Showing papers on "Equilibrium constant published in 1982"

A computer simulation method for the calculation of equilibrium constants for the formation of physical clusters of molecules: Application to small water clusters

Abstract: We present a molecular dynamics computer simulation method for calculating equilibrium constants for the formation of physical clusters of molecules. The method is based on Hill’s formal theory of physical clusters. In the method, a molecular dynamics calculation is used to calculate the average potential energy of a cluster of molecules as a function of temperature, and the equilibrium constants are calculated from the integral of the energy with respect to reciprocal temperature. The method is illustrated by calculations of the equilibrium constants for the formation of clusters of two to five water molecules that interact with each other by an intermolecular potential devised by Watts. The method is compared with other procedures for calculating the thermodynamic properties of clusters.

One-electron redox potentials of phenols. Hydroxy- and aminophenols and related compounds of biological interest

Abstract: The rate constants for reversible electron transfer between a series of substituted phenolate ions and anilines and various substituted phenoxyl or anilino radicals in aqueous solution were measured by observing the formation or depletion of the radicals involved. Nonequilibrium concentrations of the radicals were produced in the presence of the corresponding phenols or anilines by using the pulse radiolysis technique. The relaxation of the system to equilibrium was monitored by optical detection methods. From the equilibrium constants for one-electron transfer, the one-electron redox potentials (E/sup 2/) for 38 phenolic or anilino type compounds were determined, many of which are natural products. The redox potentials are strongly influenced by electron-donating or -withdrawing substituents at the aromatic system. 3 figures, 5 tables.

Determination of the rate constant of enzyme modification by measuring the substrate reaction in the presence of the modifier

Abstract: On the basis of the equations derived previously [Tsou, C. L. (1965) Sheng Wu Hua Hsueh Yu Sheng Wu Wu Li Hsueh Pao 5, 398-408, 409-417] for the substrate reaction during the course of enzyme modification, the kinetic behavior of the system chymotrypsin-substrate-modifier has been studied. The kinetics of benzoyltyrosine ester hydrolysis during the course of irreversible inhibition of the enzyme has been found to be in satisfactory agreement with equations obtained previously. The apparent rate constant between the enzyme and an irreversible inhibitor can be easily obtained in one single experiment by following the course of substrate hydrolysis in the presence of the inhibitor. The results are also in accord with the assumption that diisopropyl fluorophosphate can be classified as an irreversible competitive inhibitor. For both phenylmethanesulfonyl fluoride and L-1-[(p-toluene-sulfonyl)amino]-2-phenylethyl chloromethyl ketone, the inhibition has been found to be in agreement with the kinetics of the complexing type; i.e., a noncovalent enzyme-inhibitor complex is formed before irreversible enzyme modification. Both the equilibrium constants for the complex formation and the first-order rate constants for the irreversible modification step have been determined also by following the course of substrate hydrolysis in the presence of the irreversible inhibitor.

Selfassociation of daunomycin

Abstract: Daunomycin, a potent anthracycline antibiotic, self-associates in aqueous solution at concentrations greater than 10 microM. We report here visible absorbance, sedimentation equilibrium, and proton nuclear magnetic resonance (NMR) experiments that characterize this self-association. In contrast to earlier reports that the process is a simple dimerization, we find that an indefinite association model best fits our data, with an intrinsic association constant, Ki, equal to 1500 M-1. From the temperature dependence of the observed NMR spectra, an enthalpy of approximately -8.0 kcal/mol is calculated. The NMR data show that the aromatic protons of the anthracycline portion of the drug are most affected by aggregation, probably due to stacking of the anthracycline rings. Knowledge of the applicable model for the self-association process, and the equilibrium constant that describes the process, enables us to assess quantitatively the possible effects of drug aggregation on the interpretation of drug-DNA binding data. For the ionic conditions most commonly used in such studies, the amount of aggregated daunomycin will be slight and may safely be ignored.

