Lycopene inhibits the isomerization of β-carotene during quenching of singlet oxygen and free radicals.

The present study aimed to investigate the influence of singlet oxygen and radical species on the isomerization of carotenoids. On the one hand, lycopene and β-carotene standards were incubated with 1,4-dimethylnaphthalene-1,4-endoperoxide that produced singlet oxygen in situ. (13Z)- and (15Z)-β-carotene were preferentially generated at low concentrations of singlet oxygen, while high concentrations resulted in formation of (9Z)-β-carotene. The addition of different concentrations of lycopene led to the same isomerization progress of β-carotene, but resulted in a decreased formation of (9Z)-β-carotene and retarded degradation of (all-E)-β-carotene. On the other hand, isomerization of β-carotene and lycopene was induced by ABTS-radicals, too. As expected from the literature, chemical quenching was observed especially for lycopene, while physical quenching was preferred for β-carotene. Mixtures of β-carotene and lycopene resulted in a different isomerization progress compared to the separate β-carotene model. As long as lycopene was present, almost no isomerization of β-carotene was triggered; after that, strong formation of (13Z)-, (9Z)-, and (15Z)-β-carotene was initiated. In summary, lycopene protected β-carotene against isomerization during reactions with singlet oxygen and radicals. These findings can explain the pattern of carotenoid isomers analyzed in fruits and vegetables, where lycopene containing samples showed higher (all-E)/(9Z)-β-carotene ratios, and also in in vivo samples such as human blood plasma.