, Kx, for the disproportionation reaction among the structural units coexisting in the melts, Si2O5(2Q3)⇔SiO3(Q2)+SiO2(Q4), was calculated (Kx is the equilibrium constant derived by using mol fractions rather than activities of the structural units). From ln Kx vs l/T relationships the enthalpy (ΔHx) for the disproportionation reaction is in the range of-30 to 30 kJ/mol with systematic compositional dependence. In the potassium and sodium systems, where the disproportionation reaction shifts to the right with increasing temperature, the ΔHx increases with silica content (M/Si decreases, M=Na, K). For melts and supercooled liquids of composition Li2O·2SiO2 (Li/Si=1), the ΔHx is indistinguishable from 0. By decreasing the Li/Si to 0.667 (composition LS3) and beyond (e.g., LS4), the disproportionation reaction shifts to the left as the temperature is increased. For a given ratio of M/Si (M=K, Na, Li), there is a positive, near linear correlation between the ΔHx and the Z/r2 of the metal cation. The slope of the ΔHx vs Z/r2 regression lines increases as the system becomes more silica rich (i.e., M/Si is decreased). Activity coefficients for the individual structural units, γi, were calculated from the structural data combined with liquidus phase relations. These coefficients are linear functions of their mol fraction of the form γi=a lnX i+b, where a is between 0.6 and 0.87, and X i is the mol fraction of the unit. The value of the intercept, b, is near 0. The relationship between activity coefficients and abundance of individual structural units is not affected by temperature or the electronic properties of the alkali metal. The activity of the structural units, however, depend on their concentration, type of metal cation, and on temperature.

Solvent effects on chemical processes. Part 7. Quantitative description of the composition dependence of the solvent polarity measure ET(30) in binary aqueous–organic solvent mixtures

Abstract: The solvation effect component of the phenomenological model of solvent effects is applied to ET(30), the molar transition energy of the Dimroth–Reichardt betaine, in binary aqueous–organic solvents. The dependence of ET(30) on x2, the mole fraction of organic cosolvent, can be quantitatively described for all 17 systems studied. New data are given for 9 of these systems. It is found that a 1-parameter model suffices to describe the composition dependence of the highly polar cosolvents, whereas a 2-parameter model is needed to account for other cosolvents, which yield a nonhyperbolic composition dependence. The solvation parameters, which are exchange equilibrium constants, yield correlations suggesting their positive dependence on cosolvent hydrophobicity and electron-pair donor ability.

Kinetics of esterification of levulinic acid with n-butanol by homogeneous catalysis

Abstract: ~~/~~C~~~~0~Cc~~ ~~o - 15312e~31/RTC~B~leo3C~*o]. Experiments have shown that AHo = 7.14 f 0.87 kJ mol-l, ASo = 78.6 f 2.1 J mol-' K-l, AHl* = 51.4 f 0.4 kJ mol-l, and AHz* = 45.4 f 0.7 kJ mol-l . The equilibrium constants at 25,50,75,90, and 100 OC were found to be 0.33, 0.75, 1.50, 2.17, and 2.73, respectively.

The thermodynamics of solvent exchange.

Abstract: A model for solvation in mixed solvents, which was developed for the free energy and preferential interaction [J. A. Schellman (1987), Biopolymers, Vol. 26, pp. 549–559; (1990), Biophysical Chemistry, Vol. 37, pp. 121–140; (1993), Biophysical Chemistry, Vol. 45, pp. 273–279], is extended in this paper to cover the thermal properties: enthalpy, entropy, and heat capacity. An important result is that the enthalpy of solvation H responds directly to the fraction of site occupation. This differs from the free energy Ḡ and preferential interaction Γ32, which are measures of the excess binding above a random distribution of solvent molecules. In other words, the enthalpy is governed by K while Ḡ and Γ32 are governed by (K − 1) where K is the equilibrium constant on a mole fraction scale [Schellman (1987)]. The solvation heat capacity Cp consists of two term: (1) the intrinsic heat capacity of species in solution with no change in composition, and (2) a term that accounts for the change in composition that accompanies solvent exchange. Binding to biological macromolecules is heterogeneous but experiementalists must use binding isotherms that assume the homogeneity of sites. Equations are developed for the interpretation of the experimental parameters (number of sites nexp, equilibrium constant Kexp, and enthalpy, Δhexp), when homogeneous formulas are applied to the heterogeneous case. It is shown that the experimental parameters for the occupation and enthalpy are simple functions of the moments of the distribution of equilibrium constants over the sites. In general, nexp is greater than the true number of sites and Kexp is greater than the average of the equilibrium constants. The free energy and preferential interaction can be fit to a homogenious formula, but the parameters of the curve are not easily represented in terms of the moments of distributions over the sites. The strengths and deficiencies of this type of thermodynamic model are discussed. © 1994 John Wiley & Sons, Inc.

