Thermodynamic Association Constants Research Articles

Population genetic and biophysical evidences reveal that purifying selection shapes the genetic landscape of Plasmodium falciparum RH ligands in Chhattisgarh and West Bengal, India

BackgroundReticulocyte binding protein-like homologs (RHs) are currently being evaluated as anti-erythrocytic stage vaccine targets against Plasmodium falciparum malaria. Present study explores the possible evolutionary drivers shaping the genetic organization of Pfrhs in Indian parasite population. It simultaneously evaluates a putative gain-of-function variant of PfRH5, a keystone member of PfRH family.MethodsReceptor binding regions of Pfrh1, Pfrh2a/b, Pfrh4 and whole Pfrh5 were amplified using blood samples of P. falciparum malaria patients from Chhattisgarh and West Bengal and sequenced. Assembled sequences were analysed using MEGA7 and DnaSPv6. Binding affinities of recombinant PfRH5 proteins with basigin (BSG) were compared using in silico (CHARMM and AUTODOCK) and in vitro (Circular dichroism, fluorescence spectroscopy and isothermal titration calorimetry) methods.ResultsPfrh1 (0.5), Pfrh2a/b (0.875), Pfrh4 (0.667) and Pfrh5 (0.778) sequence changes corresponded to low frequency (< 0.05) variants which resulted in an overall negative Tajima’s D. Since mismatch distribution of none of the Pfrh loci corroborated with the model of demographic expansion, a possible role of natural selection formulating Pfrh sequence diversity was investigated. Among the 5 members, Pfrh5 displayed very high dN/dS (5.7) ratio. Nevertheless, the model of selective sweep due to presence of any advantageous substitutions could not be invoked as polymorphic nonsynonymous sites (17/18) for Pfrh5 exceeded significantly over the divergent (62/86) ones (p = 0.0436). The majority of extant PfRH5 sequences (52/83) differed from the reference Pf3D7 allele by a single amino acid mismatch (C203Y). This non-conservative alteration was predicted to lower the total interaction energy of that PfRH5variant with BSG, compared to PfRH53D7. Biophysical evidences validated the proposition that PfRH5variant formed a more stable complex with BSG. Thermodynamic association constant for interaction of BSG with PfRH5variant was also found to be higher (Kavariant = 3.63E6 ± 2.02E6 M−1 and Ka3D7 = 1.31E6 ± 1.21E6 M−1).ConclusionsTogether, the study indicates that the genetic architecture of Pfrhs is principally shaped by purifying selection. The most abundant and ubiquitous PfRH5 variant harbouring 203Y, exhibits a greater affinity for BSG compared to PfRH53D7 possessing 203C allele. The study underscores the importance of selecting the functional allele that best represents circulating strains in natural parasite populations as vaccine targets.

Theoretical interpretation of conductivity data below and above the CMC: The case of alkaline ion decanoate solutions

In this work the conductivity data of three carboxylate surfactant systems: sodium (NaDec), potassium (KDec) and cesium (CsDec) decanoates aqueous solution in the concentration range 10 −3 − 0.25 mol dm –3 , between temperatures 278.15 and 328.15 K, are analyzed by a transport theory in the frame of the mean spherical approximation (MSA). The data in the pre-micellization region, below critical micelle concentration (CMC), are reproduced well by taking into account the formation of neutral pairs involving monomers and counterions. It was found, that the values of the thermodynamic association constants, K , for NaDec and KDec are very similar and slightly smaller than values of K for CsDec. In all cases, K decreases with increasing temperature. Above CMC, the fitting of experimental data yields the effective charge of micelles, which also turned out as counterion quite insensitive, but is bigger at higher temperatures. Thus, at higher temperatures below CMC the presence of free monomers and counterions is favorable and above CMC the binding of counterion on the micelle surface is weaker. However, at studied systems no distinctive ion specific effect could be observed, thus the dominant role of Dec- as a surfactant monomer in the micellization process could be assumed. • High precision conductivities of alkaline ion decanoate salts in water at different temperatures • Experimental data were analyzed by using a Mean Spherical Approximation transport theory. • The limiting conductance of decanoate anion was determined over temperature range. • CMC, degree of dissociation α and effective charges of micelles were estimated.

