On the energetics of homolytic and heterolytic OH bond cleavage in flavonoids

Comprehensive study of three mechanisms of phenolic chain-breaking antioxidant action, i.e. hydrogen atom transfer (HAT), Single Electron Transfer–Proton Transfer (SET–PT) and Sequential Proton-Loss Electron-Transfer (SPLET), for eight naturally occurring flavonoids (polyphenols): apigenin, luteolin, fisetin, kaempferol, quercetin, epicatechin, taxifolin and cyanidin, is presented. Gas-phase OH bond dissociation enthalpies, ionization potentials, proton dissociation enthalpies, proton affinities and electron transfer enthalpies related to these mechanisms were investigated using B3LYP/6-311++G** method. Selection of flavonoids enables to evaluate the effects of various structural features, such as hydroxy groups (3′-OH, 3-OH, 5-OH), C2C3 double bond and C4O keto group, present in the molecules on studied reaction enthalpies. Lowest OH bond dissociation enthalpies (HAT) and proton affinities (SPLET) have been found mostly for 4′-OH groups at ring B. In the second step of SET–PT mechanism, formation of radicals at 4′-OH positions is also thermodynamically favored. However, for five flavonoids, lowest values of electron transfer enthalpies were found in rings A or C.