Capillary electrophoresis with chiral selectors: optimization of separation and determination of thermodynamic parameters for binding of tioconazole enantiomers to cyclodextrins.

Abstract: A systematic approach is outlined for optimization of enantiomeric separations in free solution capillary electrophoresis using chiral mobile-phase additives. Maximum electrophoretic mobility difference between the enantiomers occurs when the concentration of free selector is equal to the reciprocal of the average binding constant. General equations and data analysis methods are presented to relate mobilities to equilibrium constants in simple and competitive binding equilibria and used to determine thermodynamic parameters for host-guest complexation of tioconazole enantiomers with a range of cyclodextrin selectors. Selectivities are found to be in the reverse order of binding constants in the series dimethyl-beta-cyclodextrin (K1 = 6.9 x 10(3) M-1, alpha = 1.10) to hydroxypropyl-beta-cyclodextrin (K1 = 0.72 x 10(3) M-1, alpha = 1.29). For beta-cyclodextrin (K1 = 1.32 x 10(3)M-1, alpha = 1.20), delta H zero provides the dominant contribution to binding but delta delta H zero and T delta delta S zero terms give comparable contributions to the selectivity. Addition of alcohol does not affect the selectivity, but allows displacement of the optimum separation conditions to higher cyclodextrin concentration through either competitive binding (with cyclohexanol) or preferential solvation of reactants (with methanol).

Effects of water activity on reaction rates and equilibrium positions in enzymatic esterifications

Abstract: A technique of continuous water activity control was used to examine the effects of water activity on enzyme catalysis in organic media. Esterification catalyzed by Rhizopus arrhizus lipase was preferably carried out at a water activity of 0.33, which resulted in both maximal initial reaction rate and a high yield. When Pseudomonas lipase was used as catalyst it was beneficial to start the reaction at high water activity (giving the optimal reaction rate with this enzyme) and then shift to a lower water activity toward the end of the reaction to obtain a high yield. The apparent equilibrium constant of the reaction was influenced by the water activity of the organic solvent. © 1994 John Wiley & Sons, Inc.

1H- and 13C-NMR-Spectroscopic Study of Chemical Equilibria in Solutions of Formaldehyde in Water, Deuterium Oxide, and Methanol

Abstract: Reliable information on chemical equilibria in formaldehyde solutions is needed for the design of separation process for formaldehyde containing mixtures. Chemical equilibria of the poly(oxymethylene) glycol formation in formaldehyde solutions in water (and deuterium oxide) and of the poly(oxymethylene) hemiformal formation in methanolic formaldehyde solutions were studied by [sup 1]H and [sup 13]C-NMR spectroscopy. Overall formaldehyde mole fraction and temperature range from 0.06--0.19 mol/mol, 275--357 K for solutions in water and 0.17--0.50 mol/mol, 274--317 K for methanolic solutions. Chemical equilibrium constants are determined assuming ideal solution behavior. Results from [sup 1]H and [sup 13]C-NMR spectroscopy agree well. Chemical equilibria of the poly(oxymethylene) glycol formation do not depend on whether water or deuterium oxide is used. The new experimental results confirm only some of the literature data.

