Kourosh Motevalli - Department of Applied Chemistry, Islamic Azad University, South Tehran Branch, Tehran, Iran
Zahra Yaghoubi - Faculty of Industrial Engineering, Islamic Azad University, South Tehran Branch, Tehran, Iran

In This letter, A special study has been done upon some toxic metal ion complexes. these kind of researches are useful for understanding the role played by the active site cavities in biological molecules and the bonding behavior of protein residues with the metal ions. The species refined and their relative concentrations under the experimental conditions represent the possible forms of aminoacids in biofluids. In this letter, An Equilibrium study upon the complex formation of L-histidine with As(III), 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 models containing different numbers of species were refined by using the computer program MINIQUAD75. The species distribution and the plausible equilibria for the formation of the species are also presented. The computer program SCPHD was used to calculate the correction factor. The following conclusions have been drawn from the modeling studies of the L-Histidine complexes of As(III), Cd(II) and Hg(II) in DMSO-water mixture:(1) L-Histidine forms both protonated and unprotonated complexes under pH range 2.0–9.0. The species detected for the toxic metals As(III) and Cd(II) are:ML(2) H(4) , ML(2) H(3) , ML(2) H(2) , ML(2) H, and ML(2) . ML(2) H(4) , ML(2) H(3) , ML(2) , and ML species are detected the case of Hg(II).(2) The linear variation of stability constants as a function of 1/D of the medium indicates the dominance of electrostatic forces over nonelectrostatic forces. The linear increasing trend with DMSO content supports the dominance of the structure forming nature of DMSO over its complexing ability.(3) The order of ingredients in affecting the magnitudes of stability constants due to incorporation of errors in their concentrations is alkali > acid > metal> ligand > volume > log F.(4) Some species are stabilized due to electrostatic interactions and some are destabilized due to the decreased dielectric constant.

L-Histidine Complexes; Toxic Metal Ions; stability constants; computer program