INFLUENCE OF THE RATIO COPPER(II) TO LIGAND CONCENTRATIONS AND THE NATURE OF ENTERING AND LEAVING LIGANDS ON THE LABILITY OF COPPER(II) COMPLEXES

The influence of the ratio Cu:L on the lability of copper(II) complexe s with simple ligands of different thermodynamic stabilities (L=EDTA, NTA, cysteine, proline or glycine) was investigated in the range 4: 1- 1:3. Chelex-100 and chitin were used parallel and batch wise, to cause the dissociation of the complexes, and the results were compared. A f ew experiments were also performed with the yeast, Saccharomyces cerev isiae. The effect of the exposure time, between 5 and 30 min, and mass (or number of sites) of the particulate matter (PM), always in large excess, were also studied. For all systems, but Cu-cysteine with chiti n, the lability of Cut showed no marked dependence on the mass of PM: when the mass of PM was duplicated, an increase in lability by less th an or equal to 10% was found. For cysteine and proline with Chelex-100 , and cysteine and glycine with chitin, the lability of Cu-L complexes markedly increased with the time of exposure, which indicated that th e kinetics of the substitution reactions were relatively slow. These r esults indicate the role of both the leaving and entering ligands in t he kinetics of the ligand substitution reactions. The effect of Cu : L on the complex dissociation was more marked with chitin than with Che lex-100. When the leaving ligand is in excess (Cu:L<1) the magnitude o f that excess did not affect markedly the lability. But, when Cu : L>1 , the lability decreased with the increase of the excess of metal for cysteine or glycine with Chelex-100, and for all ligands in presence o f chitin. The heterogenous behaviour of the PM may be partially respon sible for these results. A maximum lability was observed for 1:1 ratio , particularly for the complexes that are thermodynamically more stabl e. In the presence of yeast cells, the tendencies mentioned above were confirmed. The present results reinforce the operational character of the speciation studies based on lability data, and that relationship between lability and bioavailability must be analysed for each individ ual case and should not be extrapolated to different PM or experimenta l conditions.