pH Dependent Electrochemical Characterization, Computational Studies and Evaluation of Thermodynamic, Kinetic and Analytical Parameters of Two Phenazines

Abstract

The electrochemical response of two biologically important phenazine derivatives was examined by using cyclic, differential pulse and square wave voltammetry in a wide pH range of 2.0 -12.0. Cyclic voltammetry was employed for the determination of heterogeneous electron transfer rate constant and diffusion coefficient of the analytes. The value of electron transfer rate constant was used for evaluation of Gibbs free energy, enthalpy and entropy changes of redox reactions of the compounds. Differential pulse voltammetry was used for the determination of number of protons and electrons involved in the redox processes. Limits of detection and quantification were assessed by square wave voltammetry. The detailed pH dependent redox mechanisms of the compounds were proposed on the basis of experimental results which were further supported by theoretical calculations. These redox mechanistic pathways are expected to play a key role in understanding the necessary aspects of structure-activity relationships and exploring the hidden routes by which this class of compounds exert its biochemical actions. Phenazines have broad range applications in biology and chemistry.1 This class constitutes a large group of nitrogen containing heterocyclic compounds, the members of which differ in their chemical and physical properties on the basis of type and position of functional groups present. Naturally, phenazines are produced by limited genera of bacteria including Pseudomonas, Burkholderia and Brevibacterium.2 Starting from different parent materials, phenazines can be synthesized by various reactions including oxidative condensation, electrochemical reduction, reductive cyclization and photochemical reactions.From the last 50 years, phenazines are the subject of extensive research investigations because of their broad range applications in pharmaceutical and clinical research.3 Such compounds have been reported for their wide variety of biological activities such as antitumor, anti-parasitic and antibiotic activities. Benzo [a] phenazine acts as efficient DNA intercalating ligand with antitumor activity in leukaemia and solid tumors. 4 1-Hydroxyphenazine has been reported to have promising antifungal activity against various microbes such as aspergillus fumigatus and candida albicans.5 Macrolactone synthesized from benzo [a] phenazine shows good activity against mycobacterium tuberculosis. 6 In general, natural and synthetic phenazines find useful applications against malaria 7 and hepatitis C viral replication. 8The biological activity of phenazines is due to their redox reactions leading to the formation of toxic radicals. 9 Different functional groups attached to phenazine determine their redox potential and solubility. The redox behavior and concomitant modulation of biological activities of phenazines by changing the nature and position of substituents attached to their aromatic rings have been reported by several research groups.10-12 Phenazines can alter cellular redox states and act as transporters of electrons to altern...

pH- and temperature-responsive redox behavior of hydroxyanthracenediones

“…Oxidation becomes more facile at higher pHs, as indicated by a shift of E p towards more negative potentials. This suggests a stabilization of oxidized species by hydroxyl ions present in the solution [29]. The DPVs of HACAD recorded in the potential range of 0 to +1.5 V show three independent signals in acidic media and two peaks in neutral and under alkaline conditions.…”

“…2B were used for the determination of protons accompanying electron transfer processes. The slope of plots of E p versus pH of a*, 1a and 2a shows the involvement of a single proton transfer during the electron transfer reactions [29]. The presence of a third peak under strongly acidic conditions in case of HAD (see Fig.…”

“…As electrochemistry mainly focuses on the kinetics of the reactions at the electrode surface [29], therefore, the values of different kinetic parameters such as diffusion coefficient (D) and heterogeneous electron transfer rate constant (k sh ) were obtained from CV by monitoring the variation of the peak current with the scan rate. The Randles-Sevcik equation relates the magnitude of the peak current (I p ) to the temperature (T), the concentration (C), the electrode area (A), the number of electrons involved in the oxidation or in the reduction (n), the diffusion coefficient (D), and the scan rate [29,30].…”

“…The Randles-Sevcik equation relates the magnitude of the peak current (I p ) to the temperature (T), the concentration (C), the electrode area (A), the number of electrons involved in the oxidation or in the reduction (n), the diffusion coefficient (D), and the scan rate [29,30]. The temperature effects of the scan rate on the redox properties of the three anthracenediones were monitored at pH 7.0 at different temperatures.…”

“…An examination of Table 1 reflects that the diffusion coefficients are affected by molecular mass and temperature. The values of D decrease with increasing the molecular mass because a heavier molecule of larger radius moves slowly towards the electrode according to the StokesEinstein equation [29]. As temperature increases, the solvent viscosity decreases, which in turn increases the mobility of the analytes.…”