DFT Study of the Reactions between Singlet-Oxygen and a Carotenoid Model

Carotenoids such as beta-carotene are one of the most efficient singlet-oxygen (O-1(2)) quenchers. They quench catalytically O-1(2) (a highly reactive and toxic form of oxygen) through an almost diffusion-controlled energy transfer process (physical pathway): O-1(2) + carotenoid --> O-3(2) + (3)carotenoid --> O-3(2) + carotenoid (+ heat) (eq 1). In contrast to physical quenching, less efficient but concomitant processes exist, involving real chemical reactions. For example. chemical oxidation reactions, which result in the destruction of carotenoids and thus in the loss of antioxidant protection, have been observed (chemical pathway): O-1(2) + carotenoid --> chemical pathway (eq 2). To obtain more detailed information about the reactions between carotenoids and singlet-oxygen, we have performed a DFT computational study of the reaction mechanisms involved in the attack of O-1(2) to the all-trans-decaottanonaene (P-9), a polyene with 9 conjugated double bonds, chosen as carotenoid model. We have found that, together with the main energy transfer pathway (eq 1) which is almost barrierless, there are secondary but concomitant reactions (eq 2) with low-energy barriers leading to biradical intermediates via direct addition of O-1(2) to P-9. These biradicals may give ring closure to form 1,2-addition dioxetane products whose decomposition leads to the observed carbonyl chain cleavage oxidation fragments. However, these biradicals seem to be also responsible, through an S-0 --> T-1 intersystem crossing, of an alternative chemically mediated catalytic quenching of the singlet-oxygen which is returned to its triplet deactivated ground state through a dissociation process on T-1.