Role of radicalization in the antioxidant and phototoxic properties of curcuminoids.

Abstract

Curcumin exhibits antioxidant properties as well as phototoxic potential. Formation of curcumin radicals could be invoked to explain both features. In this work we present a systematic computational study on the tendency of curcumin to form radicals upon photosensitization to its excited singlet state. Concomitantly, experimental evidence of photoinduced curcumin radical production is provided through LC-HRMS and UV-visible absorption spectroscopy studies exploiting the radical-targeting spin-trap TEMPO. Because curcuminoids undergo extensive intramolecular hydrogen bonding, we investigate the role of hydrogen bonding patterns in fostering radical formation. Namely, we assess by TR-DFT calculations the differential proneness to radicalize through H-atom abstraction in correspondence of the enol and phenyl hydroxyl groups of curcumin and three substituted curcuminoids. The latter were chosen either to selectively prevent radicalization of each of the three hydroxyl moieties under quest or to hinder the formation of the H-bonds in which these moieties are involved in the parent compound. A mechanism potentially able to rationalize previous and current experimental results is, eventually, proposed, basing on the natural S1 → T1 intersystem crossing of photoexcited carbonyl groups, the T1 state being able to extract hydrogen atoms to form allyl-type radicals. These findings promise to open new perspectives towards full exploitation of curcuminoids photobiological activity.

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