Syntheses, characterizations and antioxidant activities of copper complexes of quercetin as influenced by redox states

Abstract

Copper is an essential element for living but imbalance between Cu⁺ and Cu2⁺ produces free radicals. Quercetin is an effective metal chelator and its chelates are often better free radical scavengers than free quercetin. In this study, the syntheses, characterizations, antioxidant activities of quercetin as well as its Cu (I) and Cu (II) complexes using UV-VIS, IR spectrometric methods, and 2,2-diphenyl 1-picrylhydrazyl (DPPH) scavenging assay were investigated. 1:1 and 2:1 stoichiometric ratios were obtained for the dark-brown Cu⁺ and green Cu2⁺ quercetin complexes respectively. Bathochromic shifts from 268 nm (band II) and 362 nm (band I) in quercetin were observed at 302 nm and 386 nm for Cu (I) ^–quercetin complex and at 281 nm and 394 nm for Cu (II) ^- quercetin complex. FT-IR showed shifts in the positions of carbonyl and hydroxyl groups with presence of bands at 932.01 cm^-1 in Cu (I) ^– quercetin and 928.57 cm^-1 in Cu (II) ^– quercetin confirming complexation. Both complexes were better free radical scavengers than free quercetin with Cu (I)^-quercetin complex having the highest antioxidant activity. Higher antioxidant activity and differences in spectra characters of Cu (I) quercetin complex are related to relative stability of fully filled orbital of Cu⁺ over Cu2⁺.
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Keywords: Oxidation state, relative stability, DPPH, Stoichiometric ratio, complexation