Mixed ligand complexes of essential metal ions with L-glutamine and succinic acid in sodium dodecyl sulfate-water mixtures

Speciation of mixed ligand complexes of Co(II), Ni(II) and Cu(II) with L-glutamine and succinic acid was studied in varying amounts (0.0-2.5 %, w/v) of sodium dodecyl sulfate (SDS) in aqueous solutions while maintaining an ionic strength of 0.16 mol L-1 (NaCl) at 303.0 K. Titrations were performed in the presence of different relative concentrations (M:L:X = 1:2:2, 1:4:2 and 1:2:4) of metal (M) to L-glutamine (L) to succinic acid (X) using sodium hydroxide. The stability constants of the ternary complexes were refined with MINIQUAD75. The best-fit chemical models were selected based on statistical parameters and residual analysis. The species detected were ML2X, MLX, MLXH and MLXH2 for Co(II), Ni(II) and Cu(II). The increased stability of the ternary complexes compared to their binary complexes was believed to be due to electrostatic interactions of the side chains of the ligands, charge neutralization, chelate effect, stacking interactions and hydrogen bonding. The species distribution with pH at different compositions of SDS and plausible equilibria for the formation of the species are also presented.