RESUMO
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)O2 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 â¼ 535 nm, by comparison of the initial triplet signal amplitude of perinaphthenone acting as the (1)O2 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 â¼ 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.
Assuntos
Betacianinas/química , Oxigênio Singlete/química , Cromatografia Líquida de Alta Pressão , Oxirredução , Fotólise/efeitos da radiação , Espectrometria de Massas em Tandem , Raios UltravioletaRESUMO
The photophysical properties of betanin in aqueous and alcoholic solutions were determined at room temperature using ultrafast UV-vis-NIR transient absorption spectroscopy (λexc = 535 nm). Its S1 â Sn (n > 1) absorption bands appear with maxima at about λ â¼ 450 and 1220 nm. The short betanin S1 state lifetime (6.4 ps in water) is mainly determined by the efficient S1 â S0 radiationless relaxation, probably requiring a strong change in geometry, since the S1 lifetime grows to 27 ps in the more viscous ethylene glycol. The fluorescence quantum yield is very low (Φf â¼ 0.0007 in water), therefore this deactivation path is of minor importance. Other processes, such as S1 â T1 intersystem crossing or photoproduct formation, are virtually absent, since full S0 â S1 ground state recovery is observed within tens of picoseconds after photoexcitation. The observed fast light-to-heat conversion in the absence of triplet excited state formation supports the idea that betanin is a photoprotector in vivo.