Excitation energy- and electron transfer-processes in the photosynthetic reaction center after replacement of the accessory bacteriochlorophyll by 132OHNiBChl

Hartwich, Gerhard; Haeberle, T.; Langenbacher, Thomas; Lossau, H.; Rousseau, Gerhard; Ogrodnik, A.; Scheer, Hugo; Michel‐Beyerle, M. E.

doi:10.1016/0162-0134(95)97356-u

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“…This is remarkable as the distance between H and P is substantially greater than between B and P. This much greater distance, along with the negligible spectral overlap between H and P, makes the Förster mechanism for direct energy transfer between 1 H and P impossible. A more reasonable model is that energy transfer occurs sequentially, first from 1 H to B and then from 1 B to P …”

Section: Discussionmentioning

confidence: 99%

Excited State Energy Transfer Pathways in Photosynthetic Reaction Centers. 1. Structural Symmetry Effects

1996

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Ultrafast excited state energy transfer to the primary electron donor or special pair in photosynthetic reaction centers has been measured following excitation of the lowest electronic state of the other chromophores. This was achieved by observing the rise time and induced anisotropy in the spontaneous fluorescence from the special pair using fluorescence up-conversion at 85 K. Very fast energy transfer is observed when exciting either of the bacteriochlorophyll monomers. Energy transfer from the bacteriopheophytins, which are considerably further from the special pair than the bacteriochlorophyll monomers, is about 50% slower. The rate, distance, and temperature dependence of energy transfer in both cases are very different from what is predicted by conventional Fo ¨rster dipole-dipole theory. By working at low temperature and with the reaction center mutant (M)L214H (the beta mutant), which assembles with a bacteriochlorophyll monomer in place of a bacteriopheophytin in the H L binding site (the location of the primary electron acceptor), it is possible to selectively initiate the energy transfer process on the functional and nonfunctional sides of the reaction center. The observed rates of energy transfer to the special pair are found to be similar. Thus energy transfer rates are comparable on the functional and nonfunctional sides, while electron transfer rates differ by at least 2 orders of magnitude. This suggests that the dominant source of functional asymmetry for electron transfer involves differences in the association of the functional-side chromophores with their environment (e.g. free and reorganization energy differences), rather than differences in electronic coupling.

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Section: Discussionmentioning

confidence: 99%