Reduced minimum model for the photosynthetic induction processes in photosystem I

• A detailed kinetic model of PS I is systematically reduced to coarse-grained model.
• Characteristic pattern of I820 signal in induction time is reproduced appropriately.
• The pattern of I820 reflects the accumulations of P700, Pc+ and P700FA-FB-.
• Charge recombination processes become active when PS I is doubly reduced.

Photosystem I (PS I) is one of the most important protein complexes for photosynthesis, which is present in plants, algae and cyanobacteria. A variety of mechanisms for environmental response in and around PS I have been elucidated experimentally and theoretically. During the photosynthetic induction time, the congestion of electron occurs in PS I and then the over-reduced PS I states are realized. This means that the degree of freedom of the redox states of PS I becomes large and thus the understanding of phenomena based on the model describing PS I in the state space becomes difficult. To understand the phenomena intuitively, we have reduced the complicated PS I model which has the multi-timescale property for electron and excitation-energy transfer processes into a simple one which has only the mono-timescale property through the use of hierarchical coarse-graining (HCG) method. The coarse-grained model describes the state of PS I by seven variable states, while the original model describes the PS I by 3 × 27 (= 384) states. Based on the derived model, the I820 (820 nm transmittance signal) curve in photosynthetic induction term, which indicates the accumulations of P700+ and Pc +, is simulated and analyzed in comparison with experiment. With respect to this signal curve, it is revealed that the initial increase up to the shoulder at 10-3 s, the increase from that point to the peak at 2 × 10-2 s, and the decay after that peak reflect the accumulations of P700+, Pc+ and P700FA-FB- (PS I state in which P700 , FA- and FB- are observed.), respectively. Besides, the important role of the charge recombination processes from P700+ A0A- and P700+ A1A- states for the dissipation of the extra absorbed energy in photosynthetic induction period is confirmed.