The second acidic constant of salicylic acid

The second dissociation constant of salicylic acid (H2L) has been determined, at 25 degrees C, in NaCl ionic media by UV spectrophotometric measurements. The investigated ionic strength values were 0.16, 0.25, 0.50, 1.0, 2.0 and 3.0 M. The protolysis constants calculated at the different ionic strengths yielded, with the Specific Interaction Theory, the infinite dilution constant, log beta1(0) = 13.62 +/- 0.03, for the equilibrium L2- + H+ <==> HL-. The interaction coefficient between Na+ and L2-, b(Na+, L2-) = 0.02 +/- 0.07, has been also calculated.     

Metalloprotein association,self-association,and dynamics governed by hydrophobic interactions: simultaneous occurrence of gated and true electron-transfer reactions between cytochrome f and cytochrome c(6) from Chlamydomonas reinhardtii

Noninvasive reconstitution of the heme in cytochrome c(6) with zinc(II) ions allowed us to study the photoinduced electron-transfer reaction (3)Zncyt c(6) + cyt f(III) --> Zncyt c(6)(+) + cyt f(II) between physiological partners cytochrome c(6) and cytochrome f, both from Chlamydomonas reinhardtii. The reaction kinetics was analyzed in terms of protein docking and electron transfer. In contrast to various protein pairs studied before, both the unimolecular and the bimolecular reactions of this oxidative quenching take place at all ionic strengths from 2.5 through 700 mM. The respective intracomplex rate constants are k(uni) (1.2 +/- 0.1) x 10(4) s(-1) for persistent and k(bi) (9 +/- 4) x 10(2) s(-1) for the transient protein complex. The former reaction seems to be true electron transfer, and the latter seems to be electron transfer gated by a structural rearrangement. Remarkably, these reactions occur simultaneously, and both rate constants are invariant with ionic strength. The association constant K(a) for zinc cytochrome c(6) and cytochrome f(III) remains (5 +/- 3) x 10(5) M(-1) in the ionic strength range from 700 to 10 mM and then rises slightly to (7 +/- 2) x 10(6) M(-1), as ionic strength is lowered to 2.5 mM. Evidently, docking of these proteins from C. reinhardtii is due to hydrophobic interaction, slightly augmented by weak electrostatic attraction. Kinetics, chromatography, and cross-linking consistently show that cytochrome f self-dimerizes at ionic strengths of 200 mM and higher. Cytochrome f(III) quenches triplet state (3)Zncyt c(6), but its dimer does not. Formation of this unreactive dimer is an important step in the mechanism of electron transfer. Not only association between the reacting proteins, but also their self-association, should be considered when analyzing reaction mechanisms.     

Modelling the adsorption of Cd(II), Cu(II), Ni(II), Pb(II), and Zn(II) onto Fithian illite

Illite samples from Fithian, IL were purified and saturated with Na(+) ions. The acid-base surface chemistry of the Na-saturated illite was studied by potentiometric titration experiments with 0.1, 0.01, and 0.001 M NaNO(3) solutions as the background electrolyte. Results showed that the titration curves obtained at different ionic strengths did not intersect in the studied pH range. The adsorption of Cd(II), Cu(II), Ni(II), Pb(II), and Zn(II) onto illite was investigated as a function of pH and ionic strength by batch adsorption experiments. Two distinct mechanisms of metal adsorption were found from the experimental results: nonspecific ion-exchange reactions at lower pH values on the basal surfaces and 'frayed edges' and specific adsorption at higher pH values on the mineral edges. Ionic strength had a greater effect on the ion-exchange reactions. The binding constants for the five heavy metals onto illite were determined using the least-square fitting computer program FITEQL. Linear free energy relationships were found between the surface binding constants and the first hydrolysis constants of the metals.     

