Antioxidant and free radical scavenging activity of purpurin

Density functional theory (DFT) calculations were performed to estimate the antioxidant activity of purpurin (1,2,4-trihydroxyanthraquinone-9,10-dione), a naturally occurring anthraquinone pigment. All conformations of purpurin and corresponding radicals, anions, and radical cation were analyzed using the M052X/6-311+G(2df,p) level of theory. The most stable rotamer of purpurin involves three internal hydrogen bonds. The most stable rotamers of purpurin radical and anion were obtained by dehydrogenating hydroxyl groups at C1 and C2, in gaseous and aqueous phases. The antioxidant activity of purpurin was elucidated by its bond dissociation enthalpy and ionization potential. Proton dissociation enthalpies were also computed. On the basis of these values, a one-step hydrogen atom transfer mechanism was invoked, rather than sequential proton loss-electron transfer or electron transfer-proton transfer, to explain the antioxidant activity of purpurin in the gas phase and in nonpolar solvents. In aqueous solution, the sequential proton loss-electron transfer mechanism plays a significant role.