UNIVERSALITY OF ENERGY AND ELECTRON-TRANSFER PROCESSES IN PHOTOSYSTEM-I

Femtosecond transient absorption spectroscopy has been used to investi gate the photoinduced energy and electron transfer processes in photos ystem I (PS I) particles from cyanobacteria, green algae, and higher p lants. At room temperature, the kinetics observed in all three species are very similar: Following 590 nm excitation, an equilibration proce ss(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-w avelength pigments (pigments that absorb at longer wavelengths than th e primary electron donor) were observed. The difference spectrum assoc iated with reduction of the primary electron acceptor [(A(0)(-) - A(0) ) difference spectrum] was obtained for all three species. The (A(0)(- ) - A(0)) difference spectra obtained from measurements using detergen t-isolated PS I particles from spinach and Chlamydonmonas reinhardtii are similar but clearly different from the (A(0)(-) - A(0)) difference spectrum obtained from measurements using PS I cyanobacterial membran e fragments. In all three species the reduced primary electron accepto r (A(0)(-)) is reoxidized extremely rapidly, in about 20 ps. The diffe rence spectrum associated with A(0) reduction appears to contain contr ibutions from more than a single chlorophyll pigment.