Thermodynamics study of the dimerization equilibria of rhodamine B and 6G in different ionic strengths by photometric titration and chemometrics method

The dimerization constants of rhodamine B and 6G have been determined by studying the dependence of their absorption spectra on the temperature in the range 20-80 degrees C at different total concentrations of rhodamine B (5.89 x 10(-6) to 2.36 x 10(-4)M) and rhodamine 6G (2.34 x 10(-5) to 5.89 x 10(-4)M) and in different concentrations of LiCl, NaCl and KCl salts as supporting electrolytes. The monomer-dimer equilibrium of rhodamine B and 6G have been determined by chemometrics refinement of the absorption spectra obtained by thermometric titrations performed at different ionic strengths. The quantitative analysis of the data of undefined mixtures, was carried out by simultaneous resolution of the overlapping spectral bands in the whole set of absorption spectra. The dimerization constants are varied by changing the ionic strength and the degree of dimerization are decreased by increasing of the ionic strength of the medium. The enthalpy and entropy of the dimerization reactions were determined from the dependence of the equilibrium constants on the temperature (van't Hoff equation). From the thermodynamic results the TDeltaS degrees -DeltaH degrees plot was sketched. It shows a fairly good positive correlation which indicates the enthalpy-entropy compensation in the dimerization reactions (compensation effect).     

Stability constants of aluminium fluoride complexes

Pointwise titrations of aluminium nitrate solutions with sodium fluoride were made at ionic strengths of 0.5, 0.2, 0.1 and 0.05M and at temperatures of 25 degrees and 37 degrees . The total Al(3+) concentration was kept constant at 5 x 10(-4)M and the total F(-) concentration covered the range 5-20 x 10(-4)M with a minimum of 20 points per titration. It is shown that only mononuclear complexes were present with these solution compositions. Determinations of H(+) and F(-) in solution were made by the use of quinhydrone and lanthanum fluoride electrodes, respectively. Formation constants were calculated by three different procedures. Best results were obtained by a non-linear generalized least-squares method that (a) permits calculation of the formation constants with the simultaneous presence of all the complexes being taken into account, (b) does not require segregation of data according to the values of the formation function (n), and (c) yields standard errors that reflect the errors in all the experimental measurements. Values for the formation constants at infinite dilution were obtained by a linear least-squares adjustment of the values obtained for the four finite ionic strengths. The formation constants thus obtained for the reaction AlF(n-1) + F <==> AlF(n) (charges omitted) at 25 degrees have logarithmic values of 6.69 +/- 0.02, 5.35 +/- 0.03, 3.68 +/- 0.01 and 2.75 +/- 0.04 for n = 1, 2, 3 and 4, respectively. The corresponding values at 37 degrees are 6.68 +/- 0.10,5.34 +/- 0.02,3.94 +/- 0.04, and 3.29 +/- 0.05.     

EFFECTS OF IONIC STRENGTH and pH ON THE BINDING OF SANGUINARINE TO DEOXYRIBONUCLEIC ACID

Abstract— The interaction of sanguinarine with DN A has been studied in buffers of various ionic strengths and pH values where the physicochemical properties of DNA remain unchanged. Spectrophotometric analysis using absorption and fluorescence techniques indicates that the complex formed between sanguinarine and DNA is a function of ionic strength and pH. Increasing the salt concentration minimizes the importance of intercalator charge and extrapolation to 1 M [Na+] salt reveals the intercalative abilities, as reflected in binding constants, to be of the same order of magnitude as that of ethidium bromide. The fluorescence of sanguinarine bound to DNA is quenched and can be decreased by [Na +], [Mg²+] and [Ca²+] ions. The influence of pH on the fluorescence spectrum of sanguinarine with increasing concentration of DNA was studied. It is concluded that the binding of sanguinarine to DNA contains a large favourable non-electrostatic interaction and the alkaloid binds more DNA in buffer of low ionic-strength and acidic pH.     