Organic electrolyte solutions: Modeling of deviations from ideality within the binding mean spherical approximation

Deviations from ideality in organic electrolyte solutions are described within the Binding Mean Spherical Approximation (BiMSA) theory, in which ions are regarded as charged hard spheres and unlike ions may associate to form an ion pair. Association is modeled within the Wertheim theory (as done in SAFT-type models). The model includes a mass action law with a thermodynamic association constant. The literature about the thermodynamic properties of this type of solution is reviewed. Besides, it is attempted to gain some insight into the solvation of ions by computing their Stokes hydrodynamic diameters from conductivity experiments, and by employing an original analysis introduced by Fawcett. The BiMSA model is used to represent the osmotic coefficient of 1-1 electrolytes in pure methanol, ethanol, 2-propanol, acetone and acetonitrile. The optimized cation sizes in the solvents are compared with their hydrodynamic diameters. The regressed association constants are compared with literature values derived from conductivity and vapor pressure experiments.

Determination of the NaFaq association constant for the NaF–NaCl–H2O System at 25–75°C by means of potentiometry

Potentiometric measurements are made for the NaF–NaCl–H2O system at 25, 50, and 75°C using Na+- and F–-sensitive electrodes and a Ag/AgCl reference electrode in both liquid junction and liquid junction free cells. A correction for the liquid junction potential is introduced to the liquid junction cells. The activity coefficients are calculated using the Debya–Huckel equation in the third approximation. The NaF thermodynamic association constants are determined (logK° =–0.375,–0.29, and–0.25) with an error of ±0.1 log units at 25, 50, and 75°C, respectively.

Spectrophotometric, FTIR and Theoretical Studies of the Charge-transfer Complexes Between Nicotinamide and the Acceptors (p-Chloranil, Chloranilic Acid and DDQ) in Acetonitrile, Their Association Constants, Thermodynamic Properties and Other Related Properties

Nicotinamide, an important vitamin in our bodies, was found to form beautifully colored charge-transfer complexes with p-chloranil, chloranilic acid and 2,3-dichloro-4,5-dicyano benzoquinone in acetonitrile with absorption maxima at 474.5, 519.0 and 614.0 nm, respectively. The compositions of the complexes were determined to be 1:1 from Job’s method of continuous variation. The measured FTIR spectra of the solid complexes formed between nicotinamide and the acceptors and the corresponding individual constituents showed considerable shifts in absorption peaks, changes in intensities of the peaks and formation of the new bands upon complexation. The thermodynamic association constants K DA and other thermodynamic parameters (ΔG°, ΔH° and ΔS°) of the complexes, determined spectrophotometrically taking D (donor) and (acceptor) A in varying ratios (2:8–8:2) and also in equimolar ratios, showed the complex formation to be spontaneous. The experimental hν CT and the theoretical hν CT values (calculated using density function theory) of the charge-transfer complexes are compared. The hν CT (experimental) values of the transition energies of the complexes in acetonitrile differ from hν CT (theoretical) values in the gaseous state. The $$ I_{\text{D}}^{\text{V}} $$ value of nicotinamide was calculated. A comparative study of the energetics of the partial electrovalent charge-transfer complexes with the organic dative ions D+ and A− (in the excited state) and true electrovalent compounds between M+ and X− is presented.

Direct Determination of Thermodynamic Association Constants of Charge Transfer Complexes Using High Performance Thin Layer Chromatography

