Comparison between the Light-Harvesting Mechanisms of Type‑I Photosynthetic Reaction Centers of Heliobacteria and Photosystem I: Pigment Site Energy Distribution and Exciton State

The
photosynthetic pigment system on the anoxygenic type-I reaction
center of heliobacteria (hRC), which has a symmetrical structure,
was analyzed. The excitonic coupling among all of the bacteriochlorophyll g (BChl-g) molecules and chlorophyll a (Chl-a) molecules and the site energy
for each pigment were calculated using Poisson-TrESP and the charge
density coupling (CDC) methods. The obtained theoretical model reproduced
the optical absorption and circular dichroism spectra. It also interpreted
the decay-associated spectra upon the photoselective laser excitation,
which represent the ultrafast excitation energy migration process,
better than the simpler hRC models that assumed constant pigment site
energy shifts. Spatial movements of excitation energy on pigments
on hRC upon the laser excitation were visualized. The energy dissipation
by carotenoid molecules in hRC was also predicted. The hRC model was
compared with the model of the reaction center of cyanobacterial photosystem
I (PSI), which carries 95 Chl-a on the analogous
type-I structure with significantly different amino-acid sequences,
pigment species, and output redox powers. It is shown that the locations
and site energy values of pigments in hRC resemble those in the core
of PSI, except for the red-Chl-a sites, suggesting
common functional mechanisms implicated in their evolution.