Polarographic catalytic nickel prewave induced by some cysteine-containing dipeptides.
The catalytic nickel prewave produced by Cys-Gly, Cys-Tyr and Cys-Phe was studied in acetate buffer at pH 4-6.5. These peptides are more active than cysteine, especially the last two. The catalytic activity is depressed by decrease of pH owing to protonation of the catalyst. However, Cys-Phe and Cys-Tyr are still active at pH < 5, in contrast to Cys-Gly and Cys. This difference is accounted for by the interaction of the aromatic ring with the electrode surface at potentials more positive than the potential of zero-charge. This effect enhances the surface concentration of the catalyst in the case of Cys-Phe or Cys-Tyr. At pH < 5, a catalyst concentration 10(-5)M and large excess of nickel, the current is controlled by the rate of accumulation of catalyst in the adsorbed state. Under these conditions the reducible complex is a 1:1 Ni(2+)-catalyst chelate. The catalytic wave recorded at pH 4.8 allows the determination of Cys-Phe or Cys-Tyr (10(-6)-10(-4)M) in the presence of the constituent amino-acids.
Banica
F G
FG
Department of Analytical Chemistry, Bucharest Polytechnic, Spl. Independenţei 313, Bucharest, Romania.