Few accurate thermodynamic association constant K DA values of charge transfer complexes (DA) taking donor (D) and acceptor (A) in stoichiometric ratios are available and there is no method to determine directly the concentration of the complexes in solution. HPTLC, an extensively used technique to estimate the amount of trace organics, had been found to be a valuable method to directly determine the compositions and the association constants K DA of the complexes when D and A are in their stoichiometric ratios. HPTLC with a video densitometer was successfully utilized for estimating the concentrations of charge-transfer complexes between isoniazid/nicotinamide with acceptors like p-chloranil, chloranilic acid, tetracyanoethylene and dichlorodicyanobenzoquinone in acetonitrile solution. The compositions of the complexes and the thermodynamic association constants were determined (donor and the acceptors in stoichiometric ratios) by comparing the mobility of the migrated donors (i.e., isoniazid/nicotinamide) in acetonitrile solution from the complexes with the respective blanks. The areas of spots of the donors were measured densitometrically. The K DA values determined using the HPTLC technique were verified and validated using a widely accepted spectrophotometric method. The method can also be utilized for determination of equilibrium constants of the complexes. It is a rapid, economic and eco-friendly method.

Conductometric and UV–Visible Spectroscopic Studies on Association in Dilute Aqueous Solutions of Dq2Fe(CN)6

Electrical conductivities and electronic absorption spectra of Dq2Fe(CN)6 (where Dq2+ is diquat, the N,N′-ethylene-2,2′-bipyridine cation) were measured in dilute solutions in water. Experimental data were interpreted in terms of a molecular model which includes the presence of 1:1 and 2:1 associates. Initially, electrical conductivities and absorption spectra were treated using the Quint–Viallard conductivity equations and modified Hemmes algorithm in a limited concentration range, when only minor 2:1 complex formation is expected. The thermodynamic association constant of the 1:1 complex (\( K_{ 1}^{{^\circ }} \) = 17,800 ± 1800 dm3·mol−1) and the corresponding contact distance (rmin = 5.31 ± 0.07 A) were evaluated from the spectral data. Analysis of the conductivity data resulted in \( K_{ 1}^{{^\circ }} \) = 16,500 ± 1500 dm3·mol−1 (rmin = 5.38 ± 0.07 A). Both contact distances were found to agree with the Fe···N distances in crystalline DqH2Fe(CN)6·4H2O and Dq2Fe(CN)6·6H2O. This agreement allowed us to use the same rmin value to estimate the association constant of the 2:1 complex in accordance to the Fuoss–Krauss theory. Both \( K_{ 1}^{{^\circ }} \) and \( K_{ 2}^{{^\circ }} \) were used to calculate electrical conductivities and optical absorbancies in the concentration range up to 5 mmol·dm−3. Satisfactory agreement was achieved between calculated and experimental data.

Uncovering the thermodynamics of monomer binding for RNA replication.

The nonenzymatic replication of primordial RNA is thought to have been a critical step in the origin of life. However, despite decades of effort, the poor rate and fidelity of model template copying reactions have thus far prevented an experimental demonstration of nonenzymatic RNA replication. The overall rate and fidelity of template copying depend, in part, on the affinity of free ribonucleotides to the RNA primer–template complex. We have now used 1H NMR spectroscopy to directly measure the thermodynamic association constants, Kas, of the standard ribonucleotide monophosphates (rNMPs) to native RNA primer–template complexes. The binding affinities of rNMPs to duplexes with a complementary single-nucleotide overhang follow the order C > G > A > U. Notably, these monomers bind more strongly to RNA primer–template complexes than to the analogous DNA complexes. The relative binding affinities of the rNMPs for complementary RNA primer–template complexes are in good quantitative agreement with the predictions of a nearest-neighbor analysis. With respect to G:U wobble base-pairing, we find that the binding of rGMP to a primer–template complex with a 5′-U overhang is approximately 10-fold weaker than to the complementary 5′-C overhang. We also find that the binding of rGMP is only about 2-fold weaker than the binding of rAMP to 5′-U, consistent with the poor fidelity observed in the nonenzymatic copying of U residues in RNA templates. The accurate Ka measurements for ribonucleotides obtained in this study will be useful for designing higher fidelity, more effective RNA replication systems.

