Conformation-dependent antioxidant properties of β-carotene

The antioxidant capacity of β-carotene has been studied in terms of H-atom abstraction reactions using quantum chemical methods. These oxidation reactions are studied for the all-trans as well as 15,15'-cis isomers (15Z) of β-carotene, as the latter is only ∼10 kJ mol^-1 less stable than the all-trans isomer in the gas phase and about 9 kJ mol^-1 less stable in aqueous solution. Hydrogen abstraction from the rotamers obtained through C-C single and double bond rotations has been shown to play an important role in determining the antioxidant capacity of β-carotene. Hydrogen abstraction from the C4 and C5-CH₃ positions of the β-ionone rings and the C7 and C9 positions along the polyene chain of β-carotene by the hydroxyl radical have been studied. In the all-trans form the most favorable H-atom abstraction reaction occurs at the C4 position of the terminal regions of the polyene π-system of β-carotene, closely followed by hydrogen abstraction from the C5 methyl position. The H-atom abstraction reactions are more exothermic in water than in the gas phase due to solvation energies for the water product.