METAL ION-BUFFER INTERACTIONS - STABILITY OF BINARY AND TERNARY COMPLEXES CONTAINING 2-AMINO-2(HYDROXYMETHYL)-1,3-PROPANEDIOL (TRIS) AND ADENOSINE 5'-TRIPHOSPHATE (ATP)

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 Km, where Δlog Km= log KM(ATP)TrisM(ATP)− log KM(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. Copyright © 1979, Wiley Blackwell. All rights reserved