The formation constants of the 1:1 and 1:2 complexes of Hg(II) with glutathione and their protonated species have been determined by using a competitive potentiometric titration with the competing ligand diethylenetriaminepentaacetic acid (DTPA). The formation constants of the 1:1 complex and its protonated species have not been reported previously. The formation constant of the 1:2 complex of Hg(II) and glutathione is substantially smaller than the accepted values that has been reported in the literature. These results have important implications in the models that have been employed to explain the mobilization and distribution of Hg(II) in biological systems.

No abstract

The equilibria between gallium(II1) ion and selected hydroxyaromatic and dihydroxyaromatic ligands at 2S"C, p = 0.100 M (KNO,) have been determined. Potentiometric measurements on 1: 1,2: 1, and 3: 1 molar ratios of ligand to Ga(II1) have been made as a function of degree of neutralization over the entire accessible -log [H'] scale. Calculations were carried out so as to take account of competing hydrolytic reactions, and formation constants of gallium(II1) with chromotropic acid, 8-hydroxyquinolineS-sulfonic acid, 5-sulfosalicylic acid, and 1,2-dihydroxybenzene-3,5-disulfonic acid were obtained. Stable hydroxo chelates do not form under the reaction conditions employed. The protonation constants of the ligands and the formation constants of the gallium chelates are discussed and compared with previously published work on these gallium chelates and on chelates of "analogous" metal ions such as those of Fe(II1) and AI(II1).

Nuclear magnetic resonance (n.m.r.) spectroscopy was used to determine the preferred protonation sites in TTHA. For its 1 :1 complex with PbOI) the following equilibrium constants for protonation were obtained (triethylenetetraaminehexaacetic acid = H6A)The non-protonated complex is considered to have four coplanar (or nearly coplanar) metal-nitrogen bonds with the center carboxylate groups coordinated above and below this plane, and with the terminal carboxylate groups playing only a small part in the coordinate bonding. The first and second protonations of the complex occur preferentially at the terminal and center nitrogen atoms, respectively, on the same side of the complex, accompanied by breaking of the respective metal-nitrogen bonds. This causes partial unwrapping of the complex from one side. Rapid interconversion between configurations in which unwrapping and rewrapping occurs first from one side of the molecule and then from the other leads to simplified n.m.r. spectra.La spectroscopie r.m.n. a 6td utilis6e pour determiner les sites prkfdrentiels de protonation dans le TTHA. Dans le cas d'une complexation 1 :1 avec le Pb(I1) les constantes d'dquilibre suivantes ont ttd obtenues pour la protonation (acide tri6thylenetdtraamineh6xaacdtique = H6A) PbA4-+ H + e PbHA3-log K = 8.25 + 0.2 PbHA3-+ H + PbH2A2-log K = 4.25 i-0.25Dans le complexe non protone, on considere que les quatre liaisons mdtal-azote sont coplanaires (ou presque coplanaires), que les groupes carboxylates centraux sont coordonnds au-dessus et en dessous du plan et que les groupes carboxylates terminaux n'interviennent que trks peu dans les liaisons coordonn6es. La premiere et la seconde protonation du complexe se font prdf6rentiellement et respectivement sur les atomes d'azote terminaux et centraux et sur le meme c6td du complexe provoquant ainsi la rupture respective des liaisons metal-azote. Ceci provoque un ddroulenient du complexe i partir d'un cdtC. L'interconversion rapide entre les configurations ou I'enroulement et le dCroulement sefait d'abord d'un c6td de la molkule puis de I'autre, conduit i simplifier les spectres r.m.n.

An integrated senior chemistry experimentMost chemistry majors in their junior or senior year study the properties of aminocarboxylic acids. This may come about in numerous ways; that is, their preparations or structures, their dipolar behavior (ampholytes), their metal complexing abilities (EDTA), their ionization constants, solubilities, iso-electric points, etc. It follows that an experiment involving an aminocarboxylic acid could be used for the integrated laboratory course where most branches of chemistry would be represented adequately. This paper is limited to a protonation study of an aminocarboxylic acid and its metal complexes; however, the reader will soon realize that it can be expanded to cover topics from instrumental techniques to a study of the kinetics of metal-ligand bond formation. Diethylenetriaminepentaacetic acid was chosen because of its symmetry (as will be shown later) and its commercial availability in a pure form at a relatively low cost.