Electrochemical Characterization Of Biologically Important Electroactive Metal Ligand Complexes With Multi-electron Transfer Reactio

Abstract

The redox behavior of Cu (II) only and Cu (II) with aspartic acid (Apa), L-phenl alanine (Phe), 3-nitrobenzene sulfnate (NBS) ; Zn(II) only and Zn(ll) with Apa, Phe, NBS and Catechol ((ate) has been studied Cyclic voltammetry (CV), Differential pulse voltammetry (DPV) and Chronamperometry (CA) techniques. The voltammetri technique demonstrates the interaction of biologically important metals (Cu, Zn) with the ligands (Apa, Phe, NBS, Cate) that is formed of metal-ligand complexes. The interaction studies have been carried out in variation of metal ion concentration, ligand concentration, Buffer solution of different pH and scan rate. In all the studies of Cu(II) and Zn(II) complexes, with some exception, Cu(II )-ligand systems and Zn(II)-ligand systems have been found to undergo quasireversible electrode reaction with EC mechanism. For all the ligands, Apa and Phe are electroinactive ligands whereas Cate and NBS are electroactive ligands. Both the anodic and cathodic voltamnietric peaks were shifted and sometimes developed new peaks with the addition of Apa, Phe, NBS, Cate in Cu(II) and Zn(II) solutions. On the addition of Apa, Phe and NBS in Cu(II) solution, the peak positions of the voltammogram of Cu(II)-Apa, Cu(II)-Phe and Cu(II)-NBS, first anodic peak was shifted positively but the second oxidation peak was shifted negatively with respect to that of only Cu ( II). The peak current decreases significantly compared with that for free Cu(II) in the same experimental conditions, This behavior confirms the interaction and complexation between Cu(II) and Apa, Phe. The effect of pH of Cu(II)-Apa, Cu(II)-Phe and Cu(II)-NBS were studied by varying pH from 3.5 to 7.0. The peak current of Cu(II)-Apa, Cu (II)-Phe and Cu(II)-NBS increases with the decrease of pH indicating that at lower pH the Cu(II)-Apa, Cu(II)-Phe and Cu(II)-NBS is highly electroactive. The maximum peak current was obtained at pH 3.5. This shows that the electrochemical oxidation of Cu(II)-Apa, Cu(II)-Phe and Cu(II)-NBS is aicilitated in acid media and hence the rate of electron transfer is faster.The average diffusion coefficient, D of Cu(II)- Apa or Cu(II)-Phe is found to he 4.5 x l0-6cm2s-1. The cyclic voltammogram of Zn(II) only and Zn(II) with Apa, Phe, NBS and Cate in aqueous and in buffer solution of different pH were taken at different scan rates. The CV of Zn (II) shows at pH (1 .5, 3 and 4.5) one well defined anodic and cathodic peak at different scan rate hut in aqueous solution only Zn(II) showed very weak anodic peak. Upon the addition of Apa, Phe, NBS, Cate with Zn(II) the first anodic peak shifted positively and the cathodic peak is shifted negatively which indicates the formation of Zn (II)-Apa or Zn ( II)- Phe complex. The intensity of the anodic and cathodic peak current decreases with the increasing of Apa or Phe or NBS or Cate suggesting that is formed of more Zn( 11 )-Apa or Zn(II)- Phe or n(ll)- NBS complex. The effect of pH of Zn(II)-Apa, Zn(II)-Phe, Zn(II)-N BS and Zn(II)-Cate were studied by varying pH from 1.5 to 11. The peak current decreases with the increase of pH .The maximum peak current was obtained at pH 3-4.5.At higher pH(7-11), the anodic peak disappeared. This shos that the electrochemical oxidation of Zn(II) complexes is hindered in basic media. For the comparison CV of Zn(II) only and Zn( II) —Asp, Zn(II)-Phe, Zn( II)- Cate, Zn( II)-NBS at similar condition (pH 3, scan rate 0.1 V/s). it is seen that the anodic peak current of Zn(II)-Asp and Zn(II)-Phe are lower than that of Zn(II) only hereas the anodic peak current of Zn(II) - Cate and Zn(II)-NBS is higher than that of Zn(II) only. When Cate and NBS are coordinated with Zn(II) both metal and ligands are electroactive, So constructive interference of peak appeared whereas Asp and Phe are coordinated with Zn(II), here ligands are eleciroinactive so the overall peak currents are lower than Zn(II) only. The slopes of the plots of Ep against pH of all studied metal ligand complexes ere determined graphically as anodic peak 25- 35 mV/pH at 0.1 V/s, which is close to the theoretical value of 30 mV for a two-electron, two-proton transfer process which indicates that the oxidation of all metal-ligand complexes proceeded via the 2e-2H+ processes. This suggests that during the reaction not only electron but also protons are released from the metal-ligand complexes. For all the studied system, with the increasing of metal: ligand composition from 1:1 to 1:5 (metal fixed), the current decreases linearly but after more addition of ligands, the current intensity change is constant. This indicates that the availability of metal is limited for the formation of the metal-ligand complex at saturation point. When the composition of metal increases with the fixed of the composition of ligands from 1:1 to 5:1. after certain concentration, the voltammogram pattern reflected into the only metal, because of the deficiency, of ligands in solution. The relation between metal - ligand concentration and cyclic voltammetric anodic and cathodic peak current (Ipaand Ipc) is linear. The proportionality of the anodic and cathodic peak currents with square root of scan rate of all the studied metal ligand complexes with few exceptions suggests that the peak current of the difièrent complexes at each redox reaction is controlled by diffusion process. From the studied of all systems it is seen that current functions (1/v-1/2) decreased with the increasing of' scan rate except Zn(II)- Cate. So the behavior of electrode reaction of all Cu(II)- ligands and Zn(II)- ligands are of ECE mechanism except Zn(II)- Cate is of CE mechanism.