Photoprotective properties of natural antioxidant flavonoids: A DFT and TD-DFT study on acridone derivatives

Antioxidant and photoprotective properties of two natural acridone derivatives namely Paratrimerin C (1) and Citrusinine-I (2) have been studied by DFT and TD-DFT at the M05-2X/6-311++G(3df, 3p)//M05-2X/6-31+G(d) level of theory. Three common mechanisms characterized for the antioxidant properties including H-atom transfer (HAT) and proton transfer (PT) towards HOO●/HO● radicals and single electron transfer (SET) were investigated in the gas phase, water and pentyle thanoate. The DFT results revealed that both compounds can efficiently scavenge HOO● and HO● radicals via HAT mechanism in all media. Particularly, the HAT reactions of 2 with HO● radical in water are the most favored reaction (ΔH -37.7 kcal/mol). In addition, the efficient UV-absorption ability of the studied compound was elucidated by TD-DFT. All compounds can absorb UV radiations in the range of 200-335 nm, for which the easiest excitations are at 334-332 nm and the strongest absorptions were found at 234-227 nm, for 1 and 2, respectively. The HOMO to LUMO and HOMO-3 to LUMO (π-π^*) transitions are assigned for the corresponding UV-absorption. (author)