EXTRACTION BY N,N,N',N-TETRAALKYL -2 ALKYL PROPANE -1,3 DIAMIDES. I. H2O, HNO3 and HClO4

Abstract: The extracting properties of substituted diamides were investigated. The auto-association of N,N'-dimethyl dibutyl tetradecyl malonamide (DMDBTDMA) : (C4H9(CH3)NCO)2CHC14H29 in benzene and in TPH (aliphatic hydrocarbon) was shown by NMR using mass-action model. The degree of aggregation depends on the diluent, the diamide concentration and the acidity. The extraction of HNO3 and HClO4 was investigated by distribution measurements and IR spectroscopy. HNO3 extraction is explained by the competing formation of four adducts : whose the equilibrium constants for 0,72 mol.l-1 amide in TPH are 0.167, 0.215, 5.19-10-3 and 3.7-10-4, respectively, on a molal scale. HClO4 is extracted by a reaction involving the competing formation of the two adducts . Their equilibrium constants for 0.697 mol.1-1 amide in TPH are 0.649 and 1.71, respectively, on a molal scale. V(C=O) IR absorption shifts indicate that in are linked by hydrogen bonds, whereas in is transferred from one molecule of acid to one C=O of the DM...

Photochromism, thermochromism and solvatochromism of some spiro[indolinoxazine]-photomerocyanine systems: effects of structure and solvent

Abstract: Three spiro[indoline-naphthoxazines] and a spiro[indoline-phenanthroxazine], which exhibit photochromic and thermochromic properties, have been investigated. Solvent and structure effects on the absorption spectra of the merocyanines produced under UV irradiation and kinetic parameters for the ring-closure and ring-opening reactions were studied. Positive solvatochromism was found, indicating that the opened form is a weakly polar species. Equilibrium constants and rate constants for the forward and back reactions spiroxazine ⇄ merocyanine increase with increasing the solvent polarity and with electron-donating groups in the oxazine moiety. The reaction is endothermic by 10–20 kJ mol–1 and almost isoentropic. The activation entropy is generally negative, while the activation Gibbs energy is approximately independent of solvent and structure.

Transient kinetic analysis of rhodamine phalloidin binding to actin filaments

Abstract: We have characterized the binding of rhodamine phalloidin to actin filaments and actin filaments saturated with either myosin subfragment-1 or tropomyosin in 50 mM KCl, 1 mM MgCl2 buffer at pH 7.0. Direct transient kinetic measurements of rhodamine phalloidin binding to actin filaments indicate an association rate constant of 2.8 x 10(4) M-1 s-1 and a dissociation rate constant of 4.8 x 10(-4) s-1. The ratio of the rate constants yields a dissociation equilibrium constant of 17 nM. From equilibrium measurements, the apparent affinity of rhodamine phalloidin for actin filaments is 116 nM. The difference between the affinities determined by equilibrium and kinetic experiments is attributed to the depolymerization of filaments at low actin concentrations in the equilibrium samples. The binding stoichiometry is one rhodamine phalloidin molecule per actin subunit. When myosin subfragment-1 and tropomyosin are bound to actin filaments, the rate constants for rhodamine phalloidin binding are the same as for actin alone and in agreement with the binding affinities measured in equilibrium experiments. Presumably these proteins stabilize the filaments. Neither substitution of CaCl2 for MgCl2 nor the inclusion of 20 mM phosphate altered the rate or equilibrium constants.

Compositional heterogeneities in hydrogen-bonded polymer blends: infrared spectroscopic results

Abstract: The equilibrium constants describing phenolic OH/ester carbonyl hydrogen bonds are experimentally determined for polymer blends, mixtures of low molecular weight analogues of the polymer repeat units, solutions of the polymers in the low molecular weight analogues, and random copolymers of the original blend segments. These equilibrium constants are determined by IR spectroscopy, and it is demonstrated that the interactions in the low molecular weight analogues and polymer solutions are (within error) described by the same equilibrium constant. The blend equilibrium constant has a value that is much smaller (about 25% of the solution value), while the copolymer value is intermediate between these two extremes

On the probability of finding a water molecule in a nonpolar cavity

Abstract: Measurements of solubility and vapor pressure indicate that the cost of making a cavity in a nonpolar solvent that is large enough to accommodate a single water molecule is just balanced by the attraction between the water molecule and the walls of the cavity. As a result of this unexpected coincidence, the equilibrium constant for entry of an isolated water molecule from the vapor phase into cyclohexane is almost exactly unity at room temperature. Molecules smaller than water prefer the vapor phase, whereas molecules larger than water prefer cyclohexane. In equilibrium with liquid water, the tendency of single water molecules to enter small nonpolar cavities, such as may be present in mutant proteins, is expected to be vanishingly small.

