Poly(L-cysteine) as an electrochemically modifiable ligand for trace metal chelation

The short-chain (n approximately equal to 50) homopolymer poly(L-cysteine) (PLC) has been previously studied for use as a novel metal chelator. PLC exhibits reversible oxidation-reduction chemistry involving the thiol groups of the cysteine (Cys) residues. Previously, chemical oxidation of the PLC immobilized on silica showed that metal binding capacity was minimal in the oxidized state. In this study, Cys and PLC are immobilized on a glassy carbon disk electrode (GCE) to study these redox processes and how they impact metal binding and release. Voltammetric and chronoamperometric methods were employed to demonstrate nearly monolayer coverage of both immobilized Cys monomer and immobilized PLC on GCE. The PLC-GCE exhibited a maximum metal binding capacity for Cd2+ of approximately 11 Cd2+ ions/chain. No detectable metal binding capacity was observed for oxidized PLC. The bound metals were capable of being efficiently released through disulfide bond formation and tertiary structure changes by means of repetitive oxidative pulses. The Cys-modified electrode exhibited a metal binding capacity for Cd2+ of approximately 1 Cd2+/Cys. Oxidized Cys did retain a significant capacity following oxidation, likely as a result of complexation with the terminal carboxylate site and unoxidized thiols. A glycine (Gly)-modified electrode was also evaluated as an amino acid control. Minimal Cd2+ binding was observed. Further metal binding studies were conducted using PLC-GCE with single metal solutions of Co2+, Cu2+, Ni2+, and Pb2+, as well as a multimetal solution composed of equal concentrations of all five target metals. The observed metal binding trend was as follows: Cu2+ > Cd2+ > Ni2+ > Pb2+ > Co2+. All metals were quantitatively released upon oxidation of PLC using the same anodic potential, 600 mV vs Ag/AgCl.