Voltammetry of the interaction of metronidazole with DNA and its analytical applications

Voltammetric methods were used to probe the interaction of antimicrobial drug metronidazole (MTZ) with calf thymus DNA. Binding constants (K) and binding site sizes (s) were determined from the voltammetric data, i.e., shifts in potential and changes in limiting current with the addition of DNA. MTZ showed appreciable electrostatic binding to DNA in solution with K=2.2(+/- 1.3) x 10(4) M(-1) and s=0.34 bp. One reduction peak of MTZ at the bare glassy carbon electrode (GCE) split into two peaks at the DNA modified GCE (DNA/GCE). These changes in the cyclic voltammogram can only be due to the interaction of MTZ with the surface-confined DNA. In addition, the peak current of MTZ at the DNA/GCE was nearly 8-fold of the response at the bare GCE. The low detection limit of 2.0 x 10(-8) M made the DNA/GCE a promising biosensor for MTZ determination. And this method was successfully applied with high precision and accuracy compared with spectroscopic methods (relative error < 6%) for estimation of the total MTZ drug content in pharmaceutical dosage forms.     

Dynamics and heterogeneity of Pb(II) binding by SiO2 nanoparticles in an aqueous dispersion

Pb(II) binding by SiO(2) nanoparticles in an aqueous dispersion was investigated under conditions where the concentrations of Pb(2+) ions and nanoparticles are of similar magnitude. Conditional stability constants (log K) obtained at different values of pH and ionic strength varied from 4.4 at pH 5.5 and I = 0.1 M to 6.4 at pH 6.5 and I = 0.0015 M. In the range of metal to nanoparticle ratios from 1.6 to 0.3, log K strongly increases, which is shown to be due to heterogeneity in Pb(II) binding. For an ionic strength of 0.1 M the Pb(2+)/SiO(2) nanoparticle system is labile, whereas for lower ionic strengths there is loss of lability with increasing pH and decreasing ionic strength. Theoretical calculations on the basis of Eigen-type complex formation kinetics seem to support the loss of lability. This is related to the nanoparticulate nature of the system, where complexation rate constants become increasingly diffusion controlled. The ion binding heterogeneity and chemodynamics of oxidic nanoparticles clearly need further detailed research.     

Solubility and first hidrolysis constants of europium at different ionic strength and 303 K

The solubility of europium at 0.02M, 0.1M and 0.7M NaClO₄ ionic strength solutions was determined by a radiometric method and pEu_s-pC_H diagrams were obtained. Hydrolysis constants were also determined at the same ionic strengths by pH titration and the values found were log *₁ = -7.68±0.11, -8.07±0.10 and -8.20±0.11. The log K _sp values were -23.5±0.2, -22.7±0.2 and -21.9±0.2 for 0.02M, 0.1M and 0.7M NaClO₄ ionic strengths, respectively, at 303 K under CO₂-free conditions and the extrapolated value at zero ionic strength was log K _sp ⁰ = -24.15. The working pC_H ranges for the calculation of the hydrolysis constants were selected from the pEu_s-pC_H diagrams in the region where precipitation of europium oxide or hydroxide was less than 20%. Europium removal from aqueous solutions with zeolites was explored.     

Study of interactions of anthraquinones with DNA using ethidium bromide as a fluorescence probe

The interactions of fish sperm deoxyribonucleic acid (DNA) with anthraquinones, such as chrysophanol, physcion and 1,8-dihydroxy anthraquinone, were investigated by using ethidium bromide (EB) as fluorescence probe. The binding constants of anthraquinones and DNA were obtained by the fluorescence quenching technique. Further, the binding mechanisms on the reaction of the three anthraquinones with DNA and effect of ionic strength on the fluorescence property of the system have also been investigated. The results of the assay indicate that the binding modes of chrysophanol, physcion and 1,8-dihydroxy anthraquinone with DNA were evaluated to be groove binding. And the binding constants of chrysophanol, physcion and 1,8-dihydroxy anthraquinone with DNA-EB complex were 1.64x10(4), 3.04x10(4) and 2.88x10(5) l mol(-1), respectively.     

