Role of β-carotene in the reaction centres of Photosystems I and II of spinach chloroplasts prepared in non-polar solvents

Abstract

Spinach chloroplasts have been prepared nonaqueously using non-polar solvents ( n-hexane, CCl 4, n-heptane) and the β-carotene content extracted in a controlled manner. This procedure is reproducible and does not result in large structural or spectral changes of the chloroplasts. The organisation of the chlorophyll-proteins is unaltered, as fragmentation with digitonin results in the appearance of the same fractions as found previously for aqueously-prepared chloroplasts, including the pink zone containing cytochromes f and b 6 in the ratio 1:2. The chloroplasts possess both Photosystem I activity ( P-700 photo-bleaching, and NADP + photoreduction) and Photosystem II activity (parabenzoquinone reduction with Mn 2+ as electron donor, and chlorophyll fluorescence induction). Use of moderate intensity red illumination has allowed a study of the role of β-carotene in photochemistry separate from its roles in energy transfer and photoprotection. Removal of the fraction of β-carotene closely associated with the Photo-system I reaction centre caused the rate of NADP + photoreduction to fall to a low, but significantly non-zero level. Thus, in the complete absence of β-carotene, photochemistry can still be observed, however the specific association of β-carotene with the reaction centre is required for maximal rates. We propose that β-carotene bound at the reaction centre decreases the rate of transfer of excitation energy away from the reaction centre, and increases the rate of photochemistry. It is possible that this occurs via formation of an exciplex between ground state β-carotene and chlorophyll in the first excited state.