Two coupled β-carotene molecules protect P680 from photodamage in isolated Photosystem II reaction centres

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.