Anion Binding Constants Research Articles

Prediction of surface charge on oxides in salt solutions: Revisions for 1:1 (M +L −) electrolytes

Quantitative characterization of the development of proton surface charge on the surfaces of minerals is necessary for a fundamental understanding of reactions between minerals and aqueous electrolyte solutions. Despite many experimental studies of charge development, few attempts have been made to integrate the results of such studies with a theoretical framework that permits prediction. The present study builds on a theoretical framework to analyze a total of 55 sets of proton surface charge data referring to wide ranges of ionic strengths, and types of electrolyte and oxide. The resulting parameters were interpreted with the aid of crystal chemical, electrostatic, and thermodynamic theory, which enable a number of generalizations. Prediction of values of the pH ZPC and Δ pK n θ reduces the number of triple-layer parameters to be estimated. New standard states for the equilibrium constants for electrolyte adsorption ( K M + θ and K L − θ) permit direct comparison of samples with a range of surface areas or site densities. Predicted cation binding on high dielectric constant solids (e.g., rutile) shows K M + θ, increasing in the sequence Cs +, Rb +, K +, Na +, Li +. In contrast, on low dielectric constant solids (e.g., amorphous silica), the predicted sequence is Li +, Na +, K +, Rb +, Cs +. The opposite sequences are attributable to the large solvation energy contribution opposing adsorption on low-dielectric constant solids. Cation and anion binding constants are in general different, which enables direct prediction of the point-of-zero-salt effect ( pH PZSE ) relative to the pristine point-of-zero charge. The inner and outer capacitances in the triple-layer model ( C 1 and C 2) are predictable parameters consistent with physically reasonable distances and interfacial dielectric constants for water. In summary, all the parameters in the triple-layer model can be estimated with the revised equations of this study, which enables prediction of proton surface charge for any oxide in 1:1 electrolyte solutions independent of experiments. Such predictions can serve as a complement to the experimental study of new oxide/electrolyte systems, or more complex systems, where additional mechanisms of charge development are likely.

Synthesis and Anion Binding Properties of Bifunctional Urea Derivative of Calix[4]diquinone.

The simultaneous binding of cationic and anionic guest species by ditopic receptors is a rapidly developing new field for ion pair recognition of environment and biological importance. By taking advantage of calix[4]arene framework, a few neutral bifunctional receptors were developed and showed the enhanced binding constants of hydrophilic anions in the presence of cations in organic media. As a consequence of their upper and lower rim of topologies the calixarenes are attractive host molecules to modify and so create unique geometries for the multifunctional receptors. The combination of two binding sites, urea for anions and two podand groups for cations, yields receptor that is capable of binding anions and alkali metal cations simultaneously. In addition to ion binding ability, two quinone moieties are introduced for the redox switchable receptors. We report here the synthesis of a new bifunctional receptor calix[4]diquinone 4 containing two urea moieties and two podand groups and the simultaneous binding properties of cation and anion guests were studied.

Synthesis and Anion Binding Properties of Bifunctional Urea Derivative of Calix[4]diquinone

The simultaneous binding of cationic and anionic guest species by ditopic receptors is a rapidly developing new field for ion pair recognition of environment and biological importance. By taking advantage of calix[4]arene framework, a few neutral bifunctional receptors were developed and showed the enhanced binding constants of hydrophilic anions in the presence of cations in organic media. As a consequence of their upper and lower rim of topologies the calixarenes are attractive host molecules to modify and so create unique geometries for the multifunctional receptors. The combination of two binding sites, urea for anions and two podand groups for cations, yields receptor that is capable of binding anions and alkali metal cations simultaneously. In addition to ion binding ability, two quinone moieties are introduced for the redox switchable receptors. We report here the synthesis of a new bifunctional receptor calix[4]diquinone 4 containing two urea moieties and two podand groups and the simultaneous binding properties of cation and anion guests were studied.