The thermochemical properties of gas‐phase transition metal ion complexes

Abstract: Gas phase ion clustering reactions of the form A+(L)n+L⇄A +(L)n+1 were studied using high pressure mass spectrometry Equilibrium constants together with enthalpy, entropy, and free energy changes were determined for stepwise clustering reactions of water about the Ag+ ion with n=0 to5 and the Cu+ ion with n=2 to4, ammonia about the Ag+ and Cu+ ions from n=1 and n=2 to4, respectively, and pyridine about the Ag+ and H+ ion from n=2 and n=1 to 3, respectively These results, together with those of other studies, show evidence for the existence of structure in some cluster ions Considerations of bonding and entropy, together with comparisons with the results of ion chemistry in the liquid phase, suggest that such structures can be attributed to several types of cluster ion systems, including gas phase analogs of traditional ’’coordination complexes’’ known in the aqueous phase

Accumulation of heavy metals in unicellular algae

Abstract: A model for metal uptake by microorganisms based on surface adsorption has been developed, and then applied to the uptake of cadmium by Chlorella vulgaris. A linear equilibrium relationship between metal in the solution and that adsorbed on the cell surface is assumed and confirmed at low cadmium concentrations by short-term uptake experiments. When it incorporates a description of cell growth, the model predicts an initial rapid uptake and a subsequent slow uptake. Such behavior has often been observed in experiments with growing microorganisms. This indicates that the slow uptake, sometimes thought to be active or metabolic, could be due to the simultaneous effects of growth and surface adsorption. The model shows that initial metal uptake is fast and approaches equilibrium within a few seconds. This prediction is in agreement with experimental results in a batch system: Equilibrium is reached before the first samples are taken (at 10 min) and there is then no measurable change until growth provides a significant increase in cell surface (after several hours). Thus the equilibrium constant can be calculated from experimental results of uptake at 10 min. The equilibrium is found to be affected by phosphate concentration; the amount of cadmium adsorbed on the cell decreases as the concentration of phosphate is increased. Long-term uptake experiments in growing cultures show a greater metal accumulation than predicted by the adsorption model, suggesting the involvement in the slow long-term uptake of some mechanism other than adsorption. This is confirmed by experiments in which uptake in cultures exposed to cadmium throughout the growth period is compared with short-term uptake in similar cultures grown in the absence of cadmium. The modeling approach to metal adsorption provides a basis for further development. A model combining description of adsorption and of intracellular accumulation is necessary to provide a more complete description. Such a model, with precise definitions of system parameters and means of evaluating these parameters from experimental results, will be a powerful tool in investigation of metal uptake by microorganisms.

Conformational analysis of β-D-ribo-, β-D-deoxyribo-, β-D-arabino-, β-D-xylo-, and β-D-lyxo-nucleosides from proton–proton coupling constants

Abstract: A new n.m.r. coupling constant torsion angle relation is utilized to explore the effect of exocyclic oxygen substituents at C-2′ and C-3′ on J1′2.′, J2′,3′, and J3′,4′ in the furanose ring of β-D-ribo-, β-D-arabino-, β-D-xylo-, and β-D-lyxonucleosides. The five vicinal couplings in β-D-deoxyribonucleosides are also investigated. Calculated coupling constants for the full pseudorotational itinerary at different values of the puckering amplitudes are given. It is shown that experimental coupling constants, taken from the literature, can be satisfactorily explained in all cases on the basis of a two-state (N/S)-model of the furanose pseudorotation. Good estimates of both N- and S- geometry and the conformational equilibrium constant in five-membered rings of ribose, deoxyribose, xylose, lyxose, and arabinose derivatives are given. The judicious use of geometrical information obtained by X-ray crystallographic studies appeared indispensable for this type of analysis.

