A novel relationship between the radicalscavenging activity of flavonoids and enthalpy of formation revealed with Hartree-Fock computations and thermochemical deduction

Objective: The present study aims to establish the relationship between the reported radical-scavenging activities of flavonoids and some enthalpy changes that may occur during flavonoids' reactions with free radicals. Method: Eight flavonoids were chosen for the study on the basis of their structural merits and reported 1,1-diphenyl-2-picryl-hydrazyl scavenging activities. Enthalpy changes accompanying interconversions between selected conformations (including spin multiplicities) and homolytic dissociations were estimated. Results: A novel relationship exists between the total enthalpy of reaction for the abstraction of two hydrogen atoms from flavonoids, their reported radical-scavenging activities and the enthalpy of the homolytic dissociation of hydrogen molecule (104.206 kcal mol -1). Only those flavonoids which could give up two hydrogen atoms with total enthalpy changes well below 104.206 kcal mol -1 were active radical scavengers. Discussion: By appealing to equilibrium dynamics, we demonstrated that, for flavonoids to be able to donate hydrogen atoms, the change in enthalpy accompanying the abstraction of two hydrogen atoms needs to be less than 104.206 kcal mol -1. This condition does not seem to be restricted to flavonoids only but rather generally applicable to chian-breaking antioxidants. Conclusion: Thermodynamical relationships may be the most important factors governing the radicalscavenging reactions of flavonoids and possibly other compounds as well. Nevertheless, a more complete characterization of antioxidants would necessitate kinetic analysis.