Light harvesting by chlorophylls and carotenoids in the photosystem I corecomplex of Synechococcus elongatus: A fluorescence upconversion study

The photosystem I (PSI) core complex of oxygenic photosynthesis is an integ ral pigment-protein complex that incorporates both the antenna and the reac tion center (RC). It binds about 100 Chl alpha and 20 beta -carotene molecu les. In the PSI core complex of the cyanobacterium Synechococcus elongatus, a total of about 9 antenna Chi a molecules are;ed 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 techniq ue: By simultaneously analyzing the fluorescence upconversion results and t hose obtained with a streak camera with picosecond resolution and multichan nel detection (Gobets, B.; et al, Biophys. J., in press), we have mapped ou t the energy transfer processes that follow immediately after photon absorp tion. Equilibration among Chi a pigments in the bulk antenna was found to o ccur with a time constant of 360 fs. A major energy equilibration phase bet ween bulk Chi a and the red-shifted antenna Chls occurs in 3.6 s. A slow ph ase in energy equilibration takes place in 9.8 ps, after which the excitati ons are trapped by the RC in 38 pst Fluorescence anisotropy measurements in dicated an initial anisotropy of 0.30, which decayed biphasically with a ma jor fast phase of 160 fs and a minor slow phase of 1.8 ps to a final anisot ropy of 0.06. The 160 fs phase is assigned to elementary energy transfer st eps in the bulk Chi a antenna, and the 1.8 ps phase to further equilibratio n processes, possibly involving energy transfer to or among red-shifted Chl s. Energy transfer from p-carotene to Chi a was found to proceed both from the S-2 State and the S-1 state, with the majority of transferred excitatio ns (60%) originating from the S2 state, resulting in an estimated overall y ield of similar to 90%. A comparison is made with the PSII core antenna pro tein CP47, which binds the same pigments but has a substantially lower caro tenoid-Chl a energy transfer yield of similar to 35% (van Dorssen R. J.; et al. Biochim. Biophys. Acta 1987, 893, 267).