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Abstract
Curcumin has been believed to have effective medicinal properties such as anti-cancer, anti-Alzheimer, anti-inflammatory, and antioxidant…, in which the free radical scavenging activities play a crucial role in its treating mechanisms. Although the antioxidant properties of curcumin and its derivatives have been widely studied in the literature, a systematical investigation of the thermodynamics and kinetics of the reaction towards hydroperoxide (HOO•), the standardized free radicals, has still been lacking. This work investigated the HOO• radical scavenging activities of two curcumin derivatives, namely curcumin I (Cur-I) and curcumin III (Cur-III), in water and pentyl ethanoate (PEA) solutions using Density functional theory (DFT) approaches. The antioxidant properties of the neutral and anionic forms of two tautomers, including keto-enol and diketone of curcumin, were investigated via three common mechanisms, i.e., hydrogen transfer (HT), radical adduct formation (RAF) and single electron transfer (SET). Intrinsic parameters, thermochemical parameters, and kinetics of the curcumin-HOO radical reactions were systematically characterized. As a result, the overall rate constant for the reaction in the water of Cur-I (9.36 107 M-1 s-1) is about three times higher than the one of Cur-III (2.60 107 M-1 s-1). Meanwhile, the ones in the PEA solvent are less significant, being 4.02 101 M-1 s-1 and 8.16 102 M-1 s-1, respectively. Because of the dominant molar fraction of the keto-enol form compared to that of the diketone, the reaction rates were contributed mainly by the keto-enol form. Finally, the chemical nature of the HT processes was analyzed in detail, and it was found that all the most predominant HT reactions at the phenolic -OH groups (i.e., O22H and O23H) occurred via the proton-coupled electron transfer (PCET) process.