Determination of the compositions and accurate thermodynamic association constants of charge-transfer or EDA complexes of TNT and amines using HPTLC with scanning video densitometer

Determination of the compositions and accurate thermodynamic association constants of charge-transfer or EDA complexes of TNT and amines using HPTLC with scanning video densitometer

Comparative conductimetric studies of salicylic acid in methanol–water mixtures at 25 °C

Conductivity data of salicylic acid in methanol–water mixtures were measured at 25°C. The data were analyzed in two methods, the Hsia–Fuoss’s and Fuoss 78’s conductance equations and a comparison was made. The two methods concern the derivation of thermodynamic association constants and limiting molar conductivities for all solvent compositions. The limiting equivalent conductance decreases with the increase of methanol content in the binary mixtures over the whole range of the solvent composition, but the variation does not give a constant value of Walden product. The electrolytes were found to be practically completely associated in all studied solvent mixtures. The association constant of acid decreases with the increase in relative permittivity of the mixtures. The values of ionic coefficients of self diffusion and the ionic conductance at infinite solutions were estimated.

Comparative conductimetric studies of salicylic acid in methanol-water mixtures at 25 °C

The conductivity of salicylic acid in methanol-water mixtures was measured at 25 °C. Experimental data were analyzed using the Hsia-Fuoss and Fuoss-78 conductance equations and a comparison was made. The Hsia-Fuoss and Fuoss-78 methods were also used to deter-mine the thermodynamic association constants and the limiting molar conductivities for all solvent compositions. The limiting equivalent conductance decreases with an increase in the methanol content in the binary mixtures over the whole range of solvent compositions, but the variation does not give a constant value of Walden product. The electrolytes were found to be practically completely associated in all solvent mixtures studied. The association constant of salicylic acid decreases as the dielectric constants of the mixtures increase.

Spectrophotometric, FTIR and theoretical studies of the charge-transfer complexes between isoniazid (pyridine-4-carboxylic acid hydrazide) and the acceptors (p-chloranil, chloranilic acid and tetracyanoethylene) in acetonitrile, their association constants, thermodynamic properties and other related properties

Spectrophotometric, FTIR and theoretical studies of the charge-transfer complexes between Isoniazid (pyridine-4-carboxylic acid hydrazide) and the acceptors (p-chloranil, chloranilic acid and tetracyanoethylene) in acetonitrile, their association constants, thermodynamic properties and other related properties were studied. Isoniazid (INH), a widely used anti tubercular agent was found to form beautifully colored charge-transfer complexes with p-chloranil, chloranilic acid and tetracyanoethylene in acetonitrile. The absorption maxima of the complexes were 484, 519 and 479nm, respectively (isoniazid had no absorption, but the acceptors had absorption in these regions). The composition of the complexes were determined to be 1:1 from Job’s method of continuous variations depending on the time period of experiments as the stability of some of the complexes (p-chloranil and tetracyanoethylene complexes) was time dependent. Solid complexes formed between isoniazid and the acceptors were isolated but p-chloranil was found to form two different complexes. FTIR spectra of the complexes and the acceptors were measured. FTIR spectra of the complexes showed considerable shift in absorption peaks, changes in intensities of the peaks and formation of the new band (probably due to hydrogen bonding) on complexation.The thermodynamic association constants and other thermodynamic parameters of the complexes were determined spectrophotometrically taking D and A in varying ratios (2:8–8:2) and also in equimolar ratios. The complex formation was found to be spontaneous and associated with negative changes of ΔG0, ΔH0 and ΔS0.The energies hνCT of the charge-transfer complexes were compared with the theoretical values of hνCT of the complexes obtained from HOMO and LUMO of the donor and the acceptors. Density function theory utilizing different basis sets was used for calculation. hνCT (experimental) values of the transition energies of the complexes in acetonitrile differed from hνCT (theoretical) values in the gaseous state. IDV value of isoniazid was calculated.Charge-transfer complexes were assumed to be partial electrovalent compounds with organic dative ions D+ and A− (in the excited state) and attempts had been made to correlate the energy changes for the formation of the complexes with the energy changes for the formation of electrovalent compounds between M+ and X− ions.

