Redox behavior, spectroscopic characterization and biological activities of some new robustly electro-active compounds

Abstract

The correlation of biological activities of compounds with their redox properties is the subject of extensive investigations of bioelectrochemists. Schiff bases, quinones and naphthalenes contain electroactive moieties and their broad range biological activities are closely related with the ability of these compounds to donate and/or accept electrons. We synthesized Schiff bases 1-((4-bromophenylimino) methyl) naphthalen- 2-ol (BPIMN) and 1-((2,4-dimethylphenylimino)methyl)naphthalen-2-ol (HL) and used HL as a ligand for the preparation of its metallic complexes. All the synthesized compounds were confirmed by 1 H NMR, 13 C NMR, FTIR, TGA and UV–Vis spectroscopy. Structures of Schiff bases were also characterized by X-ray analysis and the experimental findings were supported by quantum mechanical calculations. The results of BPIMN were compared with a structurally related Schiff base, 1-((4- chlorophenylimino) methyl) naphthalen-2-ol (CPIMN). The photometric and electrochemical fate of all these Schiff bases were investigated in a wide pH range and the obtained results helped in proposing the redox mechanistic pathways. The synthesized compounds were subjected to numerous biological applications and the results revealed that Schiff base HL and its metal complexes other than oxovanadium complex remarkably decrease the blood glucose, triglyceride and cholesterol levels. The metal complexes were found to exhibit significant inhibition against alkaline phosphatase enzyme as compared to Schiff bases. The zinc complex was found as the most potent inhibitor of bacteria/fungi while the vanadyl product displayed the least activity among all the metal coordinated products. Quinones, another biologically important class were also investigated due to their robust electrochemical properties and wide range of biological activities. The laxative and therapeutic activities of quinones are related to their redox characteristics. Electrochemically unexplored hydroxy substituted quinones including 4-hydroxy-5-methoxynaphthalene-1-ylacetate (HMNA), 1,4-dihydroxy-2-(3-hydroxy- 3-(trichloromethyl)pentyl)-8-methoxyanthracene-9,10-dione 2(hydroxymethyl)anthracene-9,10-dione anthracenedione acetate (DHDN) (HACAD), (HAC), (HCAQ), 1-hydroxy- 1,8-dihydroxy-4,5-dinitro 4,8-dihydroxy-9,10-dioxo-9,10-dihydroanthracen-1-yl 1,4,5-trihydroxyanthracene-9,10-dione (HAD) and 1,4,5- trihydroxy-2-methyl-3-(3-oxobutyl)anthracene-9,10-dione (HOAD) were selected and their redox behavior was studied in a wide pH range using modern electrochemical techniques. Kinetic parameters such as diffusion coefficient and iiiheterogeneous electron transfer rate constant and thermodynamic parameters of the electron transfer processes such as ∆G # , ∆H # and ∆S # were electrochemically evaluated. Their redox mechanisms were proposed on the basis of experimental findings supported by computational calculations. Moreover, a detailed UV–vis spectroscopy was carried out in a wide pH range for photometric characterization and acid-base dissociation constant, pK a determination. Though naphthalene by itself is toxic, however, some of its derivatives are bestowed with medicinal properties. Two biologically important naphthalene derivatives, naphthalene-2,3-dicarboxylic acid (NDA) and 1,8-dimethoxynaphthalene (DMN) were characterized by electrochemical techniques and screened for their antioxidant and anti-diabetic activities. NDA was found less toxic to HeLa cells and biological antioxidant studies revealed it as a more effective antioxidant as compared to DMN and standard antioxidant, ascorbic acid. Both NDA and DMN significantly increased the cholesterol level in blood but showed varied biological activities as regards to glucose and triglyceride concentrations. The cytotoxicity results evidenced DMN to significantly inhibit the cell proliferation in a dose dependent manner. Like the biological antioxidant studies, the electrochemical results also witnessed NDA as stronger antioxidant than DMN. pH dependent oxidation of NDA revealed its antioxidant role to be exerted both by the donation of electrons and protons. Although the oxidation potential of NDA is greater than the widely used natural antioxidant, ascorbic acid, yet it is capable of donating two electrons as compared to one electron donating ability of ascorbic acid. The redox mechanistic pathways proposed in this work are expected to provide useful insights about the unexplored mechanisms by which Schiff bases, quinones and naphthalenes exert their biochemical actions.