Al 071 a i slope 3 x 10 12 s 1 intercept adapted from

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al: 0.71 A -I slope; 3 x 10 12 S -1 intercept. Adapted from References 162 and 163. protein structure for the best pathways coupling two redox sites (the pathways between the histidines (33, 39, 62, 72, 79) and the heme are shown in Figure 6.35). A given coupling pathway consisting of covalent bonds, H-bonds, and through-space jumps can be described in terms of an equivalent covalent path- way with an effective number of covalent bonds (neff)' Multiplying the effective number of bonds by 1.4 A/bond gives a-tunneling lengths (al) for the five pathways (Table 6.7) that correlate well with the maximum rates (one-bond
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358 6 / ELECTRON TRANSFER limit set at 3 x 10 12 S -I; slope of 0.71 A -I) (Figure 6.35). The 0.71 A -I decay accords closely with related distance dependences for covalently coupled donor- acceptor molecules. 73,77 E. Bacterial Photosynthetic Reaction Centers Photosynthetic bacteria produce only one type of reaction center, unlike green plants (which produce two different kinds linked together in series), and are therefore the organisms of choice in photosynthetic electon-transfer research. 171- 176 As indicated in Section LB, the original reaction center structure (Figure 6.15) lacked a quinone (QB)' Subsequent structures for reaction centers from other photosynthetic bacteria 177,178 contain this quinone (Figure 6.36 See color plate section, page C-13.). The Rps. sphaeroides reaction center contains ten cofactors and three protein subunits. (Note that the Rps. viridis structure con- tains a cytochrome subunit as well.) The cofactors are arrayed so that they nearly span the 40-A-thick membrane (Figure 6.37 See color plate section, page C-13.). The iron atom is indicated by the red dot near the cytoplasmic side of the membrane (bottom). In spite of the near two-fold axis of symmetry, electron transfer proceeds along a pathway that is determined by the A branch. In partic- ular, BChl B and BPheB do not appear to play an important role in the electron transfers. It was demonstrated long ago that (BChlh is the primary electron donor and that ubiquinone (or metaquinone) is the ultimate electron acceptor. Transient flash photolysis experiments indicate that several electron-transfer steps occur in order to translocate the charge across the membrane (Figure 6.38). Curiously, the high-spin ferrous iron appears to play no functional role in the QA to QB electron transfer. 179 In addition, the part played by BChl A is not understood-it may act to promote reduction of BPheA via a superexchange mechanism. 180 ,181 Cytochromes supply the reducing equivalents to reduce the special pair (BChlh + . Estimated rate constants for the various electron-transfer steps, together with approximate reduction potentials, are displayed in Figure 6.39. For each step, the forward rate is orders of magnitude faster than the reverse reaction. The rapid rates suggest that attempts to obtain x-ray structures of intermediates (es- pecially the early ones!) will not be successful. However, molecular dynamics methods are being explored in computer simulations of the structures of various intermediates. 182 ,183
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