Femtosecond transient absorption spectroscopy has been used to investigate the photoinduced energy and electron transfer processes in photosystem I (PS I) particles from cyanobacteria, green algae, and higher plants. At room temperature, the kinetics observed in all three species are very similar: Following 590 nm excitation, an equilibration process(es) with a 3.7-7.5 ps lifetime was observed, followed by a 19-24 ps process that is associated with trapping. In all three species long-wavelength pigments (pigments that absorb at longer wavelengths than the primary electron donor) were observed. The difference spectrum associated with reduction of the primary electron acceptor [(Ao- - A0) difference spectrum] was obtained for all three species. The (Ao- - Ao) difference spectra obtained from measurements using detergent-isolated PS I particles from spinach and Chlamydomonas reinhardtii are similar but clearly different from the (A0- - A0) difference spectrum obtained from measurements using PS I cyanobacterial membrane fragments. In all three species the reduced primary electron acceptor (A0-) is reoxidized extremely rapidly, in about 20 ps. The difference spectrum associated with A0 reduction appears to contain contributions from more than a single chlorophyll pigment.
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