Kinetics of Excitation Transfer and Trapping in Purple Bacteria

Abstract

The transfer and trapping of excitation energy in photosynthetic purple bacteria, as it appears from time-resolved and steady-state spectroscopy, are discussed. As a background to the dynamics we first briefly describe the structure and organization and spectroscopy of purple bacterial antenna pigments. The concept of spectral inhomogeneous broadening and its consequences for understanding the spectroscopy and dynamics is thoroughly discussed. We start the discussion of the dynamics by describing the overall energy equilibration and trapping processes in antenna-reaction center systems of varying complexity. These processes occur on the time scale of several picoseconds to tens of picoseconds. The most conspicuous results are that excitation energy is equilibrated very rapidly, ≤10 ps, over the entire antenna, but transfer of the energy from the antenna to the reaction center (the special pair) is a relatively slow process (30–40 ps), both at room temperature and 77 K. The reaction center is not a perfect trap for the excitation energy; at room temperature ∼25% of the energy returns to the antenna in Rhodospirillum rubrum, upon selective excitation of the reaction center pigments. At low temperatures (≤77 K) the back transfer to the antenna is negligible. With recently available subpicosecond and femtosecond laser pulses the most fundamental steps of energy transfer has been resolved. It is shown that energy transfer between a pair of bacteriochlorophyll molecules typically occur on the timescale 0.2–0.5 ps. On this extremely short timescale it becomes necessary to also take into account various intramolecular processes, like vibrational relaxation and vibrational energy redistribution. For such very fast energy transfer processes it may be necessary to question the applicability of conventional Fürster theory of electronic energy transfer. In order to test particular structural and dynamical aspects of the energy transfer and trapping, molecular biology methods are invaluable tools to introduce specific changes into the pigment-protein systems. Several examples of advanced spectroscopy on genetically modified systems are given.