Solution Equilibria of Binary and Ternary Complexes Involving Zinc(II) with 2,6-Diaminopyridine and Various Biologically Relevant Ligands

The complexing properties of 2,6-diaminopyridine (DAP) with zinc(II) were investigated pH-metrically at 25 °C and at ionic strength of 0.1 mol·dm−3 (NaNO3). Binary and ternary complexes of Zn(II) involving DAP and various biologically relevant ligands containing different functional groups are investigated. The ligands used (L) are amino acids, dicarboxylic acids, amides and DNA unit constituents. The ternary complexes are formed by simultaneous reactions. The results showed the formation of Zn(DAP)(L) complexes with amino acids and dicarboxylic acids. Amides form both Zn(DAP)(L) complexes and the corresponding deprotonated amide species Zn(DAP)(LH−1). The concentration distributions of the various complex species formed in solution were also evaluated as a function of pH. The effect of dioxane as a solvent on the protonation constant of DAP and the formation constants of Zn(II)–DAP complexes were discussed. The effect of ionic strength on the protonation constants of DAP is also evaluated.