Light Harvesting by Chlorophylls and Carotenoids in the Photosystem I Core Complex of Synechococcus elongatus: A Fluorescence Upconversion Study

Abstract

The photosystem I (PSI) core complex of oxygenic photosynthesis is an integral pigment-protein complex that incorporates both the antenna and the reaction center (RC). It binds about 100 Chl a and 20 ‚-carotene molecules. In the PSI core complex of the cyanobacterium Synechococcus elongatus, a total of about 9 antenna Chl a molecules are red-shifted with respect to the primary electron donor, which absorbs at 700 nm. We have studied energy transfer and trapping processes in trimeric PSI complexes of this species at femtosecond resolution by means of the fluorescence-upconversion technique. By simultaneously analyzing the fluorescence upconversion results and those obtained with a streak camera with picosecond resolution and multichannel detection (Gobets, B.; et al. Biophys. J., in press), we have mapped out the energy transfer processes that follow immediately after photon absorption. Equilibration among Chl a pigments in the bulk antenna was found to occur with a time constant of 360 fs. A major energy equilibration phase between bulk Chl a and the red-shifted antenna Chls occurs in 3.6 ps. A slow phase in energy equilibration takes place in 9.8 ps, after which the excitations are trapped by the RC in 38 ps. Fluorescence anisotropy measurements indicated an initial anisotropy of 0.30, which decayed biphasically with a major fast phase of 160 fs and a minor slow phase of 1.8 ps to a final anisotropy of 0.06. The 160 fs phase is assigned to elementary energy transfer steps in the bulk Chl a antenna, and the 1.8 ps phase to further equilibration processes, possibly involving energy transfer to or among red-shifted Chls. Energy transfer from ‚-carotene to Chl a was found to proceed both from the S2 state and the S1 state, with the majority of transferred excitations (60%) originating from the S 2 state, resulting in an estimated overall yield of 90%. A comparison is made with the PSII core antenna protein CP47, which binds the same pigments but has a substantially lower carotenoid-Chl a energy transfer yield of 35% (van Dorssen R. J.; et al. Biochim. Biophys. Acta1987, 893, 267).