Bond dissociation free energy as a general parameter for flavonoid radical scavenging activity

Notwithstanding multiple mechanisms of radical scavenging (RS), measured RS activities (RSA) of flavonoids are usually related to O–H bond dissociation enthalpy (BDE) for hydrogen atom transfer (HAT). For 12 flavonoids the reaction free energies were calculated for: (1) HAT, (2) single electron transfer–proton transfer (SET-PT) and (3) sequential proton loss electron transfer (SPLET) in gas and aqueous phases. Aqueous free energies, like bond dissociation (BDFEaq), ionisation (IFEaq) and deprotonation (ΔGdeprot,aq) free energies were estimated using thermochemical cycles. While in gas HAT is a RS mechanism (BDFEg < IFEg < ΔGdeprot,g), in water SPLET can be concurrent or dominant mechanism depending upon pH since ΔGdeprot,aq < BDFEaq and ETFEaq ⩽ BDFEaq. For 12 flavonoids, BDFEaq has been correlated with ΔGdeprot,aq and ETFEaq with r = 0.74 and 0.87 respectively. This reveals why BD(F)E parameter explains most of variance in variously measured RSA data even if the underlying mechanism is SPLET.