Hence hundreds of these light harvesting pigments

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suitable absorption cross section for sunlight. Hence, hundreds of these light- harvesting pigments function as molecular antennas; an x-ray structure 35 of one subunit of a bacteriochlorophyll-protein complex is displayed in Figure 6.14. Absorption of a photon by an antenna pigment promotes the pigment into an electronically excited state, which can return to the ground state by a variety of relaxation processes, including fluorescence or resonance transfer of excita- tion energy to a nearby pigment at picosecond rates. As much as 100 ps may elapse between the photon absorption and the arrival of the light energy at a reaction center. During this time, the energy may "migrate" in a random-walk fashion among hundreds of pigments. The energy of the excited state is converted into electrochemical potential energy at the reaction center, which contains a primary electron donor P that transfers an electron to a nearby acceptor Al within the same protein (and P
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328 t c o -a o If) .n ro 400 500 wavelength (nm) 600 700 Figure 6.13 Absorption spectra of the photosystem pigments. Figure 6.14 Structure of a subunit of a bacteriochlorophyll-protein complex. Reproduced with permission from Reference 35.
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I. ELECTRON TRANSFERS IN BIOLOGY 329 becomes oxidized to P+): (6.5) This charge separation is of paramount importance. The key problem is main- taining the charge separation, which involves minimization of the energy-wast- ing back reaction. Reaction centers contain an ordered array of secondary elec- tron acceptors (AI, A z , A 3 "') that optimize the ilGo that occurs at each step: (6.6) Thus, the back reaction is circumvented by optimizing forward electron transfers that rapidly remove electrons from A I -. As the acceptors are separated by greater and greater distances from P +, the probability of the back electron transfer to P + decreases. Put another way, the overlap of P + and each acceptor orbital decreases in the order P + fA I > P + fA z - > P + fA 3 - . Photosynthetic bacteria contain only one type of reaction center (l00 kDa). The solution of the x-ray structure (at 2.9 A resolution) of the Rps. viridis re- action center was reported 36 in 1984, providing conclusive proof that electrons can "tunnel" over 10-20 A distances through protein interiors. The reaction- center protein contains many cofactors (Figure 6.15): two bacteriochlorophylls (BChl) in close proximity (the so-called "special pair"), two further bacterio- chlorophylls that are spectroscopically identical, two bacteriopheophytins (BPh), two quinones (QA and QB), and one iron center. (QB was lost during isolation of the Rps. viridis reaction center and thus does not appear in Figure 6.15.) The reaction center contains an approximate two-fold rotation axis. Despite this strikingly high symmetry in the reaction center, one pathway of electron flow predominates, as the cartoon in Figure 6.16 indicates.
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