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 CM-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.

Automated summary

The study analyzed the photosynthetic pigment system on the anoxygenic type-I reaction center of heliobacteria, showing excitonic coupling, energy migration processes, and spatial movements upon laser excitation. A theoretical model was developed to better explain the optical absorption and circular dichroism spectra. Comparison with the cyanobacterial PSI model revealed similarities in pigment locations and energy values, suggesting common functional mechanisms in evolution.