Interaction of Cu(II) and Cd(II) ions with DNA from Spirulina platensis

Equilibrium dialysis and atomic absorption analysis were used to obtain adsorption isotherms and determine the stoichiometric binding constants of Cu(II) and Cd(II) ions to DNA from Spirulina platensis in solutions. The stoichiometric constants of Cu(II) and Cd(II) ions with DNA from S. platensis in 3 mM NaCI are 15.56⋅10⁴ and 14.40⋅10⁴, respectively. Effect of ionic strength and DNA GC content on binding constants of Cu(II)- and Cd(II)-DNA complexes were studied out. It was showed that the binding constants of Cu(II)- and Cd(II)-DNA complexes decrease with increase of ionic strength. The empirical dependences of logK on the GC content has been derived for Cd(II)- and Cu(II)-DNA complexes.     

Effective concentration difference model to study the effect of various factors on the effective diffusion coefficient in the dialysis membrane

Cellulose acetate dialysis membrane (CDM) has been used in the diffusive gradients in thin films (DGT) technique, where accurate diffusion coefficients are essential for the assessment of the concentrations of labile metal in solution. Effective concentration difference model (ECDM), based on the assumption that the effective diffusion coefficient of metal ion in the dialysis membrane is determined by the effective concentration difference (ΔC(e)) across the dialysis membrane, is proposed and applied to study the effect of ionic strength, binding agent, ligands and Donnan potential on the effective diffusion coefficient. The effective diffusion coefficients of Cd(2+) through the dialysis membrane immersed in receptor solutions with binding agent were almost the same as those in receptor solutions without binding agent at higher ionic strengths (0.01-1 M) but much higher than those at lower ionic strengths (0.001-0.0001 M). The effective diffusion coefficients of Cd(2+) through the dialysis membrane immersed in deionized water receptor solutions with binding agent were not significantly different from those in synthetic receptor solutions (receptor solutions with various ionic strengths) with binding agent. The DGT-labile fractions were measured in synthetic solutions and natural waters, which indicated that the effective diffusion coefficients, through the dialysis membrane immersed in the deionized water solution with binding agent as receptor solution and in the spiked natural water as source solution, were more suitable for DGT application.     

Sequence-dependent interactions of cationic naphthalimides and polynucleotides

The binding interactions of three naphthalimide derivatives with heteropoly nucleic acids have been evaluated using fluorescence, absorption and circular dichroism spectroscopies. Mono- and bifunctionalized naphthalimides exhibit sequence-dependent variations in their affinity toward DNA. The heteropoly nucleic acids, [Poly(dA-dT)]2 and [Poly(dG-dC)]2, as well as calf thymus (CT) DNA, were used to understand the factors that govern binding strength and selectivity. Sequence selectivity was addressed by determining the binding constants as a function of polynucleotide composition according to the noncooperative McGhee-von Hippel binding model. Binding affinities toward [poly(dA-dT)](2) were the largest for spermine-substituted naphthalimides (Kb = 2-6 x 10(6) M(-1)). The association constants for complex formation between the cationic naphthalimides and [poly(dG-dC)]2 or CT DNA (58% A-T content) were 2-500 times smaller, depending on the naphthalimide-polynucleotide pair. The binding modes were also assessed using a combination of induced circular dichroism and salt effects to determine whether the naphthalimides associate with DNA through intercalative, electrostatic or groove-binding. The results show that the monofunctionalized spermine and pyridinium-substituted naphthalimides associate with DNA through electrostatic interactions. In contrast, intercalative interactions are predominant in the complex formed between the bifunctionalized spermine compound and all of the polynucleotides.     

