Electrochemistry as an investigation tool in bioinorganic chemistry: from toxicology to the simulation of enzyme catalyzed reactions

This contribution presents the results of a polarographic investigation of the electrochemical processes in the presence of nickel(II) and penicillamine (Pen: R2C(SH)-CH(NH2)-COOH; R = CH3). Pen is one of the most efficient agent for nickel detoxification and this investigation shows that this is due to the outstanding kinetic stability of the [Ni(Pen)2]2- comples. This behaviour strongly contrast that of cysteine (Cys, R = H), despite of the minor difference in the thermodynamic stability of the corresponding complexes and allowed to suggest that kinetic rather that thermodynamic factors are deciding the detoxification efficiency of the ligand. On the other hand, it was possible to make a direct identification of the nickel complex which is responsible for the occurrence of the catalytic hydrogen evolution on the mercury electrode by a mechanism that simulates the enzymatic H+ reduction. This complex is a labile, monoligand species, with the same structure in the case of both Cys and Pen.