Abstract
Abstract A pH-titration study, at 25·0 ± 0·1°C and μ = 0·1 M (NaClO 4 ), of equilibria in solutions of one metal, Al 3+ , Ca 2+ or Cd 2+ , and ligands taken in twos and threes from cysteine, orthophosphoric acid and either citric or nitrilotriacetic acids is reported. It is shown that Al 3+ and Ca 2+ ions have little affinity for cysteine in the presence of phosphate and carboxylate ions, in accord with what is known of their properties in plant and animal tissues. Aluminium ions do not compete for Ca 2+ sites, probably on account of the ability of Ca to accommodate eight or nine non-directional bonds to oxygen. Cadmium forms soluble (phosphate)Cd(NTA) ternary complexes in three-ligand systems below pH 7·0 and soluble quaternary complexes above pH 7·0. In three-ligand systems involving citrate, (phosphate)Cd(citrate) predominates at pH values up to the pH of precipitation (4·2) but at pH 7·5 the Cd precipitate is the system Cd(cysteine)-(citrate), phosphate ion remaining almost entirely (free) in solution. NTA could lead to greater dispersion of toxic Cd ions in living tissues. A suggestion is made that (phosphate) M (citrate) complexes approaching roots of plants bind to ADP in the plasmalemma to form an ATP complex and that this is hydrolysed by an ATPase to deliver phosphate and metal nutrients to the cytoplasm.
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