What is β–carotene doing in the photosystem II reaction centre?

Abstract

During photosynthesis carotenoids normally serve as antenna pigments, transferring singlet excitation energy to chlorophyll, and preventing singlet oxygen production from chlorophyll triplet states, by rapid spin exchange and decay of the carotenoid triplet to the ground state. The presence of two β–carotene molecules in the photosystem II reaction centre (RC) now seems well established, but they do not quench the triplet state of the primary electron–donor chlorophylls, which are known as P ₆₈₀ . The β–carotenes cannot be close enough to P ₆₈₀ for triplet quenching because that would also allow extremely fast electron transfer from β–carotene to P ⁺ ₆₈₀ , preventing the oxidation of water. Their transfer of excitation energy to chlorophyll, though not very efficient, indicates close proximity to the chlorophylls ligated by histidine 118 towards the periphery of the two main RC polypeptides. The primary function of the β–carotenes is probably the quenching of singlet oxygen produced after charge recombination to the triplet state of P ₆₈₀ . Only when electron donation from water is disturbed does β–carotene become oxidized. One β–carotene can mediate cyclic electron transfer via cytochrome b 559. The other is probably destroyed upon oxidation, which might trigger a breakdown of the polypeptide that binds the cofactors that carry out charge separation.