Mixed ligand complexes of toxic metal ions with L-glutamic acid and L-methionine in urea-water mixtures

Abstract

Speciation of mixed ligand complexes of Pb(II), Cd(II) and Hg(II) with L-glutamic acid and L-methionine was studied in varying amounts (0.0-36.83%>w/v) of urea in aqueous solutions maintaining an ionic strength of 0.16 mol dm -3 (NaNO3) at 303 K. Titrations were carried out in the presence of different relative concentrations (M : L : X = 1 : 2: 2, 1 : 2 : 4, 1 : 4: 2) of metal (M) to L-glutamic acid (L) to L-methionine (X) with sodium hydroxide. Stability constants of ternary complexes were refined with MINIQUAD75. The best-fit chemical models were selected based on statistical parameters and residual analysis. The species detected were MLXH for Pb(II), MLX and ML2X for Cd(II), and MLX and MLXH2 for Hg(II). Extra stability of ternary complexes compared to their binary complexes was believed to be due to electrostatic interactions of the side chains of ligands, charge neutralization, chelate effect, stacking interactions and hydrogen bonding. The species distribution with pH at different compositions of urea and plausible equilibria for the formation of species were also presented. The bioavailability of the toxic metal ions is explained based on the speciation.