Metal complexes of taurine. The first reported solution equilibrium studies for complex formation by taurine at physiological pH; the copper(II)–glycylglycinate–taurine and the copper(II)–glycylaspartate–taurine systems

The first solution studies at physiological pH for the formation of metal complexes of taurine, +NH3CH2CH2SO3−, one of the most abundant low molecular weight organic compounds in the animal kingdom, are reported. The complexes Cu(Gly-GlyH−1) (1) and [Cu(Gly-AspH−1)]− (2) react with taurine to give the ternary complexes [Cu(Gly-GlyH−1)taurine]− (3) (log K=2.95±0.03, I=0.2 M, T=25.0°C) and [Cu(Gly-AspH−1)taurine]2− (4) (log K=2.68±0.02) in which taurine acts as an N-donor ligand, most likely monodentate, without involvement of the sulphonate group in coordination. The results of the pH-metric studies are confirmed by visible and EPR spectrophotometric studies. The taurine complexes are less stable than the analogous complexes of β-alanine due to the decreased basicity of the amino group in the former ligand, and in the case of the Cu(Gly-GlyH−1) complexes due to involvement of the carboxylate group of β-alanine in axial coordination.