Temperature dependent ultraviolet‐visible absorption cross sections of N2O and N2O4: Low‐temperature measurements of the equilibrium constant for 2NO2 ⇌ N2O4

Abstract: A study of the temperature dependence of the ultraviolet-visible absorption cross sections for NO2 has been made in the temperature range 213–298 K and between 310 and 570 nm using a diode array spectrometer. Analysis of the experimental data allowed the simultaneous measurement of the NO2 and N2O4 cross sections and the equilibrium constant for the dimerization of NO2. The absorption cross sections were measured at a spectral resolution of 0.54 nm full width at half maximum (FWHM) and are presented for use in the analysis of differential absorption spectra of atmospheric NO2. Averaged cross section data for NO2 are presented for use in photolysis rate calculations. Values of cross sections for N2O4 and equilibrium constant for the association of NO2 are presented in the range 213 to 263 K. No temperature effect was observed on the overall shape of the NO2 absorption spectrum or on the averaged cross section values used for calculating atmospheric photolysis rates. Changes in the fine structure of the NO2 spectrum with temperature are observed and this has important implications for measurements of atmospheric NO2 made by ultraviolet-visible absorption techniques.

Crystal structures of true enzymatic reaction intermediates: aspartate and glutamate ketimines in aspartate aminotransferase.

Abstract: The crystal structures of the stable, closed complexes of chicken mitochondrial aspartate aminotransferase with the natural substrates L-aspartate and L-glutamate have been solved and refined at 2.4- and 2.3-A resolution, respectively. In both cases, clear electron density at the substrate-coenzyme binding site unequivocally indicates the presence of a covalent intermediate. The crystallographically identical environments of the two subunits of the alpha 2 dimer allow a simple, direct correlation of the coenzyme absorption spectra of the crystalline enzyme with the diffraction results. Deconvolution of the spectra of the crystalline complexes using lognormal curves indicates that the ketimine intermediates constitute 76% and 83% of the total enzyme populations with L-aspartate and L-glutamate, respectively. The electron density maps accommodate the ketimine structures best in agreement with the independent spectral data. Crystalline enzyme has a much higher affinity for keto acid substrates compared to enzyme in solution. The increased affinity is interpreted in terms of a perturbation of the open/closed conformational equilibrium by the crystal lattice, with the closed form having greater affinity for substrate. The crystal lattice contacts provide energy required for domain closure normally supplied by the excess binding energy of the substrate. In solution, enzyme saturated with amino/keto acid substrate pairs has a greater total fraction of intermediates in the aldehyde oxidation state compared to crystalline enzyme. Assuming the only difference between the solution and crystalline enzymes is in conformational freedom, this difference suggests that one or more substantially populated, aldehydic intermediates in solution exist in the open conformation. Quantitative analyses of the spectra indicate that the value of the equilibrium constant for the open-closed conformational transition of the liganded, aldehydic enzyme in solution is near 1. The C4' pro-S proton in the ketimine models is oriented nearly perpendicularly to the plane of the pyridine ring, suggesting that the enzyme facilitates its removal by maximizing sigma-pi orbital overlap. The absence of a localized water molecule near Lys258 dictates that ketimine hydrolysis occurs via a transiently bound water molecule or from an alternative, possibly more open, structure in which water is appropriately bound. A prominent mechanistic role for flexibility of the Lys258 side chain is suggested by the absence of hydrogen bonds to the amino group in the aspartate structure and the relatively high temperature factors for these atoms in both structures.

Emulsion liquid membranes for wastewater treatment: equilibrium models for some typical metal-extractant systems.