Evaluating the Conformational Role of an Allosteric CuII Ion in Anion Recognition and Catalysis by a Tricopper Complex

Structural and anion binding studies have provided an insight into the conformational role of an allosteric CuII ion in the trinuclear catalyst (L 2–2H). L 2 is a novel trinucleating ligand with pyridyl, pyrimidyl and amide donor groups. (L 2–2H)Cu (1) has been characterized by X-ray crystallography. The Cu ion is located at the allosteric site and coordinated by two amide N and two pyrimidine N atoms, the CuN4 plane is tetrahedrally distorted, and the complex adopts a helically twisted conformation. In contrast, the (L 2–2H)Cu3(μ4-C2O4) subunit of the dodecanuclear complex [(L 2–2H)4Cu12(μ4-C2O4)2(μ-OH)4(μ-Cl)4Cl4(H2O)2] (5) is roof-shaped, and the allosteric Cu is located on the top of a square-based pyramid. The oxalate coligand is coordinated by the two catalytic Cu ions in an unusual 1,4-μ-O,O bridging mode with an O ⃛O “bite length” of 2.6 Å and a Cu ⃛Cu distance of 6.4 Å. Intramolecular transesterification of the phosphodiester 2-(hydroxypropyl)-p-nitrophenyl phosphate (HPNP) by [(L 2–2H)Cu3]4+ was investigated, in comparison with the closely related complex [(L 3–2H)Cu3]4+ in which the ligand framework is somewhat less flexible. From a kinetic analysis of cleavage rate at varying HPNP concentrations, K HPNP (the equilibrium constant for binding of HPNP to the complex) and k cat (first-order rate constant for cleavage of HPNP when bound to the complex) parameters were derived: K HPNP=190 M-1 ([(L 2–2H)Cu3]4+) and 305 M-1 ([(L 3–2H)Cu3]4+), k cat=10×10-3 s-1 ([(L 2–2H)Cu3]4+) and 3.3×10-3 s-1 ([(L 3–2H)Cu3]4+). Anion binding constants of the complexes were determined by monitoring competitive inhibition of HPNP cleavage. The complexes have a high affinity to , , and , which appear to be of the appropriate size for bridging coordination, while “smaller” anions and are bound less efficiently. [(L 3–2H)Cu3]4+ has a higher affinity than [(L 2–2H)Cu3]4+ to HPNP but a lower affinity to the rather large anion . This is interpreted as a consequence of the reduced flexibility of [(L 3–2H)Cu3]4+, which slightly disfavours widening of the Cu ⃛Cu distance for incorporation of perrhenate. Similarly, the somewhat lower reactivity of [(L 3–2H)Cu3]4+ is attributed to the larger energy gap between the ground state and sterically more demanding (and less efficiently stabilized) transition state.

Potentiometric studies of the rutile–water interface: hydrogen-electrode concentration-cell versus glass-electrode titrations

Abstract This paper represents a comparison of surface protonation studies of rutile in NaCl media obtained using a conventional glass-electrode autotitrator system from 10 to 50 °C, and hydrogen-electrode concentration cells from 25 to 250 °C [J. Colloid Interface Sci., 200 (1998) 298]. Experimental conditions were matched as closely as possible between the two techniques, permitting a direct comparison of the results. Values for the pH of zero net proton charge (pH znpc ) of the rutile surface obtained at 10 and 35 °C were consistent with the temperature trends observed previously using hydrogen-electrode concentration cells. The pH znpc of rutile decreases systematically from 5.7 to 4.2 as temperature increases from 10 to 250 °C. Moreover, the experimentally determined pH znpc values agree with independent estimates of the pH at the pristine point of zero charge (pH znpc ) calculated from an extension of the revised Multi-Site Complexation (MUSIC) Model of Hiemstra et al. [J. Colloid Interface Sci., 184 (1996) 680]. Surface protonation curves obtained from the glass-electrode titration results were rationalized using surface protonation constants derived from the MUSIC Model, in conjunction with a Basic Stern representation of the electrical double layer (EDL) structure. Best-fit parameters (Stern layer capacitance values, and electrolyte cation and anion binding constants) are consistent with those obtained from fits to titration curves obtained using hydrogen-electrode concentration cells at 25 and 50 °C. Consequently, this comparison demonstrates that independent conventional glass-electrode and hydrogen-electrode concentration cell titrations provide completely compatible results despite the intrinsic differences in the two techniques (pH calibration, equilibration times, stirring rates, gas phase composition, etc).

