Stability constants buffer complexes

Obviously one could measure the pH of a known concentration of a weak acid and obtain a value of its hydronium ion activity, which would permit a direct evaluation of its dissociation constant. However, this would be a one-point evaluation and subject to greater errors than by titrating the acid halfway to the equivalence point. The latter approach uses a well-buffered region where the pH measurement represents the average of a large number of data points. Similar arguments can be made for the evaluation of solubility products and stability constants of complex ions. The appropriate expression for the evaluation of solubility products again is based on the half-equivalence point of the titration curve for the particular precipitation reaction [AgI(OH2)2h represents the titrant] ... 

The most controversial issue is the number and exact stoichiometries of the iron(III)-sulfito complexes formed under different experimental conditions. Earlier, van Eldik and co-workers reported the formation of a series of [Fe(SO ) ]3-2" (n = to 3) complexes and the [Fe(S03)(0H)] complex (89,91,92). The stability constants of these species were determined by evaluating time resolved rapid-scan spectra obtained from the sub-second to several minutes time domain. The cis-trans isomerization of the complexes was also considered, under feasible circumstances. In contrast, Betterton interpreted his results assuming the formation and linkage isomerization of a single complex, [Fe(SC>3)]+ (93). In agreement with the latter results, Conklin and Hoffmann also found evidence only for the formation of a mono-complex (94). However, their results were criticized on the basis that the experiments were made in 1.0 M formic acid/formate buffer where iron(III) existed mainly as formato complex(es). Although these reactions could interfere with the formation of the sulfito complex, they were not considered in the evaluation of the results (95). Finally, van Eldik and co-workers re-examined the complex-formation reactions and presented additional data in support of... 

The equilibrium constant for this reaction depends on the stability constants of the ionophore-M+ complexes and on the distribution of ions in aqueous test solution and organic membrane phases. For a membrane of fixed composition exposed to a test solution of a given pH, the optical absorption of the membrane depends on the ratio of the protonated and deprotonated indicator which is controlled by the activity of M+ in the test solution (H,tq, is fixed by buffer). By using a to represent the fraction of total indicator (Ct) in the deprotonated form ([C]), a can be related to the absorbance values at a given wavelength as... 

The equilibrium constant of an enzyme-catalyzed reaction can depend greatly on reaction conditions. Because most substrates, products, and effectors are ionic species, the concentration and activity of each species is usually pH-dependent. This is particularly true for nucleotide-dependent enzymes which utilize substrates having pi a values near the pH value of the reaction. For example, both ATP" and HATP may be the nucleotide substrate for a phosphotransferase, albeit with different values. Thus, the equilibrium constant with ATP may be significantly different than that of HATP . In addition, most phosphotransferases do not utilize free nucleotides as the substrate but use the metal ion complexes. Both ATP" and HATP have different stability constants for Mg +. If the buffer (or any other constituent of the reaction mixture) also binds the metal ion, the buffer (or that other constituent) can also alter the observed equilibrium constant . ... 

Preliminary rate measurements should allow one to make a plot of initial velocity Vq versus [metal ion], and this should provide information on the optimal metal ion concentration. (For many MgATP -dependent enzymes, the optimum is frequently 1-3 mM uncomplexed magnesium ion.) Then, by utilizing pubhshed values for formation constants (also known as stability constants) defining metal ion-nucleotide complexation, one can readily design experiments to keep free metal ion concentration at a fixed level. To compensate properly for metal ion complexation in ATP-dependent reactions, one must chose a buffer for which a stability constant is known. For example, in 25 mM Tris-HCl (pH 7.5), the stability constant for MgATP is approximately 20,000 M Thus, one can write the following equation ... 

In this case, the metal ion is titrated with a standard solution of EDTA. The solution containing the metal ion is buffered to an appropriate pH at which the stability constant of the metal-EDTA complex is large. The free indicator has a different colour from that of the metal-indicator complex. 

