PROTECTION OF CHLOROPHYLL a BY CAROTENOID FROM PHOTODYNAMIC DECOMPOSITION

The order of inhibition of the photooxidation of chlorophyll a in ethanol and ethanol‐benzene is as follows: β‐carotene, α‐tocopherol, benzoquinone, DABCO, menadione, cholesterol and KI. The quenching of singlet oxygen by β‐carotene occurs by a collisional quenching mechanism with a diffusion‐controlled rate of 1.7 × 10 10 M ‐1 s ‐1 . Photodecomposition of Chi a is faster in ethanol‐D 2 O than in ethanol‐H 2 O. Photoirradiation (660 nm) of the peridinin‐Chl a ‐protein complex, a photosynthetic light‐harvesting pigment isolated from marine dinoflagellates, did not show any photo‐decomposition of its Chi a in H 2 O or D 2 O, even after an extended period (12 h) of irradiation. However, the carotenoid, peridinin, in the photosynthetic antenna pigment was photobleached (ca. 10%) during the irradiation. We conclude that the singlet oxygen formed as a result of the Chi photosensitization is immediately quenched by the low‐lying triplet state of four peridinin molecules (per Chl a ) bound within the same protein crevice. The carotenoid thus effectively protects Chl a from photodynamic damage, providing a direct proof for the protective role of carotenoids in the photosynthetic pigment complex.