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

Abstract

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.

Theoretical Model of the Far-Red-Light-Adapted Photosystem I Reaction Center of Cyanobacterium Acaryochloris marina Using Chlorophyll d and the Effect of Chlorophyll Exchange

“…Using the same strategy, we have constructed the exciton models of natural/modified photosynthetic systems of PSI and Heliobacterial RC (hRC) in silico and studied the structure− function relationship. 11,16,17 As a result, our exciton model reproduced the absorption, CD, and decay-associated spectra of hRC and revealed that hRC processes a funnel-like distribution of site energy, in contrast to the non-funnel-like one in TePSI. 17 In this study, we elucidate the light-harvesting mechanism of AmPSI based on its structural information and examine its functional and evolutionary differences.…”

“…11,16,17 As a result, our exciton model reproduced the absorption, CD, and decay-associated spectra of hRC and revealed that hRC processes a funnel-like distribution of site energy, in contrast to the non-funnel-like one in TePSI. 17 In this study, we elucidate the light-harvesting mechanism of AmPSI based on its structural information and examine its functional and evolutionary differences. The P-TrESP/CDC method was used to obtain the excitonic coupling strength and site energy shift of the exciton Hamiltonian of AmPSI to reproduce the absorption/CD spectra.…”

“…It is contrary to the weak correlation detected between hRC and TePSI. 17 In Figure 2b, the value of the negative or positive site energy shifts ΔE m X for each pigment was plotted against the distance from the center of a special pair, shown as a red circle or a blue square in AmPSI and TePSI, respectively. The distance distributions of the site energy shift are compared between AmPSI and TePSI in Figure 2b.…”

“…The number is nearly equal to that of 16 found in hRC and is smaller than 25 found in TePSI. 17 Transient absorption spectra induced by 630 and 740 nm laser pulses were recorded for AmPSI. 54,55 We calculated these spectra with eq 6, as shown in Figure 6.…”

“…The physical properties and spatial arrangements of the pigments in the antenna/RC complexes seem to optimize the efficiencies of excitation energy transfer (EET) and electron transfer (ET) . Extensive experimental/theoretical studies of the elementary processes in RCs have revealed optimization mechanisms under diverse natural environments. − …”