Uptake of methylphosphonic, aminomelhyl-phosphoniCs hydroxymethyl-phosphonic. l-hydro ycthane-(l, 1-diphosphonic), iminodi-(methylphosphonic). nitnlotris-(methylene-phosphonic). elhylcncdimtnlotetrakis-(methylcnephosphonic), and diethylenetruu-tnlopentakis-(methylenephosphonic) acids was interpreted in terms of formation of 2-9 different surface species whose stability constants are interrelated, namely, log K = (1145 + 7.31 nH2.53 + 0 46n>Z where n is the surface protonation level and Z IS the surface complex charge, and fully deprotonated anions are componenls. Adsorplion isotherms at constant pH were also obtained in the presence of buffers lEP at pH 7 2 in dispersion titrated with Na COj. pristine lEP at pH 8.5. 

The stability constant of a complex of pyrophosphate with Ni (presumably NiPjO though this is not specified by the authors) was measured by an amperometric titration procedure. Small amounts of the nickel nitrate salt were titrated with the pyrophosphate in an ammonium nitrate buffer solution (0.1 M), adjusted to a pH value of 8 with aqueous ammonia. A Pb02 indicating electrode was used with a platinum foil coimter electrode and a saturated calomel reference electrode. The procedure was found to be very sensitive to pyrophosphate even in the presence of phosphate. 

An 1,8-naphthyridine q-aminonitrile moiety serves both as an effective donor-acceptor array for complexation of creatinine and as an intrinsic chromophore and fluorophore. In the pH range of 4.1-4.6 the monoprotonated form apparently predominates in 70 % aqueous methanol, producing the absorption spectrum shown in Figure 14. Under these conditions creatinine exists as a mixture of protonated and unprotonated forms, since its pK is approximately 4.2 in this solvent mixture. Such proton-transfer equilibria complicate the calculation of specific stability constants, but under buffered conditions absorption and emission changes result only from complexation, not from proton transfer. As shown in Figure 14, addition of creatinine to a buffered solution decreases the intensity of the 442 nm absorption band attributed to the protonated receptor. Creatinine complexation also quenches the yellow-green fluorescence of the protonated receptor and titration experiments in progress may yield the effective stability constant of the complex. This receptor exemplifies the manner in which intrinsic chromophores and fluorophores may be incorporated into hosts for reversible complexation of clinically important analytes (26). 

Figure 7 pH dependence of conditional stability constants, of some analytically important EDTA-metal complexes. The dashed line indicates the effect of 1 mol I ammonia/ammonium ion buffer on the conditional constants of the Zn-EDTA complex. 

Abstract. Crown ethers derived from tartaric acid present a number of interesting features as receptor frameworks and offer a possibility of enhanced metal cation binding due to favorable electrostatic interactions. The synthesis of polycarboxylate crown ethers from tartaric acid is achieved by simple Williamson ether synthesis using thallous ethoxide or sodium hydride as base. Stability constants for the complexation of alkali metal and alkaline earth cations were determined by potentiometric titration. Complexation is dominated by electrostatic interactions but cooperative coordination of the cation by both the crown ether and a carboxylate group is essential to complex stability. Complexes are stable to pH 3 and the ligands can be used as simultaneous proton and metal ion buffers. The low extractibility of the complexes was applied in a membrane transport system which is a formal model of primary active transport. 

The tweezer-type receptor 43 that was developed by Kilbum and coworkers contains a disnbstitnted guanidinium group in the middle of the chain which was expected to bind to the terminal carboxylate gronp of peptidic guests (Scheme 22). The two peptide arms that are arranged in a parallel fashion serve to induce substrate selectivity. To test this idea, 43 was incubated in aqueous sodium borate buffer (pH 9.2, 16.7% DMSO) with a 1000-member library of tripeptides attached to a TentaGel resin via the amino terminus. Mainly hydrophobic amino acid residues were incorporated into these tripeptides to ensure that receptor substrate interactions are largely due to hydrophobic interactions. Receptor 43 was found to bind to about 3% of the library members and showed 95% selectivity for Val at the carboxylate terminus of the tripeptides and 40% selectivity for Glu(OfBu) at the amino terminus. A stability constant of 4 x lO M- was determined for the complex between 43 and Z-Glu(OrBu)-Ser(OfBu)-Val-0 in 16.7% DMSO/water (1 mM sodium borate buffer, pH 9.2) by means of isothermal titration microcalorimetry. 

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