Conductivity is a contrivance to explore ion-pair and triple-ion structure of ethanoates in tetrahydrofuran, dimethyl sulfoxide and their binaries

Precise electrical conductance measurements are reported for some ethanoates, viz. ammonium, lithium, sodium and potassium in pure tetrahydrofuran (THF) and dimethyl sulfoxide (DMSO) and their binary mixtures at 298.15K. The conductance data have been analyzed by the Fuoss conductance–concentration equation to evaluate the limiting molar conductance (Λo), the thermodynamic association constant (KA) and the association diameter (R) for ion-pair formation. The limiting ionic conductances have been estimated from the appropriate division of the limiting molar conductivity value of the “reference electrolyte” Bu4NBPh4. Furthermore, the conductance data for pure THF have been analyzed by the Fuoss–Kraus theory of triple ions and the values of the ion-pair and triple-ion formation constants (KP and KT) were made and the results have been discussed in terms of molecular scale model.

Nanoscale Mapping and Affinity Constant Measurement of Signal-Transducing Proteins by Atomic Force Microscopy

Atomic force microscope (AFM) was used to measure the interaction force between two signal-transducing proteins, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and Ras homologue enriched in brain (Rheb), and to analyze the binding of glyceraldehyde-3-phosphate (Gly-3-P) to GAPDH. To enhance the recognition efficiency and avoid undesirable multiple interactions, the AFM probe and the substrate were each modified with a dendron, glutathione S-transferase (GST)-fused proteins were employed, and reduced glutathione (GSH) was conjugated at the apex of each immobilized dendron. The resulting median specific force between GAPDH and Rheb was 38 ± 1 pN at a loading rate of 3.7 × 10(3) pN/s. The measurements showed that the GAPDH-Rheb interaction was inhibited by binding of Gly-3-P. An adhesion force map showed individual GADPHs on the surface and that the number density of GAPDH decreased with the concentration of Gly-3-P. Maps obtained in the presence of various Gly-3-P concentrations provided information on the binding behavior, yielding a thermodynamic association constant of 2.7 × 10(5) M(-1).

Electrochemical Studies on cis-[CrIII(bipy)2(SCN)2]I3 (Where bipy Denotes 2,2′-Bipyridine) in Acetonitrile

The molar conductivities (Λ) of solutions of bis(2,2′-bipyridine)bis(thiocyanate)chromium(III) triiodide [CrIII(bipy)2(SCN)2]I3 (where bipy denotes 2,2′-bipyridine, C10H8N2), [ $\mathrm{A}^{+}\mathrm{I}_{3}^{-}$ ], were measured in acetonitrile (ACN) at the temperatures 294.15, 299.15, and 305.15 K. In addition, cyclic voltammograms (CVs) of [ $\mathrm{A}^{+}\mathrm{I}_{3}^{-}$ ] were recorded on platinum, gold, and glassy carbon working electrodes in ACN, using n-tetrabutylammonium hexafluorophosphate (NBu4PF6) as the supporting electrolyte, at scan rates (v) ranging from 0.05 to 0.12 V⋅s−1. Furthermore, electrochemical impedance spectroscopic (EIS) measurements were carried out in the frequency range 50 Hz<f<50 kHz using these three working electrodes. The measured molar conductivities (Λ) demonstrate that [ $\mathrm{A}^{+}\mathrm{I}_{3}^{-}$ ] behaves as uni-univalent electrolyte in ACN over the investigated temperature range. The Λ values were analyzed by means of the Lee-Wheaton conductivity equation in order to estimate the limiting molar conductivities (Λ o), as well as the thermodynamic association constants (K A), at each experimental temperature for formation of [A+ $\mathrm{I}_{3}^{-}$ ] ion-pairs. The limiting ionic conductivities ( $\lambda_{\pm}^{\mathrm{o}}$ ), the diffusion coefficients at infinite dilution (D ±), as well as the Stokes’ radii (r St) were determined for both A+ and $\mathrm{I}_{3}^{-}$ ions. The thermodynamic parameters for the ionic association process, i.e. the Gibbs energy ( $\Delta G_{\mathrm{A}}^{\mathrm{o}}$ ), enthalpy ( $\Delta H_{\mathrm{A}}^{\mathrm{o}}$ ), and entropy ( $\Delta S_{\mathrm{A}}^{\mathrm{o}}$ ), were also determined. The mobility and diffusivity of the A+ ion increase linearly with increasing temperature because the solvent medium becomes less viscous as the temperature increases. The K A values indicate that significant ion association occurs that is not influenced by temperature changes. The ion-pair formation process is exothermic ( $\Delta H_{\mathrm{A}}^{\mathrm{o}}<0$ ), leading to the generation of additional entropy ( $\Delta S_{\mathrm{A}}^{\mathrm{o}}>0$ ). As a result, the Gibbs energy $\Delta G_{\mathrm{A}}^{\mathrm{o}}$ is negative ( $\Delta G_{\mathrm{A}}^{\mathrm{o}}<0$ ) and the formation of $[\mathrm{A}^{+}\mathrm{I}_{3}^{-}]$ becomes favorable. CV studies on $[\mathrm{A}^{+}\mathrm{I}_{3}^{-}]$ solutions indicated that the redox pair Cr3+/2+ appears to be quasi-reversible on a glassy carbon electrode but is completely irreversible on platinum and gold electrodes. EIS experiments confirm that, among these three electrodes, the glassy carbon working electrode has the smallest resistance to electron transfer.

