Photosystem I (PSI) is a multisubunit protein-pigment complex. It possesses Light-Harvesting Complexes (LHCs) that transfer absorbed energy to the core where the excitation energy induces charge separation in the reaction centre (RC). PSI is unique in binding red chlorophylls (Chls), with energy levels lower than that of the RC. These Chls may compete with the RC as an energy sink. The function of these red Chls is not fully known. Many time-resolved optical spectroscopy studies on PSI have revealed very heterogeneous excited state kinetics. Although this at least partly results from the intrinsic properties of PSI , including the presence of the red pigments , it might in principle also be caused by some sample heterogeneity. Here we show that the fluorescence kinetics of (highly homogeneous) crystals of PSI from pea are identical to those of PSI in solution. This justifies the structure-based modelling of spectral properties of PSI . Our new data indicate that the energy flow from LHC to the core proceeds mainly via the red Chls
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