CHARACTERIZATION OF TRIPLET-STATES IN ISOLATED PHOTOSYSTEM-II REACTION CENTERS - OXYGEN QUENCHING AS A MECHANISM FOR PHOTODAMAGE

Transient absorption spectroscopy has been used to study the isolated D1/D2/cytochrome b-559 reaction centre complex at 4°C. The D1/D2 reaction centre is observed to have an increased susceptibility to photodamage under aerobic conditions. This is attributed to oxygen quenching of a P680 triplet state, which results in the formation of highly oxidising singlet oxygen. This P680 triplet state is observed to have a lifetime of (1.0±0.1) ms under anaerobic conditions, shortening to (33±3) μs in the presence of oxygen. This state, which has a quantum yield of approx. 30%, is identified as residing upon the primary electron donor P680 by the transient bleaching of its reddest absorption band, which peaks at (680.5±0.5) nm. The shape of the P680 triplet-minus-singlet absorption difference spectrum, and particularly the (12±1) nm bandwidth of the red absorption band bleach, indicate that P680 is probably a pair of excitonically coupled chlorophyll molecules, with the P680 triplet state being localised upon one of these chlorophyll molecules. The red absorption band bleached by P680 triplet formation has a peak extinction coefficient of 133000 M-1 · cm-1 and an oscillator strength 1.1-times larger than that of the Qy-band of a chlorophyll a monomer in ether. It is shown that this P680 triplet state is formed primarily by charge recombination from the primary radical pair state at 4°C. A 3% quantum yield of a carotenoid triple state characterised by an absorption peak at 526 nm is also observed. The observed P680 triplet does not appear to be quenched by this carotenoid. © 1990.