Abstract

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 β-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 β-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 β-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 β-carotene degradation there is also, under the same conditions, an irreversible bleaching of chlorophyll absorbing at 670 nm. It is proposed therefore that β-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 β-carotene and a 1 β-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 β-carotene to P680 +, than is seen in the preparation with only one carotenoid.

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