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 Photo system II can generate singlet oxygen, O2 (1DELTA(g)). The oxygen depe ndent component of the luminescence signal, which was measured in a D2 O buffer, was quenched by either azide or H2O. Singlet oxygen was dete cted both when primary charge separation was functioning and after it had been inactivated, suggesting that O-1(2) can be generated from tri plet states formed by radical pair recombination (P680 chlorophyll tri plet) and by intersystem crossing (accessory chlorophylls). Neither az ide nor H2O was found to protect against light-induced oxygen dependen t irreversible bleaching of chlorophyll and pheophytin of reaction cen tres. We suggest that the pool of singlet oxygen detected by steady-st ate 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 oxyg en is initially generated.