Thermodynamic studies on anion binding to apotransferrin and to recombinant transferrin N-lobe half molecules

Equilibrium constants for the binding of anions to apotransferrin, to the recombinant N-lobe half transferrin molecule (Tf/2N), and to a series of mutants of Tf/2N have been determined by difference UV titrations of samples in 0.1 M Hepes buffer at pH 7.4 and 25°C. The anions included in this study are phosphate, sulfate, bicarbonate, pyrophosphate, methylenediphosphonic acid, and ethylenediphosphonic acid. There are no significant differences between anion binding to Tf/2N and anion binding to the N-lobe of apotransferrin. The binding of simple anions like phosphate appears to be essentially equivalent for the two apotransferrin binding sites. The binding of pyrophosphate and the diphosphonates is inequivalent, and the studies on the recombinant Tf/2N show that the stronger binding is associated with the N-terminal site. Anion binding constants for phosphate, pyrophosphate, and the diphosphonates with the N-lobe mutants K206A, K296A, and R124A have been determined. Anion binding tends to be weakest for the K296A mutant, but the variation in log K values among the three mutants is surprisingly small. It appears that the side chains of K206, K296, and R124 all make comparable contributions to anion binding. There are significant variations in the intensities of the peaks in the difference UV spectra that are generated by the titrations of the mutant apoproteins with these anions. These differences appear to be related more to variations in the molar extinction coefficients of the anion–protein complexes rather than to differences in binding constants.

Complex Formation and Ionic Association in Methanol Solutions of Poly(Ethylene Oxide) and Alkaline Earth Salts

Abstract Complex formation of poly(ethylene oxide) (PEO) with divalent barium and strontium salts was investigated in methanol. In these systems the complexation was accompanied by a considerable degree of ionic association. An analytical model for the polymer-ion complexation based on a one-dimensional lattice model was proposed. According to this model, the electrostatic effects between the bound ions were separated from the total free energy change of the binding. Three binding constants, i.e., the ionic association constant K A, the cation binding constant, K c, and the anion binding constant, K a, could be estimated. K A for barium and strontium salts was comparable, and the effect of counteranions on K A was not large. K c for barium salts was almost independent of the kind of counteranion and larger than that for corresponding strontium salts, indicating stronger polymer-ion interaction for barium salts. The anion binding constant, K a, was strongly dependent upon the kind of anion, and the order w...

Investigation of cobalt(II) substituted carboxypeptidase a interacting with azide and cyanate ions

Cobalt(II)-substituted carboxypeptidase A has been found to reversibly bind N3 − and NCO −, but not NCS −, in the pH range 5–10, thus including the pH range of activity of the enzyme. The pH dependence of the anion binding constant is affected by two ionizations, which are assigned as those regulating k cat and K M. The electronic and 1H NMR spectra are consistent with a substantially pseudotetrahedral geometry of the anion derivatives.

Ionic interactions and anion binding in the gramicidin channel. An electrostatic calculation

