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

A theoretical model of the far-red-light-adapted photosystem I (PSI) reaction center (RC) complex of a cyanobacterium, Acaryochloris marina (AmPSI), was constructed based on the exciton theory and the recently identified molecular structure of AmPSI by Hamaguchi et al. ( Nat. Commun., 2021, 12, 2333). A. marina performs photosynthesis under the visible to far-red light (400–750 nm), which is absorbed by chlorophyll d (Chl- d ). It is in contrast to the situation of all the other oxygenic photosynthetic processes of cyanobacteria and plants, which contains chlorophyll a (Chl- a ) that absorbs only 400–700 nm visible light. AmPSI contains 70 Chl- d, 1 Chl- d ′, 2 pheophytin a (Pheo- a ), and 12 carotenoids in the currently available structure. A special pair of Chl- d /Chl- d ′ acts as the electron donor (P740) and two Pheo- a act as the primary electron acceptor A 0 as the counterparts of P700 and Chl- a, respectively, of Chl- a -type PSIs. The exciton Hamiltonian of AmPSI was constructed considering the excitonic coupling strength and site energy shift of individual pigments using the Poisson-TrESP (P-TrESP) and charge density coupling (CDC) methods. The model was constructed to fit the experimentally measured spectra of absorption and circular dichroism (CD) spectra during downhill/uphill excitation energy transfer processes. The constructed theoretical model of AmPSI was further compared with the Chl- a -type PSI of Thermosynechococcus elongatus (TePSI), which contains only Chl- a and Chl- a ′ . The functional properties of AmPSI and TePSI were further examined by the in silico exchange of Chl- d by Chl- a in the models.