Unravelling the effect of β-diketo group modification on the antioxidant mechanism of curcumin derivatives: A combined experimental and DFT approach

Abstract

Curcumin (CU), a natural polyphenol exhibits free radical scavenging activity by three main mechanisms namely, HAT (hydrogen atom transfer), SPLET (sequential proton loss electron transfer) and SET-PT (sequential electron transfer-proton-transfer). In this study, the influence of structural modification of curcumin to its isoxazole (CI) and pyrazole (CP) on their free radical scavenging mechanism has been investigated. For this, CI and CP were first screened for their free radical scavenging ability using different free radicals like DPPH• (1,1-diphenyl-2-picrylhydrazyl), ABTS•– (2,2-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid)), O2•– (superoxide), N3• (azide), CCl3O2• (trichloromethyl peroxyl), lipid peroxyl (LOO•) radicals, FRAP (Ferric reducing antioxidant power) assay and the results were compared with CU. The reaction mechanism was explained by complementing the experimental results with molecular descriptors obtained from quantum chemical calculations. The dependence of the scavenging activity on the acidity of the solvent indicated that CU and its derivatives reacted with DPPH•, ABTS•–, O2•–, LOO• radicals and ferric ion by SPLET mechanism, while they reacted with N3• and CCl3O2• radical by SET-PT mechanism. In case of free radicals neutralized by SPLET mechanism, the IC50 value of the polyphenols (the concentration of the polyphenol required to scavenge 50% radical) increased in the order CU < CI ≈ CP. However when they neutralized the free radicals by SET-PT mechanism, their reactivity increased in the order CP > CI > CU. Using DFT calculations, the pKa of the parent molecule and their corresponding one-electron oxidized radicals were calculated. In the case of parent molecule the pKa increased in the order CU < CI ≈ CP, however in the case of the one-electron oxidized radical the trend was reversed (CP < CI < CU), indicating that modification of diketo moiety of CU to isoxazole and pyrazole govern the mechanism under SET-PT. The intrinsic stability of the phenoxyl radical formed by the free radical scavenging of CU, CI and CP was expressed in terms of radical delocalization values (RDV), which showed that the phenoxyl radical of CP was more stable as compared to CI and CU. The lower pKa value of the radical of CP as compared to CU and CI indicated that modification of CU to CP tends to orient the reaction mechanism from SPLET to SET-PT. These studies revealed that substitution of the diketo moiety with the pyrazole moiety improved the peroxyl scavenging as well as the stability of the resulting radical, serving pyrazole moiety as a useful template for the rational design of novel antioxidants that are more effective than the curcumin.