Metal Ion/Buffer Interactions

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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 (M2+) Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+ was studied by potentiometry and spectrophotometry in aqueous solution (I= 1.0 M, KNO3; 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 Kcaca(Bistris)= 2.25; the corresponding values for the Mg2+, Sr2+ and Ba2+ complexes are 0.34, 1.44 and 0.85, respectively). The ions of the 3d series follow the Irving‐Williams sequence: log KMnMn(Bistris)= 0.70, for Cu2+ 5.27 and Zn2+ 2.38. Ternary complexes containing ATP4‐ as a second ligand were also investigated: the values for Δlog KM (= log KM(ATP)M(ATP)(Bistris)–log KMM(Bistris)) are in general negative (e.g. Δlog KCa= ‐0.40 or Δllog KCu= ‐ 1.65), thus indicating that the interaction of Bistris with M(ATP)2‐ is somewhat less pronounced than with M2+. 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 Km is relatively high [for Mg2+‐ATP4‐ 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.

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