An electrostatic calculation is performed in order to examine basic features of interactions between ions (including anions) within the gramicidin channel. The calculation focuses on the effect of image forces. The substitute charge method is used for the calculation of image-force energies. Good arrangements of fictitious charges and contour points are described. Errors of calculated image-force energies are estimated not to greatly exceed 0.1%. The cases assumed are (i) the effective radius r of the channel is between 2·5 and 3·5 Å, (ii) the binding site with the highest affinity is between 1 and 3·5 Å from the channel end ( outer site), and (iii) the dimple at the channel mouth is 0–5 Å in depth. The induced energy of an ion placed at the outer site increases (and hence the affinity of the outer site decreases) with the increase in the depth of dimples, whereas the barrier height for translocation between the outer sites decreases in the presence of deeper dimples. The interactional energy between two monovalent cations placed at the outer sites is relatively small in the absence of dimples. It is large, however, in the presence of deeper dimples if the outer sites are 2·5 or 3·5 Å from the ends; but, it is still relatively small even in the presence of dimples if the sites are 1 Å from the ends. The interactional energy is very unfavorable for simultaneous occupancy by three cations. It is suggested that an ion pair may be formed at the channel mouth. Deeper positions of the outer site, smaller values of r, and deeper dimples favor the formation of the ion pair. In the presence of 5 Å dimples, the binding constant of an anion (Cl −) for a cation (Na +) which has already been bound at an outer site (with no second cation at the opposite mouth) is estimated to be 0·4–8 molal −1 if local interactions between the bound anion and the channel wall is negligibly small. The anion binding constant increases in the presence of a cation (or an ion pair) at the opposite mouth. It decreases markedly in the absence of dimples. The interactional energy is considerably unfavorable for the binding of a third cation even if the presence of an ion pair is postulated. It is still large in the presence of an ion pair at each of the mouths.

Anion binding and pH-dependent electrostatic effects in ribonuclease.

The solvent-accessibility-modified, Tanford-Kirkwood, discrete charge model for electrostatic effects is applied to both ribonuclease A and ribonuclease S. The behavior of individual titratable sites and the pH-dependent free energy of denaturation are correctly predicted. The use of the solvent-accessibility factor in reducing charge-site interactions introduces a higher Coulombic shielding for solvent-exposed sites. This shielding is interpreted as a higher local strength or alternatively a higher effective dielectric constant. Specific anion binding sites are determined by locating areas of high positive electrostatic potential at the protein solvent interface. The potential and thus the anion affinity of a given site are calculated and shown to vary with the pH-dependent charge array. pH-dependent anion binding constants are calculated for the ribonuclease S active site. These binding constants and the predicted response of the active-site histidine pK1/2 values to anion binding are shown to agree with experimental determinations.

The linkage between the four-step binding of oxygen and the binding of heterotropic anionic ligands in hemoglobin.

The linkage between the four-step binding of oxygen and the binding of heterotropic anionic ligands in hemoglobin was investigated by accurately measuring and analyzing the oxygen equilibrium curves of human adult hemoglobin in the presence and absence of various concentrations of one or two of the following materials: chloride (Cl-), 2,3-diphosphoglycerate (DPG), and inositol hexaphosphate (IHP). Each equilibrium curve was analyzed according to the Adair equation to evaluate the four-step oxygen equilibrium constants (Adair constants) and the median oxygen pressure. The binding constants of the anions for the molecular species of hemoglobin carrying j oxygen molecules, Hb(O2)j(j=0,1,...,4), were evaluated from the dependences of the Adair constants and the median oxygen pressure on the anion concentration by introducing a model which takes the competitive binding of Cl- and DPG or IHP into account. Assumptions made in the model are: (a) the hemoglobin molecule has two oxygen-linked binding sites for Cl- which are equivalent and independent and (b) no Cl- can be bound to hemoglobin to which DPG or IHP is already bound and vice versa. Thus, we could obtain values for the intrinsic binding constants of Cl- and DPG, i.e., the constants in the absence of other competitive anions. For IHP, only the binding constants and apparent binding constants for Hb and Hb(O2)2 were obtained. Values of the Cl- binding constants and apparent binding constants for DPG and IHP, i.e., the binding constants in the presence of Cl- for Hb and Hb(O2)4, were in reasonable agreement with literature values. From the binding constants we calculated anion binding curves for Hb(O2)j(J=0,1,...,4), the number of anions bound to Hb(O2)J, And the relationship between fractional anion saturation of hemoglobin and fractional oxygen saturation. The numbers of released anions are not uniform with respect to oxygenation step. This non-uniformity is the reason for the changes in the shape of the oxygen equilibrium curve with anion concentration changes and for the non-uniform dependences of the Adair constants on anion concentration, and also results in non-linear relations between anion saturation and oxygen saturation. The anion binding constants and various binding properties of the anions derived from those constants are consistent with those observed by other investigators using different techniques, indicating that the present model describes the oxygen-linked competitive anion binding well.