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%wyv) of urea in aqueous solutions maintaining an ionic strength of 0.16 mol dm À 3 (NaNO 3 ) at 303 K. Titrations were carried out in the presence of different relative concentrations (M : L : X ¼ 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 ML 2 X for Cd(II), and MLX and MLXH 2 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.

Electrometric Investigation of the Nature and Stability of Mixed Ligand Complexes of L-Ornithine and 1,10-Phenanthroline With Some Essential Metal Ions in Aqua-TBAB or PEG-400 Surfactant

“…2 and 3, respectively. The two different ligands have mutually the same degree of in uence in the formation of mixed ligand complex [14,15,34,43].…”

“…Mononuclear binary complexes involving interaction of single metal with one ligand and mixed ligand complexes that contain more than one ligand have been studied for long time [9,10]. The stabilities of the synthesized complexes are a direct function of many factors such as: dielectric constants [11][12][13][14][15][16], stacking interactions between ligands, and metal-ligands [13][14][15][16][17][18], and electrostatic and non-electrostatic interactions [7,9,[11][12][13][14][15][16]. Literature reports indicated that most of these factors were mainly dependent on the composition of organic solvents, which could alter the dielectric constant of the medium and solvating power of the solvents [12,15].…”

Cellular and signaling mechanisms supporting cadmium tolerance in salicylic acid treated seedlings

“…The most frequent compounds involved in Cd tolerance are metalbinding peptides and phytochelatins (PCs). The former are cysteine-rich, found mostly in plants, capable of neutralizing toxic Cd ions and contain glutamic acid in a δ-bond, allowing them to form thiolate bonds with Cd ions (Rao et al, 2006) and thus protect plant cells from damage. On the other hand, PCs are important not only due to their metal-binding capacity, but also because they transport the bound metal ions from the cytoplasm into the vacuoles, where they can be stored as nontoxic form, bound to organic acid ligands (Brunetti et al, 2011;.…”