The influence of the H5⋯OC4 intramolecular hydrogen-bond (IHB) on the antioxidative activity of flavonoid.

Flavonoids widely found in natural foods are characterized by acting as antioxidants compounds. There are close relationship between the antiradical activities and structural properties of flavonoids. In this work, density functional theory (DFT) methods were applied to investigate the influence of the H5⋯OC4 intramolecular hydrogen-bond (IHB) on the antiradical activity of flavonoid based on three prevalently accepted radical scavenging mechanisms: hydrogen atom transfer (HAT), single electron transfer-proton transfer (SET-PT) and sequential proton-loss electron-transfer (SPLET). The thermodynamic properties: bond dissociation enthalpy (BDE), ionization potential (IP), proton dissociation enthalpy (PDE), proton affinity (PA) and electron transfer enthalpy (ETE) related with these mechanisms were calculated to elucidate the antiradical activity. The results showed that the 5-OH group is most influenced and its antiradical capacity was weakened by the H5⋯OC4 IHB. In the gas, benzene and chloroform phases, H5⋯OC4 IHB would reduce the antiradical activity of flavonoid via increasing the bond dissociation enthalpy. While, in the DMSO and HO phases, the opposite result occurs by lowering the proton affinity.