Direct determination of the equilibrium constant and thermodynamic parameters for the reaction. C3H5+ O2⇌ C3H5O2

Abstract: The reaction of allyl radicals with oxygen has been studied using argon fluoride laser flash photolysis coupled with ultraviolet absorption detection. At temperatures between 382 and 453 K it was possible to observe directly the establishment of equilibrium between allyl, oxygen and allyl peroxy (C3H5+ O2⇌ C3H5O2). From analysis of the allyl radical decay signals the temperature dependences of both the forward and reverse rate constants, k1 and k–1, were obtained. The temperature dependence of the equilibrium constant, Kp=k1/(k–1RT) gave a value for ΔS⊖298 of –(122 ± 5) J mol–1 K–1 and for ΔH⊖298 of –(76.2 ± 2.1) kJ mol–1. The equilibrium constants were shown to be pressure-independent, although both k1 and k–1 were below their high-pressure limits. Above 453 K the form of the decay and the apparent value of the equilibrium constant deviate from the lower-temperature behaviour because of the onset of an additional reaction channel.

Fluorimetric analysis of the binding of warfarin to human serum albumin. Equilibrium and kinetic study.

Abstract: Binding of warfarin to human serum albumin results in a red shift of the UV absorption maximum, suggesting that the binding site is a hydrophobic area of the protein. The enhancement of the fluorescence of warfarin upon binding to human serum albumin was used to study the binding equilibrium and the kinetics of this drug-protein interaction. From equilibrium fluorescence measurements, contributions from free and bound warfarin could be evaluated. From the resulting Scatchard plots, equilibrium constants ranging from 4.2 X 10(5) to 3.5 X 10(5) M-1 for temperatures from 8 degrees to 37 degrees were calculated. The reaction is slightly exothermic (delta H = -1.2 kcal m mole-1) and strongly entropy-driven (delta S = +21 cal . mole-1 . K-1). The reaction rate constants of the warfarin-albumin interaction were determined by the stopped-flow technique. The association rate constant varies from 2.2 X 10(5) to 7.7 X 10(5) M-1 sec-1 from 10 degrees to 32 degrees. The corresponding activation enthalpy is 9.0 kcal . mole-1. These values are not consistent with a diffusion-controlled reaction. The dissociation of the complex was studied by making use of the direct competition between warfarin and phenylbutazone for the same binding site. The dissociation rate constant varies from 2.5 to 10.8 sec-1 in the same temperature range. Activation parameters obtained in the kinetic experiments correspond very well with the thermodynamic parameters calculated from the equilibrium study, validating the fluorescence approach to the equilibrium studies.

Equilibrium constant for the dimerization of bisulfite ion to form disulfite(2-) ion

Abstract: At high concentrations in aqueous solution, bisulfite ion dimerizes to form the species S/sub 2/O/sub 5//sup 2 -/. The equilibrium quotient for the reaction reversible 2HSO/sub 3//sup -/ ..-->.. S/sub 2/O/sub 5//sup 2 -/ + H/sub 2/O has been determined by combining Raman intensity measurements of HSO/sub 3//sup -/ with ultraviolet absorption data on S/sub 2/O/sub 5//sup 2 -/. Unlike previous determinations, the method requires no assumption about constancy of the equilibrium quotient over the range of solutions measured, although it was necessary to assume a constant molar absorptivity in the UV for S/sub 2/O/sub 5//sup 2 -/ and a constant molar Raman intensity for HSO/sub 3//sup -/. Earlier measurements reported in the literature, based only on UV measurements of S/sub 2/O/sub 5//sup 2 -/ absorption and the assumption of constant equilibrium quotient and molar absorptivity, are shown to be seriously in error. Variations of the equilibrium quotient with ionic strength and temperature are reported. A value of Q/sub d/ of 0.088 M/sup -1/ was found for a dilute solution of bisulfite in 1 M NaClO/sub 4/ solution at 25/sup 0/C. In addition to the well-characterized form of bisulfite ion having the hydrogen attached to the sulfur, Raman evidence ismore » reported for the presence of appreciable amounts of the isomer with the hydrogen bonded to one of the oxygens. 3 figures, 4 tables.« less

Phase equilibrium for carbon dioxide-benzene at 313.2, 353.2, and 393.2 K

Abstract: Bubble-point pressures have been measured for the carbon dloxldebenzene system at 313.2, 353.2, and 393.2 K and pressures fram 0.1 to 13.3 MPa (15 to 1930 psla). Vaporphase compodtlons were also measured at 313.2 K. The results differ substantially from prevlow results by Wan and Dodge and by Ohgakl and Katayama. The present data are represented well by the Soave equation of state when two optimum Interaction parameters, C12 = 0.066 and D12 = 0.043, are u W root-mean-square errors In calculated bubble-pdnl pressures are 0.06 MPa (9 pda).

