Theoretical Model of Exciton States and Ultrafast Energy Transfer in Heliobacterial Type I Homodimeric Reaction Center

A simple
theoretical model of exciton dynamics was proposed to
interpret the fast excitation energy-transfer process in the type
I homodimeric reaction center of Heliobacterium modesticaldum (hRC); this structure was recently identified and shown to resemble
that of the plant/cyanobacterial photosystem I (PSI) reaction center.
The exciton state model, which mainly relies on the geometries of
54 bacteriochlorophyll (BChl) g, 4 BChl-g′, and 2 chlorophyll (Chl) a on hRC and assumes
constant site energy values for the pigments, reproduced the absorption
spectrum of hRC rather well. The model also enabled numerical analysis
of the exciton dynamics on hRC, which can be compared with the decay-associated
spectra obtained by the laser spectroscopy experiments. The model
indicates that the stronger transition–dipole moment on BChl-g contributes to the faster energy transfer due to the higher
coherency of the delocalized exciton states on hRC compared to that
on PSI that arranges Chl-a at almost homologous locations.