DFT insight into the UV-Vis spectra and radical scavenging activity of aurantio-obtusin

Aurantio-obtusin (2,6,8-trihydroxy-1,7-dimethoxy-3-methyl anthraquinone), an anthraquinone derivative isolated from the seeds of Cassia tora L., has been studied by DFT calculations at the B3LYP/6-311+g(2df,p)//B3LYP/6-31G(d,p) level in vacuum and in methanol polarized continuum model (PCM). Eight conformers have been located by conformational search and optimization. Our results indicate that the titled compound has planar structures, and five intramolecular hydrogen bonds are found in the lowest-energy structure, including three OH⋯O and two CH⋯O hydrogen bonds. The electronic spectra in methanol have been determined by the time-dependent density functional theory (TD-DFT) approach, showing that the first singlet exited state of the title compound arises from π → π* electron transition. The antioxidant activity of aurantio-obtusin is evaluated by means of calculating O-H bond dissociated enthalpy (BDE) and the adiabatic ionization energy (IE). The calculated BDE and IE values of aurantio-obtusin are 341.8 and 732.3 kJ mol^-1, respectively, lower than the corresponding values of phenol (348.4 and 801.7 kJ mol^-1) and purpurin (360.4 and 747.8 kJ mol^-1), suggesting that aurantio-obtusin is a better antioxidant. © 2013 World Scientific Publishing Company.