On the role of the CP47 core antenna in the energy transfer and trapping dynamics of Photosystem II

The CP47-RC complex of photosystem II (PS II) has an antenna subunit of 16 chlorophyll a molecules attached to the reaction center (RC) at the side of its inactive branch. The X-ray structure of PS II revealed that the shortest interpigment distances between CP47 and RC are about 21 Angstrom, which is two-three times larger than within each subunit. Such long distances may slow down the energy transfer from CP47 to RC. In order to evaluate the influence of the CP47 antenna on the energy transfer and trapping dynamics in the RC we performed a comparative analysis of CP47-RC and RC complexes from spinach by transient difference absorption and time-resolved fluorescence spectroscopy at room temperature. Our data reveal a complex multiexponential decay of the excited states in both complexes. The main trapping lifetimes are 2-3, 30-40 and 360-460 ps in the RC and 2-6, 80-85 and 650-700 ps in the CP47-RC, the latter two phases being two to three times longer than in the RC. The data could be fitted well with a kinetic model consisting of three reversible radical pair states, the first one being nearly isoenergetic with P680*, and identical energy levels and kinetics of processes occurring within the RC in both complexes. We conclude that there is no kinetic limitation of the energy transfer between CP47 and RC on the two slowest trapping phases and that this transfer occurs in 20 ps or less. The two factors that influence the observed slower trapping in CP47-RC are the highly reversible charge separation reaction in the RC and the presence in CP47 of states with energy lower than the primary electron donor P680.