Oxidative stress, a result of an overproduction and accumulation of free radicals, is the leading cause of several degenerative diseases such as cancer, atherosclerosis, cardiovascular diseases, ageing and inflammatory diseases. Polyphenols form an important class of naturally occurring antioxidants, having innumerable biological activities such as anticancer, antifungal, antibacterial, antiviral, antiulcer and anticholesterol, to name a few. Among various polyphenols, gallic acid (3,4,5-trihydroxybenzoic acid), a naturally occurring low molecular weight triphenolic compound, has emerged as a strong antioxidant and an efficient apoptosis inducing agent. Starting from the bioavailability and the biosynthetic pathway of gallic acid, this review includes various in vitro, in vivo and in silico studies providing the mode of action, radical scavenging activity, ability to inhibit lipid peroxidation, maintenance of endogenous defense systems and metal ion chelation by this triphenolic molecule, along with a comprehensive overview of factors responsible for its high antioxidant activity. Gallic acid derivatives have also been found in a number of phytomedicines with diverse biological and pharmacological activities, including radical scavenging, interfering with the cell signaling pathways and apoptosis of cancer cells. The diverse range of applications of this simple polyphenol is due to a fine amalgam between its antioxidant and prooxidant potential. The existing literature on this dual behavior of gallic acid and its derivatives is reviewed here. This is followed by an account of their potential clinical and industrial applications.

Gallic acid: a versatile antioxidant with promising therapeutic and industrial applications

Phenolic antioxidants are ranked by reducing strength and characterized for reversibility using cyclic voltammetry at a glassy carbon electrode. Phenolics with an ortho-diphenol group show a first oxidation peak close to 400 mV (vs. Ag/AgCl) in a model wine solution (12% ethanol, 0.033 M tartaric acid, adjusted to pH 3.6), with a linear concentration dependence below 0.01 mM. Dilution of white wines 10×, and red wines 400×, gave first oxidation peak currents in the 1.5 to 2.2 μA range and 1.9 to 3.4 μC of charge passed by 500 mV, producing values for the concentrations of phenolic antioxidants with low oxidation potentials in the wines. Further peaks in the cyclic voltammograms of the diluted wines correspond to classes of phenolics with higher oxidation potentials, providing a qualitative assessment of wine phenolics based on reducing strength. Keywords: Cyclic voltammetry; wine; phenolics; antioxidants

A cyclic voltammetry method suitable for characterizing antioxidant properties of wine and wine phenolics.

Phenolic acid derivatives with potential anticancer properties--a structure-activity relationship study. Part 1: methyl, propyl and octyl esters of caffeic and gallic acids.

Antioxidant activity of Agaricus sp. mushrooms by chemical, biochemical and electrochemical assays

Graphene is usually synthesized through deoxygenation of graphene oxide (GO) by hydrazine as the most common and one of the strongest reducing agents. But, due to the high toxicity of hydrazine, it is not a promising reductant in large-scale production of graphene. Here, for the first time, we used melatonin (MLT), as a powerful antioxidant and a biocompatible competitor of hydrazine in reduction of GO suspension. X-ray photoelectron spectroscopy (XPS), current–voltage and optical characteristics of the sheets indicated that the deoxygenation efficiency of the GO suspensions by MLT and under heating in an alkaline condition for 3 h was comparable with the efficiency obtained by hydrazine in similar conditions for 30 min. Based on the XPS analysis, although the amount of nitrogen detected on the surface of the MLT-reduced GO sheets was higher than that of the hydrazine-reduced ones, smaller C–N bonds were formed on the surface of the former. The higher amount of nitrogen on the MLT-reduced GO sheets was assigned to the π–π adsorption of the oxidized MLT molecules on the reduced sheets, which resulted in more stability of the MLT-reduced GO suspension than the aggregation observed in the hydrazine-reduced one, as also confirmed by Raman spectroscopy. A mechanism describing the chemical reduction of the GO sheets by MLT was also proposed. Our results open up a promising outlook for substitution of hydrazine by a safe, biocompatible and powerful reductant for efficient deoxygenation of GO, especially in large-scale production and bio-applications.

Melatonin as a powerful bio-antioxidant for reduction of graphene oxide

Antioxidant activity of gallates: an electrochemical study in aqueous media

Lignin-based adsorbent materials for metal ion removal from wastewater: A review

Photocatalytic degradation of roxarsone by using synthesized ZnO nanoplates

Removal of boron from geothermal water by a novel boron selective resin

In the present work, polymer-enhanced ultrafiltration (PEUF) was used to study the purification of coloured wastewater. Methylene blue (MB) and methyl orange (MO) dyes were removed from aqueous solutions using water-soluble polymers: poly(2-acrylamide-2-methyl-1-propanesulfonic acid) (PAMPS) and poly(diallyldimethylammonium) chloride (PDDA), respectively. The PEUF technique has the advantage that it can be performed in homogeneous media to clean wastewater, where the novelty focuses on the water-soluble polymers used. Both polymers were first fractionated using an ultrafiltration membrane composed of regenerated cellulose with a molecular weight cut off (MWCO) of 10 kDa followed by subsequent freeze-drying. The removal of both dyes by ultrafiltration in the absence and presence of polymer was systematically studied. The performance of the polymers was studied under different parameters, such as the effect of the MWCO for the ultrafiltration membrane, the pH of the solution, the polymer:dye molar ratio, the initial dye concentration, the interference effect of NaCl and Na2SO4, and consecutive cycles of sorption-desorption. Under optimal conditions, a maximum removal efficiency of approximately 98% and 90% was achieved for MB and MO at pH 6.0 using initial MB and MO concentrations of 3.5 mg L-1 and 80 mg L-1, respectively, together with an ultrafiltration membrane with a MWCO of 10 KDa. The presence of interferents in the PEUF process was found to be not significant for dye elimination. Finally, the reusability of both polymers was demonstrated to be effective after four sorption-desorption cycles.

Paola Santander

Papers

Antioxidant activity of gallates: an electrochemical study in aqueous media

Lignin-based adsorbent materials for metal ion removal from wastewater: A review

Photocatalytic degradation of roxarsone by using synthesized ZnO nanoplates

Removal of boron from geothermal water by a novel boron selective resin

Polyelectrolytes applied to remove methylene blue and methyl orange dyes from water via polymer-enhanced ultrafiltration