Binary and ternary complexes of Cu(II) ions with saccharin and cysteine

The square-wave voltammetric behaviour of cysteine and saccharin was studie d at a static mercury drop electrode at pH 7.4 in the presence of Cu(II) io ns. In the presence of excess Cu(II), cysteine exhibited three reduction pe aks for Hg( SR), ( - 0.086 V), free Cu(II) ( - 0.190 V) and Cu(I)SR ( - 0.6 98 V), respectively. Saccharin produced a catalytic hydrogen peak at - 1.76 2 V. In the presence of Cu(II),saccharin gave a new peak ( - 0.508 V), corr esponding to the reduction of Cu(II)-saccharinate, which in the presence of cysteine formed a mixed ligand complex ( - 0.612 V), CuL(2)A(2) (L = sacch arin and A = cysteine) The peak potentials and currents of the obtained com plexes were dependent on the ligand concentration and accumulation time. Th e stoichiometries and overall stability constants of these complexes were d etermined by Lingane's method (voltammetrically) and job's method (spectrop hotometrically). The mixed Ligand complex in the molar ratio 1:2:2 (log bet a = 33.35) turned out to be very much stronger than the 1:1 Cu(I)SR (log be ta = 21.64) and 1:2 Cu(II)-saccharinate (log beta = 16.68) complexes. Forma tion of a mixed ligand complex can be considered as a type of synergism. (C ) 2000 Elsevier Science S.A. All rights reserved.