Energy Coupling in the PSI−LHCI Supercomplex from the Green Alga Chlamydomonas reinhardtii†,‖

Energy transfer and trapping in the PSI−LHCI supercomplex from the green alga Chlamydomonas reinhardtii have been studied using femtosecond transient absorption and picosecond fluorescence spectroscopy at room temperature. Data suggest that excitations of the PSI−LHCI supercomplex at 700 nm have similar probabilities of excitation of either the primary donor in the PSI core (absorbing at 697 nm) or low-energy Chls in the LHCI (red pigments) that presumably absorb at this spectral region. Both transient absorption and picosecond fluorescence spectroscopy indicate a biphasic overall decay of the excitation in the PSI−LHCI. The process includes a photochemical trapping in the PSI core antenna occurring with a typical lifetime of 25 ± 3 ps and a significantly slower excitation decay phase in the PSI−LHCI supercomplex occurring with a lifetime of 104 ± 20 ps and maximum of absorption changes around 685 nm. The slow excitation decay process suggests presence of an energy transfer pathway from the LHCI to the PS...