pH-dependent redox mechanism and evaluation of kinetic and thermodynamic parameters of a novel anthraquinone

Abstract

The redox behavior of 1,4-dihydroxy-2-(3-hydroxy-3-(trichloromethyl)pentyl)-8-methoxyanthracene-9,10-dione (HCAQ) was investigated at a glassy carbon electrode over a wide pH range of 3–12 by using cyclic, square wave and differential pulse voltammetry (CV, SWV and DPV). CV results of HCAQ obtained at different temperatures were used for the evaluation of thermodynamic parameters like ΔG^#, ΔH^# and ΔS^#. The values of diffusion coefficient and heterogeneous electron transfer rate constant were also determined by CV. Limits of detection and quantification were determined by SWV. The numbers of electrons and protons involved in the oxidation processes were evaluated by DPV. The value of apparent acid dissociation constant (pK_a) was obtained from the intersection of two linear segments of the E_p vs. pH plot. The effect of pH on the UV-visible spectral response was also monitored which allowed the determination of pK_a of HCAQ. The values of apparent acid dissociation constant obtained from electrochemical techniques and electronic absorption spectroscopy were found to be in very good agreement. On the basis of experimental results, a pH-dependent oxidation mechanism was proposed in order to provide useful insights into the unexplored pathways by which anthraquinones exert their biochemical action.