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) complexes 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 few experiments were also performed with the yeast, Saccharomyces cerevisiae. 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 Cucysteine with chitin, the lability of CuL showed no marked dependence on the mass of PM: when the mass of PM was duplicated, an increase in lability by ≤ 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 the kinetics of the substitution reactions were relatively slow. These results indicate the role of both the leaving and entering ligands in the kinetics of the ligand substitution reactions. The effect of Cu : L on the complex dissociation was more marked with chitin than with Chelex-100. When the leaving ligand is in excess (Cu : L < 1) the magnitude of 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 of chitin. The heterogenous behaviour of the PM may be partially responsible for these results. A maximum lability was observed for 1:1 ratio, particularly for the complexes that are thermodynamically more stable. 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 individual case and should not be extrapolated to different PM or experimental conditions.