pH Dependence of reactive sites of curcumin possessing antioxidant activity and free radical scavenging ability studied using the electrochemical and ESR techniques: Polyaniline used as a source of the free radical

Curcumin is an antioxidant; its antioxidant activity and free radical scavenging ability were investigated using two electrochemical methods and electron spin resonance (ESR) technique. Polyaniline was first used as a source of the free radical to determine free radical scavenging ability of curcumin. Only one oxidation peak at 0.55 V (vs. SCE) occurs on the cyclic voltammogram of curcumin on the glassy carbon (GC) electrode at pH 6.8, however two oxidation peaks at 0.44 and 0.71 V occur on the cyclic voltammogram of curcumin at pH 8.0, indicating that one or two sites of curcumin reactivity are present in curcumin, depending on pH. The oxidation peak at less positive potential on the cyclic voltammogram arises from the phenolic OH group confirmed with cyclic voltammogram of o-methoxyphenol. Curcumin exhibits antioxidant activity for polyaniline based on the open circuit potential measurement at both pH 6.8 and 8.0. The ESR measurements directly demonstrated that curcumin can scavenge free radical of polyaniline, the scavenging ability of curcumin is attributed to the phenolic OH group at pH 6.8, and to synergistic effect of the phenolic OH group and CH group in β-keto-enol at pH 8.0. Polyaniline is a new source of the free radical that can be used to determine accurately free radical scavenging activities of antioxidants.