Binding of copper and zinc to three cyanobacterial microcystins quantified by differential pulse polarography

Microcystins are a family of potently hepatotoxic heptapeptides which have been detected in a wide range of cyanobacteria. The novel application of polarography in the analysis of these cyclic peptides has demonstrated binding of copper and zinc to microcystins in the aqueous-phase at environmentally-relevant pH values. This electroanalytical technique measured the shift in reduction potential and decrease in height of metal polarogram peaks as a function of increasing microcystin concentration. The successive formation of intermediate complexes was investigated and the strength of coordination was evaluated in terms of a formation constant (K₁) for each metal/microcystin combination. The effects of a single amino acid substitution on the formation constants of the heptapeptide was investigated with microcystin-LR, microcystin-LW and microcystin-LF. The calculated formation constants (log K₁) indicated that all three microcystins are intermediate strength metal ligands and that substitution of arginine with either tryptophan (microcystin-LW) or phenylalanine (microcystin-LF) appeared to have no discernible effect on the overall binding capacity. Reducing the pH of the supporting electrolyte strongly influenced complexation of microcystin-LR with copper, indicating that at pH 5.5 only a weak labile complex was formed. Complexation between microcystin-LR and zinc, however, was not influenced as strongly by pH.