Ultrafast Fluorescence Spectroscopy of Reaction Centers of Photosynthetic Bacteria

In the primary reaction of photosynthesis an electron is transferred through a pigment-protein complex called reaction center (RC). Starting from the primary donor, a dimer of bacteriochlorophyll molecules called special pair P, the electron reaches a quinone (Q) via two intermediate acceptors, a bacteriochlorophyll-monomer (B) and a bacteriopheophytin (H). In a number of publications this electron transfer process was investigated by transient absorption spectroscopy /1/. At least four kinetic constants of 0.9 ps, 3.5 ps, 200 ps and infinity are required to explain the absorption data /2,3/. The interpretation of these measurements is difficult since absorbance changes from different intermediates interfere. At present the most straight-forward interpretation of the data is the stepwise electron transfer model via the radical pair state P+B–: Complementary information from fluorescence up-conversion experiments is given in this contribution. By this way we follow directly the decay of the excited electronic state of the special pair. The experimental system was optimized to high time resolution and high sensitivity at low repetition rates in order to meet the requirements of the biological samples.