Hydrogen bonding association in the electroreduced intermediates of benzoquinones and naphthoquinones

Keeping in view the importance of chemical and biological functions of quinone based couples; two different series of quinones, namely benzoquinones and naphthoquinones, are investigated electrochemically. Five compounds of each series are studied systematically in dichloromethane, acetonitrile, and propylene carbonate and from there the half-wave potentials of first and second reductions are obtained through cyclicoltammetry measurements. Four different alcohols are used with increasing concentrations as hydrogen bond donors on the basis of their increasing acidity. The hydrogen-bonding power is analyzed from the positive shifts in both the waves with increasing concentrations of alcohols. The quantitative comparison is made while calculating the thermodynamic association constants and number of alcohol molecules bonded to both anion and dianion of quinones. The qualitative behavior and quantitative data both indicate the quinone-alcohol interaction as hydrogen bonding and the strength of hydrogen bond is dependant upon the nature of species involved in this couple. From the cyclic voltammetric data the relative effects of hydroxylic additives and different substituted quinones on equilibrium constant are also observed. Solvent effect is rationalized in favor of hydrogen bonding in terms of solvent polarity parameters.

Kinetics of Hybridization on Surface Oligonucleotide Microchips: Theory, Experiment, and Comparison with Hybridization on Gel-Based Microchips

The optimal design of oligonucleotide microchips and efficient discrimination between perfect and mismatch duplexes strongly depend on the external transport of target DNA to the cells with immobilized probes as well as on respective association and dissociation rates at the duplex formation. In this paper we present the relevant theory for hybridization of DNA fragments with oligonucleotide probes immobilized in the cells on flat substrate. With minor modifications, our theory also is applicable to reaction-diffusion hybridization kinetics for the probes immobilized on the surface of microbeads immersed in hybridization solution. The main theoretical predictions are verified with control experiments. Besides that, we compared the characteristics of the surface and gel-based oligonucleotide microchips. The comparison was performed for the chips printed with the same pin robot, for the signals measured with the same devices and processed by the same technique, and for the same hybridization conditions. The sets of probe oligonucleotides and the concentrations of probes in respective solutions used for immobilization on each platform were identical as well. We found that, despite the slower hybridization kinetics, the fluorescence signals and mutation discrimination efficiency appeared to be higher for the gel-based microchips with respect to their surface counterparts even for the relatively short hybridization time about 0.5–1 hour. Both the divergence between signals for perfects and the difference in mutation discrimination efficiency for the counterpart platforms rapidly grow with incubation time. In particular, for hybridization during 3 h the signals for gel-based microchips surpassed their surface counterparts in 5–20 times, while the ratios of signals for perfect-mismatch pairs for gel microchips exceeded the corresponding ratios for surface microchips in 2–4 times. These effects may be attributed to the better immobilization efficiency and to the higher thermodynamic association constants for duplex formation within gel pads.

