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Excitonic Coupling on Heliobacterial Symmetrical Type-I Reaction Center: Comparison with Photosystem I.

The excitonic couplings among 54 bacteriochlorophylls-g (BChl)-g, 4 BChl-g' and 2 Chl-aF pigments were calculated in the type-I homodimeric reaction center of Heliobacterium modesticaldum (hRC), and compared with that in photosystem I (PSI) type-I heterodimeric reaction center. The advanced combination of transition charge of electrostatic-potential with the Poisson equation (Poisson-TrESP), applied for the first time to the excitonic coupling calculation, gave a reliable model in contrast to a model calculated by simple standard dipole-dipole interaction approximation that was qualitatively valid for hRC but not for PSI. The simplest method for the calculation of the long-range contribution to the excitonic coupling on RCs is shown to be the TrESP method, which considers the distance- and orientation-independent local field/screening correction factor. The excitonic couplings of the special pairs, P800 in hRC and P700 in PSI, are also calculated by the fragment excitation difference (FED) scheme at the configuration-interaction singles level, which considers the charge-transfer characteristics of the relevant excitonic states. The calculation realized the reported parameter values for P800 and P700 better than the Poisson-TrESP calculation. Virtual exchanges between Chl-a and BChl-g on hRC and PSI indicated that the difference between hRC and PSI arises from the different electronic-structures of Chl-a and BChl-g pigments themselves and the different arrangements on hRC and PSI. The contributions of excitonic couplings to the functional properties and evolutionary modifications of hRC and PSI are also discussed.