2-COUPLED BETA-CAROTENE MOLECULES PROTECT P680 FROM PHOTODAMAGE IN ISOLATED PHOTOSYSTEM-II REACTION CENTERS

The illumination of isolated Photosystem II (PS II) reaction centres in the presence of the artificial electron acceptors, silicomolybdate (SiMo) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) leads to the irreversible bleaching of beta-carotene. The bleaching is biphasic with approx. 50% of the carotenoid being bleached rapidly and the remainder more slowly. This is attributed to the sequential bleaching of the two beta-carotenes contained in the isolated complex, due to oxidation by the primary oxidant, P680+. Using SiMo as electron acceptor we found that only when all the beta-carotene is irreversibly bleached does further illumination induce a loss of electron transfer activity. This rate of loss is exacerbated by the presence of oxygen. In addition to beta-carotene degradation there is also, under the same conditions, an irreversible bleaching of chlorophyll absorbing at 670 nm. It is proposed therefore that beta-carotene and chlorophyll 670 contained in the reaction centre protect against deleterious effects due to the photoaccumulation of the highly oxidising P680+ state by acting as electron donors. Comparison of the spectra of the normal 2 beta-carotene and a 1 beta-carotene containing preparation of the PS II reaction centre indicates that in the former these two carotenoids are excitonically coupled. The excitonic coupling allows a more rapid initial rate of electron donation by the beta-carotene to P680+, than is seen in the preparation with only one carotenoid.