Histidine and Ornithine as Ligands Towards Zinc(II)

The equilibria occurring in the systems zinc(II)-histidine and zinc(II)-ormthine have been studied at 25°C in 1.00 M NaCl as constant ionic medium by measuring the electromotive force of galvanic cells containing zinc amalgam and glass electrodes. The experimental data for both systems were accounted for by assuming the existence of the species ZnL+, ZnL2, ZnHL2 +, ZnHL2 +, ZnH2L2 2+ and ZnH2L3+, where L indicates the uninegative ligand anion (histidine or ornithine). For all the proposed species the stability constants have been determined. The protonation constants of histidine and ornithine were determined under the same experimental conditions, by using a H2 electrode. The validity of the results is supported by the good agreement between experimental data and calculated values.