The rigorous evaluation of spectrophotometric data to obtain an equilibrium constant

Abstract: This dry-lab experiment describes a method that may be used to determine the best-fit value of the 1:1 equilibrium constant to spectrophotometric data.

Hydrogen bonding of O—H and C—H hydrogen donors to Cl−. Results from mass spectrometric measurements of the ion–molecule equilibria RH + Cl− = RHCl−

Abstract: Equilibrium constants K1 for reaction [1] RH + Cl− = RHCl− in the gas phase were measured with a high pressure mass spectrometer under chemical ionization conditions. Data for some 40 compounds RH ...

Self-association of rabbit muscle phosphofructokinase: Effects of ligands

Abstract: The effects of ligands on the self-association of rabbit muscle phosphofructokinase (PFK) were investigated by velocity sedimentation at pH 7.0 and 23 degrees C. The concentration dependence of the weight-average sedimentation coefficient was monitored in the presence of these ligands. The mode of association and equilibrium constants characterizing each association step were determined by theoretical fitting of the sedimentation data. The simplest mode of association for the PFK system is M in equilibrium M2 equilibrium M4 in equilibrium M16. Ligands and temperature would perturb the various equilibrium constants without altering the mode of association. The apparent equilibrium constants for the formation of tetramer, K4app, are increased in the presence of 0.1 mM ATP and 1.0 mM fructose 6-phosphate. The value of the sedimentation coefficient for the tetramer, S4 degrees, that would best fit the data is 12.4 S instead of 13.5 S determined in the absence of substrates, thus implying a structural change in the tetramer induced by substrates. Only an insignificant amount of dimer is present under the experimental conditions. The presence of activators, ADP or phosphate, enhances the formation of tetramers, and S4 degrees assumes a value of 13.5 S. Similar results are obtained with decreasing concentrations of proton. The presence of the inhibitor, citrate, however, favors the formation of dimers. The equilibrium constants determined as a function of ADP concentration were further analyzed by the linked-function theory derived by Wyman [Wyman, J. (1964) Adv. Protein Chem. 19, 224--285], leading to the conclusion that the formation of a tetramer involves the binding of two additional molecules of ADP per monomer. Similar analysis results in a conclusion that the formation of a dimer involves the binding of one additional molecule of citrate per phosphofructokinase subunit.

Calculation of liquid junction potentials for equilibrium studies

Abstract: A detailed consideration of the assumptions underlying the Henderson equation indicates that, contrary to widely held views, the calculation of liquid junction potentials via this equation is rellable under certain well-defined conditions. In particular this is shown to be true for the constant lonic strength liquid junctions commonly used in equilibrium studies. The validity of the potentials calculated from the Henderson equation is demonstrated directly by comparison of the calculated potentials with “experimental” values and indirectly from equilibrium constant determinations. The latter indicate that accurate thermodynamic information can be obtained even In the presence of quite large liquid junction potentials. Some of the limitations and possible applications of the Henderson equation approach are briefly discussed.

Experimental determination of the equilibrium constant of the reaction CH3 + O2 ⇄ CH3O2 during the gas-phase oxidation of methane

Abstract: For the experimental determination of the equilibrium constant of the reaction CH3 + O2 ⇄ CH3O2 (1), the process of methane oxidation has been studied over the temperature range of 706–786 K. The concentration of CH3O2 has been measured by the radical freezing method, and that of CH3 from the rate of accumulation of ethane, assuming that C2H6 is produced by the reaction CH3 + CH3 C2H6 (2). The equilibrium constant of reaction (1) has been obtained at four temperatures. For the heat of the reaction the value Δ˚H298 = -32.2 ± 1.5 kcal/mol is recommended.

