CHEMIJA. 2009. Vol. 20. No. 3. P. 154–161 © Lietuvos mokslų akademija, 2009 © Lietuvos mokslų akademijos leidykla, 2009 Investigation of electrode kinetics and thermodynamics of [Zn–L-amino acidate–vitamin B 7 ] complexes by voltammetric technique Farid Khan*, Afroza Khanam Electrochemical Laboratory, Department of Chemistry, Dr. H. S. Gour University, Sagar, M. P., India * Corresponding author. E-mail: faridkhan58@yahoo.com; farid.f@redifmail.com Te voltammetric reduction of Zn 2+ using L-lysine, L-ornithine, L-threonine, L-serine, L- phenylglycine, L-phenylalanine, L-glutamic acid, L-aspartic acid and vitamin B 7 (biotin) at pH 7.30 ± 0.01 and I = 1.0 M NaClO 4 was reported at 25 and 35 °C. Te nature of the current voltage curves was quasireversible and difusion-controlled. Zn 2+ formed 1 : 1 : 1, 1 : 1 : 2 and 1 : 2 : 1 complexes with these drugs as confrmed by the Schaap and McMaster method. Te sequence of the stability constant of L-lysine < L-ornithine < L-threonine < L-serine < L-phenylglycine < L-phenylalanine < L-glutamic acid < L-aspartic acid complexes can be explained on the basis of the size, basicity and steric hindrance of ligands. Te values of the stability constant (log β) varied from 2.13 to 11.37 and confrm that these drugs, i. e. L-amino acids or in combination with vitamin B 7 or their complexes, could be used against Zn 2+ toxicity. Te thermodynamic parameters such as enthalpy (∆H), free energy (∆G) and entropy change (∆S), are also reported. Te kinetic parameters viz. the transfer coefcient (α), degree of irreversibility (λ), difusion coefcient (D) and standard rate constant (k) were calculated. Te values of α confrmed the symmetric nature of the ‘activated complex’ between oxidants and reductants in response to the applied potential between the drop- ping mercury electrode and solution interface. Key words: voltammetry, thermodynamic parameters, electrode kinetics, [Zn–L-amino acidate–vitamin B 7 ] complexes INTRODUCTION Most of L-amino acids are blood plasma ligands that form stable complexes with various essential metals in vivo [1] and play an important role in biology, pharmacy and industry [2–4]. Complexes of some metal ions with amino acids can be used as models to study the pharmaco-dynamic efects of drugs or for increasing the biocompatibility and minimizing the toxic efects of some metal ions [5]. On the other hand, L- amino acids are also involved in intracellular metabolism and operate specifc transport systems of the plasma membrane; they do not afect cardiac function under normal conditions [6, 7]. Te invention provides the use of zinc complexes of selected amino acids and other pharmacologically acceptable salts of zinc. Te use of the compound comprises adminis- tering an efective amount of said compounds for inhibiting the growth of the malarial parasite, plasmodium falciparum [8]. Vitamin B 7 (biotin or vitamin H) [9] is a member of the vitamin B complex and water-soluble. Biotin is involved in energy metabolism and plays a role in enabling the body to use glucose [10]. Biotin is sometimes chemically linked to a molecule or protein for biochemical assays [11] and earned a keen attention towards the studies of their metal complexes. Terefore, Zn complexes of these drugs are of great impor- tance. Te concentrations of zinc in vivo can be reduced by drug therapy, but the specifcity of the drug and its amount are stability-constant-dependent [12]. Terefore, the authors have undertaken the present study to determine the stability constants, thermodynamic and kinetic parameters of ternary complexes with these selected drugs for which no reference has been traced out so far in the literature. EXPERIMENTAL Instrumentation Te electrochemical experiment, i. e. a simple DC polaro- graphy, was carried out using a manual polarograph with