Abstract: Emulsion liquid membranes are capable of extracting metals from dilute waste streams to levels much below those possible by equilibrium-limited solvent extraction. Binary equilibrium data are reported for copper, nickel, and zinc with extractants that can be used in emulsion liquid membrane formulations. Predictive models that incorporate aqueous-phase nonidealities and all aqueous-phase ionic reactions have been developed. For the Cu-LIX and Zn-D2EHPA systems, a single value for the equilibrium constant (K) is accurate over a large range of pH and ionic strength. For the Ni-D2EHPA system, an average single K value was obtained for loadings (fraction of D2EHPA dimers complexed to the metal in the organic phase) less than 0.1. At higher loadings, the organic-phase nonidealities become significant, and the equilibrium constant was fitted as an exponential function of the loading. Emulsion liquid membrane extractions of Cu, Ni, and Zn from aqueous phases were successfully carried out. The effects of various parameters such as mixing speed, stripping reagent concentration, and extractant concentration on the extraction process are reported. 36 refs., 10 figs., 1 tab.

Thermodynamic and NMR Study of the Interactions of Cyclodextrins with Cyclohexane Derivatives

Abstract: Equilibrium constants and standard molar enthalpies of reaction have been determined by titration calorimetry for a series of cyclohexane derivatives (cis-1,2-cyclohexanediol, cis,cis-1,3,5-cyclohexanetriol, trans-1,2-cyclohexanediol, cyclohexanol, cyclohexanone, 2-methylcyclohexanone, 2,5-piperazinedione, and δ-valerolactam) with α-cyclodextrin and β-cyclodextrin. For the reactions involving cyclohexanol, standard molar heat-capacity changes were also determined from calorimetric measurements performed at several temperatures. The equilibrium constants for the reactions of these substances with β-cyclodextrin are in all cases larger than those for the corresponding reactions with α-cyclodextrin

On the Homogeneous Chemistry of the Thermal Decomposition of Methyltrichlorosilane Thermodynamic Analysis and Kinetic Modeling

Abstract: Equilibrium gas-phase calculations for the Si/C/Cl/H deposition system are performed over the range of conditions used to deposit silicon carbide (SiC) through the thermal decomposition of methyltrichlorosilane (MTS). The compounds that exist in significant quantities in the gas phase, and thus may influence significantly the chemistry of the process, as well as the main deposition precursors are identified. The effect of temperature, pressure, and initial composition of the reacting mixture on the equilibrium composition of the gas phase is determined, and process conditions that may lead to stoichiometric silicon carbide films are suggested. Based on the results of the equilibrium calculations, a kinetic model for the homogeneous chemistry of the decomposition of MTS is proposed. Several reaction sequences leading to the generation of carbon and silicon deposition precursors are considered and their effects on the chemistry of the system are examined. Finally, the mechanism is incorporated into the reaction and transport model of a plug flow hot-wall reactor and the overall model is used to obtain the spatial variation of the composition of the gas phase under conditions typically encountered in chemical vapor deposition reactors.

Reaction Equilibria in the Synthesis of 2-Methoxy-2-methylbutane and 2-Ethoxy-2-methylbutane in the Liquid Phase

Abstract: Tertiary ethers are used as octane-enhancing components in gasoline. Equilibrium constants for the liquid-phase synthesis of 2-methoxy-2-methylbutane (TAME) and 2-ethoxy-20methylbutane (TAEE) were measured in the temperature range 323-363 K. The equilibria were studied using the alcohol/alkene mixture in various mole ratios and the respective ether as a reagent in a batch reactor. A commercial cation exchange resin (Amberlyst 16) was used as the catalyst. The system was strongly nonideal, and the UNIQUAC estimation method was used in the calculation of the liquid-phase activity coefficients. The experimental equilibrium constants are given as a function of temperature. At 333 K the equilibrium constants K[sub a] for the synthesis of TAME were 39.6 [+-] 2.5 from methanol and 2-methyl-1-butene (2M1B) and 4.1 [+-] 0.3 from 2-methyl-2-butene (2M2B). The equilibrium constants for the synthesis of TAEE were 17.4 [+-] 1.1 from ethanol and 2M1B and 1.7 [+-] 0.1 from 2M2B. The experimental [Delta][sub r]H values for the liquid-phase synthesis of TAME were [minus]33.6[+-]5.1 kJ/mol (2M1B) and [minus]26.8 [+-] 2.3 kJ/mol (2M2B), and those for the synthesis of TAEE were [minus]35.2 [+-] 5.8 kJ/mol (2M1B) and [minus]27.3 [+-] 6.7 kJ/mol (2M2B). The results were compared with the literature values.