On the protonation equilibria of sephadex C-25 and C-50

The system H(+)Na(+) has been studied on Sephadex C-25 and C-50 at two ionic strengths by potentiometry. The data have been fitted by the H?gfeldt three-parameter model. For ionic strength 0.100 M (Na)ClO(4) an excellent fit is obtained with a standard deviation of +/-0.013 for both gels. For ionic strength 0.010 M (Na)ClO(4) a satisfactory fit could only be obtained by excluding the lowest and highest pH-value for each gel.     

Studies of interaction of emodin and DNA in the presence of ethidium bromide by spectroscopic method

Emodin interacting with deoxyribonucleic acid (DNA) has been studied by different spectroscopic techniques, such as fluorescence, ultraviolet and visible (UV-vis), and fourier transform infared (FT-IR) spectroscopies, using ethidium bromide (EB) as a fluorescence probe of DNA. The decrease in the fluorescence of DNA-EB system on addition of emodin shows that the fluorescence quenching of DNA-EB complex by emodin occurs. The binding constants of emodin with DNA in the presence of EB are 6.02x10(4), 9.20x10(4) and 1.17x10(5)Lmol(-1) at 20, 35 and 50 degrees C, respectively. FT-IR spectrum further suggests that both the phosphate groups and the bases of DNA react with emodin. The reaction of DNA with emodin in the presence of EB is affected by ionic strength and temperature. The values of melting temperature (T(m)) of DNA-EB complex and emodin-DNA-EB complexes were determined, respectively. From the experiment evidences, the major binding mode of emodin with DNA should be the groove binding.     

Competitive ion complexation to polyelectrolytes: determination of the stepwise stability constants. The Ca2+/H+/polyacrylate system

This work presents a new methodology aimed at obtaining the stepwise stability constants corresponding to the binding of ions (or other small molecules) to macromolecular ligands having a large number of sites. For complexing agents with a large number of sites, very simple expressions for the stepwise stability constants arise. Such expressions are model-independent; that is, they allow the determination of the stepwise stability constants without making any previous assumption of the detailed complexation mechanism. The formalism is first presented for a single complexing ion and further extended to competitive systems where the competing ions can display, in general, different stoichiometric relationships. These ideas are applied to the analysis of experimental titrations corresponding to competitive binding of calcium ions to poly(acrylic acid) for different pH values and ionic strengths. Intrinsic stability constants were estimated from the stepwise stability constants (by removing the corresponding statistical factor), and split into specific and electrostatic contributions (by means of the Poisson-Boltzmann equation). After this treatment, the specific proton binding energies showed almost no dependence on the coverage and ionic strength. Likewise, for the range of concentrations studied, the specific component of the intrinsic stability constants of the calcium ions, calculated assuming bidentate binding of Ca to neighboring groups of a linear chain, is almost independent of the calcium and proton coverage and ionic strength.     

The binding of ethidium bromide with DNA: interaction with single- and double-stranded structures

The pH-induced helix-coil transition of DNA and its complexes with EtBr is carried out at acidic pH in a wide interval of change of concentration ratio of EtBr/DNA. The binding isotherms of EtBr on double and single-stranded DNA at pH = 7.0 and pH = 3.0 (t = 25(o)C) are obtained by absorption and fluorimetric methods. Binding constants (K) and number of bases (n), corresponding to one binding site were determined. Non fluorescent "strong" complex with ds-DNA at pH = 7.0 and t = 25(o)C as well as "strong" and "weak" complexes with ss-DNA at pH = 3.0 and t = 25(o)C are revealed.     

Ionic strength dependence of formation constants-XVIII. The hydrolysis of iron(III) in aqueous KNO(3) solutions

The hydrolysis of iron(III) was studied potentiometrically at different ionic strengths in KNO(3) aqueous solutions, at 25 degrees C, to determine the dependence of hydrolysis constants on ionic strength (nitrate media), to check the existence of nitrate-ferric ion interactions, and to confirm the formation of high polymeric species. Under the experimental conditions 0.03 I (KNO(3)) 1M, 0.3 C 12 mM, the species Fe(OH)(2+), Fe(2)(OH)(4+)(2), Fe(OH)(+)(2) and Fe(12)(OH)(2+)(34) were found, and the hydrolysis constants log beta(11) = 2.20, log beta(12) = -2.91, log beta(22) = -5.7, log beta(12,34) = -48.9 (I = 0M) were calculated. The ionic strength dependence of hydrolysis constants is quite close to that found for several protonation and metal complex formation constants reported elsewhere.     

