Complexation equilibria of Zn(II), Pb(II) and Cd(II) with reduced Glutathione (GSH) and biologically important zwitterionic buffers

The interaction of zinc(II), lead(II), and cadmium(II) with Glutathione (5-L-glutamyl-Lcysteinylglycine) as primary ligand and zwitterionic buffers (N-[2-Hydroxyethyl]piperazine-N'-[2-ethanesulfonic acid]) (HEPES) and (N-Hydroxyethyl]piperazine-N'-[2-hydroxy-propanesulfonic acid]) (HEPPSO) as secondary ligands were studied by potentiometric-pH titration in 1: 1: 1 ratio at 25.0 degrees C and I = 0.1 mol.dm(-3) (KNO3). The formation constants of different normal and protonated binary and ternary complex species were calculated. Formation constants for-the monohydroxy, and dihydroxy complexes for the binary systems M(II) + HEPES and M(II) + HEPPSO have been evaluated. The distribution curves for the various complex species as a function of pH were constructed.