DIRECT-DETECTION OF SINGLET OXYGEN FROM ISOLATED PHOTOSYSTEM-II REACTION CENTERS

Both steady-state and time-resolved luminescence measurements at 1270 nm indicate that, when illuminated, isolated reaction centres of Photosystem II can generate singlet oxygen, O2 (1DELTA(g)). The oxygen dependent component of the luminescence signal, which was measured in a D2O buffer, was quenched by either azide or H2O. Singlet oxygen was detected both when primary charge separation was functioning and after it had been inactivated, suggesting that O-1(2) can be generated from triplet states formed by radical pair recombination (P680 chlorophyll triplet) and by intersystem crossing (accessory chlorophylls). Neither azide nor H2O was found to protect against light-induced oxygen dependent irreversible bleaching of chlorophyll and pheophytin of reaction centres. We suggest that the pool of singlet oxygen detected by steady-state luminescence at 1270 nm and quenched by azide and water is located in the bulk medium rather than in the protein matrix of the reaction centre, where the photodamage to pigments occurs, and the singlet oxygen is initially generated.