Investigation of the Electrochemical Behavior of Catechol in Presence of Glycine, Aspartic acid and Glutamic acid

Abstract

To monitor the electrochemical behavior of Catechol in presence of Glycine, L-Aspartic acid and L-Glutamic acid; cyclic voltammetry (CV), differential pulse voltammetry (DPV), controlled potential coulometry (CPC) and chronoamperometry (CA) techniques have been employed. A wide range of concentration (10mM to 150mM) of nucleophiles (Glycine, L-Aspartic acid and L-Glutamic acid), various pH media (5 to11), different electrodes ((Glassy carbon (GC), Gold (Au) and Platinum (P)) and successive scan rate (0.05 V/s to 0.5 V/s) have been used to find out the favorable condition. Catechol is an electroactive substance which shows a pair of redox peak whereas Glycine, L-Aspartic acid and L-Glutamic acid are electroinactive, hence it shows no anodic and cathodic peak in the potential range (-0.6 to 0.9 V) of investigation. By the addition of above nucleophiles in Catechol solution arises a new peak at lower potential, the corresponding redox peak shifts and peak current intensity of Catechol decreases with respect to the pure Catechol that indicates the participation of 1,4-Michael addition reaction of o-benzoquinone with mentioned nucleophiles. The formation of adducts from the reaction of Catechol with Glycine, L-Aspartic acid and L-Glutamic acid are assumed to be 2-((3,4-dihydroxyphenyl)amino)acetic acid, 2-((3,4-dihydroxy phenyl)amino)succinic acid and 2-((3,4-dihydroxyphenyl)amino)pentanedioic acid severally that go through electron transfer at more negative potentials than the Catechol. The influence of pH of Catechol in presence of Glycine, L-Aspartic acid and L-Glutamic acid has been studied by varying pH from 5 to 11. It is seen that at pH 3, 5, 9 and 11 no new anodic peak appears after repetitive cycling. In the neutral media (pH 7), the o-benzoquinone undergoes nucleophilic attack by the above nucleophiles that voltammetric new anodic peak A0 appears after repetitive cycling. The slopes of the peak potential was determined graphically from Ep vs pH plot as the anodic peaks of Catecholglycine adducts (27 mV/pH) proceeded via one step 2e−/2H+ process, Catechol-aspartic acid adducts (50 mV/pH) and Catechol-glutamic acid adducts (69 mV/pH) via two step 1e−/1H+ process.

The reaction is strongly influenced by the different pH media as well as various compositions of nucleophiles. These reactions have been carried out wide range of concentration of nucleophiles. The optimum conditions for the formation of adducts such as Catechol-glycine (pH-7, Concentration 70 mM, GC electrode and Scan rate 0.1 V/s), Catechol-aspartic acid (pH-7, Concentration 70 mM, GC electrode and Scan rate 0.1 V/s) and Catechol-glutamic acid (pH-7, Concentration 30 mM, GC electrode and Scan rate 0.1 V/s) system are observed. The effect of scan rates has been investigated on cyclic voltammogram of Catechol in presence of above nucleophile. The anodic and cathodic peak current increases proportionally with increasing square root of scan rates. This linear relationship indicates the reaction is controlled by diffusion process. The current function, Ip/v1/2 decreases exponentially with increases scan rate which determine the reaction is controlled by ECE mechanism. The electro-synthesized adducts generated from the controlled potential coulometry of Catechol with Glycine, L-Aspartic acid and L-Glutamic acid was isolated and the generated adducts supported by FTIR spectra.