Chemical quenching of singlet oxygen by betaninElectronic supplementary information (ESI) available. See DOI: 10.1039/c6pp00037a

Betanin is the best known natural dye belonging to the betacyanin family. In this work, efficient singlet oxygen quenching by betanin in deuterated water with the rate constant 1.20 ± 0.15 × 10 8 M −1 s −1 is reported, deduced from the 1 O 2 phosphorescence decays measured as a function of betanin concentration. The quenching occurs by a chemical mechanism, as confirmed by the analysis of the transient absorption kinetics at the probe λ ∼ 535 nm, by comparison of the initial triplet signal amplitude of perinaphthenone acting as the 1 O 2 photosensitizer with the final bleaching signal of betanin. The main betanin oxidation product is 2-decarboxy-2,3-dehydrobetanin, with its formation observed as the transient absorption signal at λ ∼ 445 nm. LC-MS/MS analysis of the photolyzed solutions supports the product identification as 2-decarboxy-2,3-dehydrobetanin, based on the molecular ion [M] + observed at m / z 505. Isobetanin also undergoes a similar photooxidation reaction. Singlet oxygen produced upon UV excitation of perinaphthenone in D 2 O leads to oxidation of betanin.