Metal Ion/Buffer Interactions

Abstract

The interaction of 2-amino-2(hydroxymethyl)-1,3-propanediol (Tris) with the metal ions (M²⁺) Mg²⁺, Ca²⁺, Ba²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺ was studied by potentiometry and spectrophotometry in aqueous solution (I= 0.1 or 1.0 M, KNO₃, 25°C). Stability constants of the M(Tris)²⁺ complexes were determined; those constants which were measured by both methods agreed well. Ternary complexes containing ATP⁴⁻ as a second ligand were also investigated and it is shown that in the presence of Tris, mixed-ligand complexes of the type M(ATP)(Tris)²⁻ are formed. The values for Δlog K_m, where Δlog K_m= log K_M(ATP)Tris^M(ATP)− log K_M(Tris)'^M are all negative, thus indicating that the interaction of Tris with M(ATP)^2minus; is somewhat less pronounced than with M²⁺. However, it should be noted that even in mixed-ligand system complex formation with Tris may still be considerable, hence great reservations should be exercised in employing Tris as a buffer in systems which also contain metal ions. Distributions of the complex species in dependence on pH are shown for several systems, and the structures of the binary M(Tris)^2minus; and the ternary M(ATP)(Tris)²⁻ complexes are discussed. The participation of a Trishydroxo group in complex formation is, at least for the M(Tris)²⁻ species, quite evident.