Femtosecond Infrared Spectroscopy of Photosynthetic Reaction Centers

Abstract

The reaction center (RC) from purple bacteria is the key unit to perform the primary charge separation and energy storage in photosynthesis. Two basic experimental techniques have been applied to investigate the function of this unit: (i) The structure of bacterial RC’s, a chromophore-protein complex, was determined by x-ray scattering with atomic resolution (ii) Time resolved spectroscopy in the visible or the near IR was applied to characterize the kinetics of the photoreaction as well as the electronic states of the chromophores. After light absorption, an electron is transferred from a dimer of bacteriochlorophyll molecules (special pair P) to a bacteriochlorophyll monomer B in ≈3–4 ps, to a pheophytin H in ≈1 ps and finally to a quinone Q in ≈200 ps. However, only one of the almost symmetric potential electron transfer (ET) branches seems to be essential for this reaction. This so-called ‘unidirectionality of ET’ is one of the remaining open questions of bacterial photosynthesis. Therefore, in order to obtain supplement information on structural modifications of the chromophores as well as on protein residues surrounding the chromophores, on the reaction mechanism itself, and on the electronic structure of P* and P⁺ time resolved vibrational spectroscopy in the mid-IR spectral range (1000–1800 cm^-1) was started only recently [1–4].