Excitation energy transfer and trapping processes in an iron stress-induced supercomplex of photosystem I from the cyanobacterium Synechocystis sp. PCC6803 were studied by time-resolved absorption and fluorescence spectroscopy on femtosecond and picosecond time scales. The data provide evidence that the energy transfer dynamics of the CP43′-PSI supercomplex are consistent with energy transfer processes that occur in the Ch1 a network of the PSI trimer antenna. The most significant absorbance changes in the CP43′-PSI supercomplex are observed within the first several picoseconds after the excitation into the spectral region of CP43′ absorption (665 nm). The difference time-resolved spectra (AAA) resulting from subtraction of the PSI trimer kinetic data from the CP43′-PSI supercomplex data indicate three energy transfer processes with time constants of 0.2, 1.7, and 10 ps. The 0.2 ps kinetic phase is tentatively interpreted as arising from energy transfer processes originating within or between the CP43′ complexes. The 1.7 ps phase is interpreted as possibly arising from energy transfer from the CP43′ ring to the PSI trimer via closely located clusters of Ch1 a in CP43′ and the PSI core, while the slower 10 ps process might reflect the overall excitation transfer from the CP43′ ring to the PSI trimer. These three fast kinetic phases are followed by a 40 ps overall excitation decay in the supercomplex, in contrast to a 25 ps overall decay observed in the trimer complex without CP43′. Excitation of Ch1 a in both the CP43′-PSI antenna supercomplex and the PSI trimer completely decays within 100 ps, resulting in the formation of P700+. The data indicate that there is a rapid and efficient energy transfer between the outer antenna ring and the PSI reaction center complex.
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