Comparison of immobilized poly-l-aspartic acid and poly-l-glutamic acid for chelation of metal cations Original Research Article

Poly-l-aspartic acid (PLAsp) and poly-l-glutamic acid (PLGlu) were individually immobilized onto controlled pore glass (CPG) and compared according to their metal-binding capabilities in a solution of pH 7.0. The metal-binding capacities were calculated through the analysis of breakthrough curves generated by monitoring the metal concentrations on a flow injection–flame atomic absorption system. Capacities for individual metals were comparable and in the order of Cu2+ ⪢ Pb2+ > Ni2+ ≈ Cd2+ > Co2+ > Mn2+ ⪢ Na+. Elemental combustion analysis yielded polymer coverage on the CPG of approximately 4 × 1012 to 5 × 1012 chains/cm2, when average chain lengths were used in the calculations. Formation constants and site capacities of both polymers for Cd2+ were determined through equilibrium and breakthrough studies. The maximum log K values for the strong sites were determined to be ∼13 for both PLAsp and for PLGlu. Additionally, the metal selectivity of PLAsp and PLGlu was evaluated when breakthrough curves were run with several metals present in solution at one time. Both polymers showed selectivities in the order of their single metal-binding capacities, i.e., Cu2+ > Pb2+ > Ni2+ ≈ Cd2+. Both polymers exhibited similar binding trends and binding strengths for all of the metals studied. This likely reflects the absence of a predetermined tertiary structure of the polymers on the surface and the relatively high residue-per-metal ratio (∼20:1), which places less stringent requirements on the steric hindrance between the side chains and the resultant “wrapping” of the peptide around the metal.