Magnesium ion inner sphere complex in the anticodon loop of phenylalanine transfer ribonucleic acid

Abstract: The binding of Ca2+ and Mg2+ to tRNAPhe is analyzed by equilibrium titrations and temperature-jump measurements using the Wye base fluorescence as a label. Titration experiments starting with the folded structure of the tRNA (high salt and low temperature) show that Ca2+ and Mg2+ binding detected by Wye base fluorescence changes is associated with equilibrium constants between 1 x 10(3) and 3 x 10(3) M-1. The binding of Ca2+ leads to an increase of the relaxation time associated with a conformation change of the anticodon loop and to a decrease of the corresponding amplitude. These data are represented quantitatively by a two-step reaction scheme with a preferential binding of Ca2+ to one of the anticodon conformations. When Mg2+ is added, an extra relaxation process is observed with time constants around 1 ms. This process demonstrates the formation of a Mg2+ inner sphere complex. Relaxation time constants and amplitudes are represented quantitatively by a three-step reaction scheme. Mg2+ binds preferentially to one of the anticodon conformations. In the absence of Mg2+, these conformations are populated almost equally with a transition rate constant around 5 x 10(3) s-1. The Mg2+ inner sphere complex is formed with a relatively low rate constant of (1--2) x 10(3) s-1, indicating a conformational barrier. These data strongly suggest that the Mg2+ site analyzed in the present investigation corresponds to the anticodon site with a distorted octahedral coordination characterized by X-ray analysis. The results are discussed in terms of the anticodon function and also with respect to their implications upon Mg2+ binding to nucleic acids in general.

Equilibrium and Structural Studies of Silicon(IV) and Aluminium(III) in Aqueous Solution : 4. A Potentiometric Study of Polynuclear Aluminium(III) Hydroxo Complexes with Gallic Acid in Hydrolyzed Aluminium(III) Solutions.

Abstract: The thesis is a summary and discussion of eight papers. During the last decades, precipitation has become increasingly acidic due to the extensive use of fossil fuels. In areas of poorly buffered bedrocks, e.g. Scandinavia, northeastern United States, this phenomenon has resulted in elevated amounts of Al(III) being leached into streams and lakes. Recent findings reveal that these elevated Al-concentrations could cause fish death and decreasing forest production.In the present thesis, the importance of taking naturally occurring substances into consideration when discussing Al(III) in natural waters, is emphasized. On the basis of a chemical characterization of relevant ligand classes in a natural water, the complex formation between Al^+, hydroxide ions and the inorganic ligand carbonic acid, the low-molecular weight organic ligand citric acid and the high-molecular weight model substances gallic acid, 1,2-dihydroxynaphtha-lene-4-sulfonate, 1,2-naphthoquinone-4-sulfonate, pyrocatechol and salicylic acid were investigated. The investigations were performed as series of Potentiometrie titrations and data were processed by means of the least-squares computer program LETAGROPVRID using a technique called pqr-analysis, permitting an unbiased search for complex model (and corresponding equilibrium constants) to be made. In most systems studied, the complexation at high ligand excesses can be described by a series of mononuclear complexes AIL-AIL^. Tentatively, the whole series consists of octahedrally coordinated (water and ligand oxygens) AI(III). At lower ligand excesses, the significance and in some cases even predominance of ternary mono- and polynuclear hydroxo complexes is demonstrated. In two of the systems, binary aluminium hydroxo species are evaluated.The potential importance of the substances with respect to Al-com-plexation in natural waters are indicated in a number of model calculations. The solubility of the clay mineral kaolinite is calculated as a function of -lg[H+] and ligand concentration. It is shown that citric acid, gallic acid, 1,2-dihydroxynaphthalene-4-sulfonate, pyrocatechol and salicylic acid contribute quite significant to the total solubilities, even at very low concentrations.As a complement and background to the equilibrium studies, the corrosion rate for one of the naturally occurring Al-bearing minerals, corundum, is reported. In this investigation, performed with a leach-ant solution of ground-water composition, an experimental technique was employed which made it possible to divide the corrosion into chemical and mechanical losses.