Antioxidant and UV-radiation absorption activity of aaptamine derivatives - potential application for natural organic sunscreens.

Antioxidant and UV absorption activities of three aaptamine derivatives including piperidine[3,2- b ]demethyl(oxy)aaptamine (C1), 9-amino-2-ethoxy-8-methoxy-3 H -benzo[de][1,6]naphthyridine-3-one (C2), and 2-( sec -butyl)-7,8-dimethoxybenzo[de]imidazo[4,5,1- ij ][1,6]-naphthyridin-10(9 H )-one (C3) were theoretically studied by density functional theory (DFT). Direct antioxidant activities of C1-C3 were firstly evaluated via their intrinsic thermochemical properties and the radical scavenging activity of the potential antioxidants with the HOO˙/HO˙ radicals via four mechanisms, including: hydrogen atom transfer (HAT), single electron transfer (SET), proton loss (PL) and radical adduct formation (RAF). Kinetic calculation reveals that HOO˙ scavenging in water occurs via HAT mechanism with C1 ( k app , 7.13 × 10 6 M -1 s -1 ) while RAF is more dominant with C2 ( k app , 1.40 × 10 5 M -1 s -1 ) and C3 ( k app , 2.90 × 10 5 M -1 s -1 ). Antioxidant activity of aaptamine derivatives can be classified as C1 > C3 > C2. Indirect antioxidant properties based on Cu(i) and Cu(ii) ions chelating activity were also investigated in aqueous phase. All three studied compounds show spontaneous and favorable Cu(i) ion chelating activity with Δ G 0 being -15.4, -13.7, and -15.7 kcal mol -1 , whereas Δ G 0 for Cu(ii) chelation are -10.4, -10.8, and -2.2 kcal mol -1 for C1, C2 and C3, respectively. In addition, all compounds show UVA and UVB absorption; in which the excitations are determined mostly as π-π* transition. Overall, the results suggest the potential applications of the aaptamines in pharmaceutics and cosmetics, i.e. as a sunscreen and antioxidant ingredient.