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

概要

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 (RC) of Heliobacterium modesticaldum (hRC) and compared with those in the photosystem I (PSI) type-I heterodimeric RC. The advanced combination of transition charge of electrostatic potential (TrESP) 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 a 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 scheme at the configuration-interaction singles (CIS) level, which considers the charge-transfer characteristics of the relevant excitonic states. The calculation realized that the reported parameter values for P800 and P700 were 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.

収録
The Journal of Physical Chemistry B Vol. 124, num.2, pp. 389–403 (2020)