Metal Ion/Buffer Interactions

Abstract

The acidity constant of protonated 2‐[bis(2‐hydroxyethyl)amino]‐2(hydroxymethyl)‐1,3‐propanediol (Bistris) has been measured. The influence of hydroxo groups on the basicity of Bistris and related bases is discussed. The interaction of Bistris with the metal ions (M 2+ ) Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ , and Pb 2+ was studied by potentiometry and spectrophotometry in aqueous solution ( I = 1.0 M, KNO 3 ; 25°C) and the stability constants of the M(Bistris) 2+ complexes were determined. Unexpectedly Ca(Bistris) 2+ is the most stable among the alkaline earth ion complexes (log K ca ca(Bistris) = 2.25; the corresponding values for the Mg 2+ , Sr 2+ and Ba 2+ complexes are 0.34, 1.44 and 0.85, respectively). The ions of the 3d series follow the Irving‐Williams sequence: log K Mn Mn(Bistris) = 0.70, for Cu 2+ 5.27 and Zn 2+ 2.38. Ternary complexes containing ATP 4‐ as a second ligand were also investigated: the values for Δlog K M (= log K M(ATP) M(ATP)(Bistris) –log K M M(Bistris) ) are in general negative (e.g. Δlog K Ca = ‐0.40 or Δllog K Cu = ‐ 1.65), thus indicating that the interaction of Bistris with M(ATP) 2‐ is somewhat less pronounced than with M 2+ . However, even in mixed‐ligand systems, complex formation may still be considerable, hence great reservations should be exercised in employing Bistris as a buffer in systems containing metal ions. Moreover, in several cases Δlog K m is relatively high [for Mg 2+ ‐ATP 4‐ Bistris even positive], indicating some cooperativity between the coordinated ligands, possibly hydrogen‐bond formation. Distributions of the complexes in dependence on pH are given, and the structures of the binary M (Bistris) 2+ and the ternary M(ATP) (Bistris) 2‐ complexes are discussed. The participation of Bistris hydroxo groups in complex formation is evident.