Homochiral column structure of rac- and lambda-tris(ethylenediamine)cobalt(III) cyclotriphosphate dihydrate in crystal structures and cation-anion association in aqueous solution.

rac- and Lambda-tris(ethylenediamine)cobalt(III) cyclotriphosphate dihydrate with the chemical formulas rac-[Co(en)(3)]P(3)O(9).2H(2)O (1) and Lambda-[Co(en)(3)]P(3)O(9).2H(2)O (2) were synthesized, and their crystal structures were determined by single-crystal X-ray analyses. In 1, the cationic complex molecule [Co(en)(3)](3+) with the Delta or Lambda enantiomer and cyclotriphosphate anion are alternately arrayed and connected by multiple hydrogen bonds to form a homochiral column structure. Adjacent homochiral columns with different chirality for 1 are connected by intercolumn hydrogen bonds through P(3)O(9)(3)(-) anions, as the bridging groups, to form a tetrameric cyclic cylindrical structure, while the adjacent columns with the same chirality are connected for 2 to form the cyclic cylindrical structure. All 6 amino groups per [Co(en)(3)](3+) participate in the formation of 12 hydrogen bonds, in which 8 hydrogen bonds contribute to the construction of a homochiral column and the remaining 4 hydrogen bonds contribute to the intercolumn interactions. The circular dichroism spectrum of the aqueous solution of Lambda-[Co(en)(3)](3+) changes drastically when excess P(3)O(9)(3)(-) is added, and this change is explained by ion-pair formation. The thermodynamic association constant of [Co(en)(3)](3+) with P(3)O(9)(3)(-), calculated from the conductivity data, was log K = 4.26 at 25 degrees C.

Solubility, Conductance, Association and Partition of Some Quaternary Ammonium Salts in Organic Solvent / Water Systems

In this study, the solubilities (Cs) of BzR3NX, R=Me, Et and Bu; X = Cl, Br and I,in solvents with a dielectric constant (e)range of 1.991-46.68 were measured. In this range of the dielectric constant, the solvent could affect the solubility by up to 105 –fold.For BzR3NCI , In Cs increase linearly with increasingewhene＜ 8.0. Besides the solubility, the limiting equivalent conductance (Λ°) and the thermodynamic association constant (KA) of the above salts and the salts Bu4NX, X=C1, Br, I and HSO4, in H2O, CH3CN,MeOH,C2H5NO2 and EtOH were also determined using the modified DHO conductance equation. Furthermore, for Bu4NCI, both the temperature effect onΛ° and KA and the solvent effect on KA and the partition coefficient (Kp) were investigated. The results showed that the association was exothermic; InKA increased linearly with increasing 1/e, and InKp decreased linearly with increasing 1/e.

PGSE NMR diffusion study of the self‐association of N‐methylacetamide in carbon tetrachloride

AbstractN‐Methylacetamide (NMA) is known to self‐associate in solution through the formation of hydrogen bonds. This behavior, and the presence in the molecule of an amide bond, make this an interesting model for study since the oligomer can be considered to be a protein analogue. The aims of the present study were to estimate the thermodynamic parameters of the self‐association process, through the measurement of the diffusion coefficient of NMA in carbon tetrachloride (CCl4), and to predict the population distribution of oligomers as a function of the concentration of NMA. Diffusion coefficients were measured using pulsed field gradient spin‐echo (PGSE) NMR spectroscopy. A computer model based on the Kirkwood–Riseman theory of macromolecular diffusion and an attenuative model of self‐association were used to fit the experimental data and to derive estimates of the thermodynamic association constants and bond length, and these values were, in turn, used to estimate the oligomer distribution. In addition, the temperature dependence of the diffusion coefficient was measured at three different concentrations in order to calculate the apparent Arrhenius activation energies so as to provide additional insight into the self‐association process. Finally, the concentration dependence of the viscosity of NMA in CCl4 was measured to characterize further the hydrodynamic behavior of the polydisperse system. Copyright © 2002 John Wiley &amp; Sons, Ltd.