Viscosity of sodium carboxymethylcellulose in ethylene glycol–water mixed solvent media: Separation of the influences of polyion conformation and electrostatic interactions on the reduced viscosity

Precise measurements on the viscosities of the solutions of sodium carboxymethylcellulose in water and three ethylene glycol–water mixtures containing 10, 20, and 30 mass % of ethylene glycol have been reported at 35 °C. Isoionic dilutions were performed at total ionic strengths in the range of 0.0002–0.0008 eqv L^?1 using sodium chloride to obtain the intrinsic viscosities along with the Huggins constants. The influence of the medium and the ionic strength on the intrinsic viscosities have been interpreted from the points of view of the counterion condensation and expansion/contraction of the polyion chains in solution. The variations of Huggins constants, on the other hand, provided information on the intermolecular interactions in these solutions. A convenient method has been proposed to decompose the reduced viscosity of a polyelectrolyte solution into its conformational and electrostatic components. The electrostatic reduced viscosities obtained in the present study, purely from experimental considerations, quantitatively corroborates the conclusions derived from the Huggins constants. Using the Hess and Klein theoretical approach, an expression for the reduced viscosity due to the electrostatic interactions as a function of polyelectrolyte concentration could be obtained and the reported experimental electrostatic contributions could be nicely described with the help of this approach. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1196–1202, 2010     

A new strategy for the design of water-soluble synthetic receptors: specific recognition of DNA intercalators and diamines

Water-soluble zinc bisporphyrin receptors 1 and 2 having two Lewis acidic sites (zinc) in the hydrophobic environment consisting of alkyl chains and a bisporphyrin framework, and covered with hydrophilic exterior (twelve or eighteen carboxyl groups) were prepared. The receptors show high affinity for diamines and DNA intercalators in water where the binding constants K(a) are of the order of 10(7) and 10(8) M(-1), respectively. Diamines and DNA intercalators are bound to the receptor through different mechanisms. Diamines are bound through hydrophobic interactions and zinc-nitrogen interactions, while DNA intercalators are bound through hydrophobic interactions and charge-transfer interactions. Flexible alkyl chains can make van der Waals contact with guests and create a hydrophobic environment around the bound guest by an induced-fit-type mechanism. For the binding of DNA intercalators, the following features are noteworthy: 1). Binding constants are similar between the zinc porphyrins and zinc-free porphyrins; 2). the binding constant is larger for the guest having the lower LUMO; this indicates the important contribution of charge-transfer interactions to binding; 3). the hydrophobic and cationic nature of DNA intercalators is substantially important, and 4). higher ionic strength reduced the binding affinities; this shows a moderate contribution of electrostatic interactions. The conformational instability of the receptors also contributes to the tight binding: hydrophobic and electrostatic interactions cannot both be favorable at the same time in the guest-free receptor. Enthalpy-entropy compensation observed for the binding of diamines and DNA intercalators is characterized by a relatively small slope (alpha=0.74) and a large intercept (beta=7.75 kcal mol(-1)) in the DeltaH degrees versus TDeltaS degrees plot; this shows that a conformational change of receptors and a significant desolvation occur upon binding. The receptor can competitively bind to propidium iodide to deprive DNA of the intercalated propidium iodide. These features of water-soluble receptors consisting of a rigid framework and flexible side chains with a large solvent-accessible area are in contrast to highly preorganized rigid receptors, and they can provide useful guidelines for rational design of induced-fit artificial receptors in water.