Equilibrium study on complex formation of L-histidine with Pb(II), Cd(II), and Hg(II) has been investigated pH metrically in DMSO-water mixtures (0–60% v/v) at 303？K and 0.16？mol？L？1 ionic strength. The predominant species detected for Pb(II) and Cd(II) are ML2H4, ML2H3, ML2H2, ML2H, and ML2 and those for Hg(II) are ML2H4, ML2H3, ML2, and ML. The appropriateness of experimental conditions is verified by introducing errors intentionally in the concentrations of ingredients. The models containing different numbers of species were refined by using the computer program MINIQUAD75. The best-fit chemical models were arrived at based on statistical parameters. The trend in variation of stability constants of the complexes with dielectric constant of the medium is attributed to the electrostatic and nonelectrostatic forces. The species distribution and the plausible equilibria for the formation of the species are also presented. 1. Introduction The speciation study of toxic metal ion complexes is useful to understand the role played by the active site cavities in biological molecules and the bonding behavior of protein residues with the metal ion. The species refined and their relative concentrations under the experimental conditions represent the possible forms of aminoacids in biofluids. Due to its numerous uses and high persistence, lead is a major environmental contaminant [1]. Lead is toxic even at low concentrations for living organisms, who can absorb it in various ways [2]. Lead intake by humans can be due to the consumption of crop plants grown on soils with high plant-available metal concentrations [3]. It can, however, migrate through the soil with dissolved organic matter [4] or mobilized by certain plants [5]. Moreover, carried from the air to the soils as fine particles, lead could be released more easily in soil solution [6]. Cadmium causes iron deficiency by binding to cysteine, glutamate, aspartate, and histidine ligands [7]. Cadmium inhibits enzymes that participate in bilirubine conjunction [8]. It increases urine Ca2+ excretion which can cause severe bone pathology [9]. The possible effects of long term low-level exposure to cadmium are of concern because it is readily distributed to tissues of liver and kidney, which are the main target organs in acute and chronic cadmium exposure [10, 11]. Other tissues involved in cadmium toxicity include the testis, heart, bone, eye, and brain [12]. Mercury is one of the most toxic elements and has negative health effects in human populations, highly dependent on fish consumption [13, 14]. Recent
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