Photochemical Oxidation and Reduction of Plastoquinone sensitized by Isolated Chlorophylls

Abstract

SINCE the discovery of plastoquinone in chloroplasts by Crane^1,2, many investigations have been reported on the role of the quinone in photosynthesis. Bishop³ showed that the quinone participated in the photolysis of water. Krogmann⁴ and Krogmann and Olivero⁵ demonstrated the indispensability of the quinone as a co-factor for photophosphorylation. Crane et al.⁶ and Redfearn and Friend⁷ observed photo-induced reduction and dark reoxidation of the quinone in chloroplasts. The localization of plastoquinone in the electron transport system of photosynthesis was verified by Witt's school^8,9. They indicated that the quinone acted as a primary electron acceptor from photochemically reduced chlorophyll a. Since all these investigations were carried out with chloroplasts, no evidence has been presented showing the direct interaction of plastoquinone with chlorophylls. Tollin and Green¹⁰ observed electron paramagnetic resonance signals, showing the light-induced single electron transfer reactions between chlorophyll a and various kinds of quinones, but plastoquinone was not included among the quinones they used. In this communication, we present evidence indicating that plastoquinone can be oxidized or reduced photochemically in the presence of chlorophyll a or b.