Electroanalystical investigations of the nickel(II) - penicillamine system

The structural analogy of penicillamine (Pen, (CH3)2C(S-)CH(NH2)COO-)) with cysteine (Cys) may reveal some interesting feature related to the electrochemical behaviour of the second one as far as the electrode processis mostly governed by the coordination to a metal ion. In addition, interaction of Pen with Ni(II) is interesting from t e standpoint of nickel detoxification. Compared with Cys, Pen shows the advantage of forming only two complexes ([NiL] and [NiL2]2-) with well-known stability constants, whereas Cys shows a more complicated complexation scheme. Therefore, i is possible to make a direct correlation of the electrochemical behaviour with complex species distribution in the Ni2+-Pen system. This approach demonstrated that the [NiL2]2- species is kinetically inert for L = Pen, although the analogous Cy species is labile. Moreover, it was shown that the catalytic nickel prewave depends on the occurrence of the labile [NiL] species (for L = Pen) which is the actual electron acceptor in the catalytic nickel reduction. In the case of Cys, NiL2]2- can provide the reducible [NiL] form via the fast dissociation, in addition to the direct formation of [NiL] as achemical step which is parallel to the charge transfer reaction. The third electrode process investigated in this s stem is the catalytic hydrogen evolution in the range of the catalytic hydrogen prewave previously investigated in thecase of Cys), which occurs at pH around 7 with a half-wave potential of about -1.25 V vs. SCE. This process is differen and, actually, independent on the occurrence of the well-known Brdicka catalytic hydrogen wave.