Stability and acidity constants for ternary ligand-zinc-hydroxo complexes of tetradentate tripodal ligands

Four series of tetradentate tripodal ligands containing pyridyl, 2-imidazolyl, 4-imidazolyl, amino, and/or carboxylic groups were synthesized as hydrolytic zinc enzyme models in order to elucidate the effect of various coordination environments on zinc binding and the acidity of zinc-bound water. In aqueous solution, ligands with same charges showed a good correlation between zinc binding (log K_ZnL) and zinc-bound water acidity (pK _a of LZnOH₂); the stronger the zinc binding, the higher the pK_a. The zinc-bound water acidity decreased as pyridyl groups were replaced by carboxylate groups. However, exchanging amino groups for carboxylate groups gave no change in zinc-bound water acidity regardless of the charge of the atoms in the inner coordination sphere of the metal ion. The results are consistent with the conventional notion that negatively charged carboxylate groups inherently increase zinc binding and result in decreasing zinc-bound water acidity, but also suggest that environmental effects may modulate or dominate control of acidity.