070ChemInform Abstract The stability constants for the divalent metals Mg, Ca, Mn, Co, Ni, Cu, Zn, and Cd are determined by potentiometric pH titrations at 25 rc C. The results show that the stability of most of the M(ATP)2-complexes is significantly larger than that of the corresponding complexes formed with the pyrimidine-nucleoside 5'-triphosphates (PNTP). This increased stability is attributed to the formationof outer-sphere macrochelates. Inner-and outer-sphere forms of M(ATP)2-occur for Mn, Co, Ni, Zn, and Cd in comparable amounts, Cu forms no outer-sphere species to any significant extent and Ca(ATP)2-exists only in the open, phosphate-coordinated form. Of all the M(PNTP)2-complexes, only Cu(CTP)2-forms a base-back-bound species.

7.3 f 2.0 ' 6.4 & 2.3 8.8 f 2.4 6.3 f 2.4 0.95 f 0.22' 0.94 f 0.17 0.42 & 0.16 0.91 f 0.26 0.62 f 0.12' 0'34 ' 0.34 f 0.05 0.39 f 0.08 29.5 f 3.4 ' 28.8 f 4.3 31.0 & 3.0 33.3 k 3.2 2.79 & 0.91 ' ::;: i::; 2.79 f 0.45 0.70 & 0.19' 0.57 f 0.1 1 0.64 & 0.13 0.66 f 0.13, ~~

The stability constants of the 1:l complexes between Cu2+ and Zn2+ with formate, acetate and several phenylalkanecarboxylates, i.e. C6H5-(CH2)n-C00-with n = 0 to 5, are summarized for water, 50% aqueous ethanol and 50% aqueous dioxane ( I = 0.1 M; 25°C): Complex stability depends upon carboxylate group basicity. The influence of varying amounts of ethanol or dioxane (up to 90%) on the stability of the Cu2' and Zn2+ (M") complexes with formate and acetate (CA) was measured by potentiometric pH titrations. The values for pf(,(,., and log K&-.) increase, as expected, with increasing amounts of the organic solvents, i.e. with decreasing solvent polarity. The changes in the equilibrium constants are also evaluated with regard to the mole fractions of the organic solvents and the corresponding dielectric constants. These results may be used to estimate for low dielectric cavities in proteins the equivalent solution dielectric constant on the basis of enhanced carboxylate basicity or metal ion binding capability (method 1). Furthermore, the measured stability constants are used for comparisons of the coordination tendency of carboxylate ligands towards zinc(I1)-metalloenzymes (method 2) ; in this way the equivalent solution dielectric constants in the active-site cavities of bovine carbonic anhydrase and carboxypeptidase A are estimated: the values are of the order of 35 and 70, respectively. This method seems to be generally applicable to metalloproteins.Among the prominent binding sites for metal ions in biological systems are the carboxylate, imidazole and amino groups [l -31. They are not only part of the low-molecularmass ligands [4], but occur also as side chains in proteins, sometimes located in low dielectric cavities.The acidity constants of N and 0 ligands are clearly dependent upon the polarity of the solvent [l, 51: the basicity of N ligands (with no charge) usually decreases as the polarity of the solvent decreases, while the basicity of 0 ligands (with a negative charge) increases markedly as the dielectric constant decreases. The change in the stability of complexes [l] is in accord with the change in basicity, but with N ligands the stability decreases usually only slightly, as the polarity of the solvent decreases, while with 0 ligands the increase in stability may be quite pronounced. This means that the affinity towards metal ions of an imidazole moiety is expected to change little with a change in solvent polarity, while the change Correspondence to H . Sigel, Institut fur Anorganische Chemie der Universitat Basel, Spitalstrasse 51, CH-4056 Basel, SwitzerlandAhhreviutions. Ac-, acetate; CA-, carboxylate ligand; M 2 + , general divalent metal ion. Additionally, in the Supplement: Bz-, benzoate; PAC-, 2-phenylacetate; PBu-, 4-phenylbutyrate; PCa-, 6-phenylcapronate; PPr -, 3-phenylpropionate; PVa-, 5-phenylvalerate.Miniprint supplement. Some parts of this work, including Materials and Measurements, and further details on Results and Discussion, are given in a miniprint supplement a t the end of the p...

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