In photosynthetic bacteria, light is captured by light-harvesting complexes and its energy is transferred to the protein-pigment complex termed the reaction center. In the reaction center, the formation of an excited state of the primary electron donor is followed by the transfer of an electron to a series of electron acceptors. These electron transfer reactions occur on timescales ranging from femtoseconds to microseconds with essentially every photon of light resulting in electron transfer, that is, with a quantum yield of nearly unity. By coupling the reaction center to mobile electron donors and acceptors, the electrons and accompanying protons are transferred to other components of the photosynthetic apparatus, ultimately to be converted into chemically rich compounds such as ATP and NADPH.

I n this chapter, the properties of the reaction center are presented with an emphasis on those structural features that are conserved among purple bacteria. The overall architecture is first discussed, namely the arrangements of the protein subunits and the embedded pigments that serve as the electron transfer chain participants, including the primary electron donor. Many of the presented aspects are based upon extensive mutagenesis studies that have been performed on the reaction center from purple bacteria, for which the influence of specific protein-cofactor interactions can be probed due to our ability to perform a wide variety of spectroscopic investigations as well as the availability of high-resolution X-ray structures. Once the protein and cofactor framework is established, the bacterial reaction center is compared to the evolutionarily related complex, Photosystem II, the site for the oxidation of water in oxygenic photosynthesis (Chapter 4). Finally, the use of bacterial reaction centers as key components in biologically inspired technological devices is briefly summarized.

276 | 12 Reaction Centers from Purple Bacteria 12.2

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