Publication date: 8th October 2020
In our work, we study and compare the electron transfer (ET) reactions inside the photosystem I (PSI) cores from different photosynthetic organisms prepared in solution and immobilized on conducting glass (fluorine-doped tin oxide; FTO) forming a biophotoelectrode. The decay kinetics and the time scales of ET recombination reactions in the PSI incorporated in the photovoltaic systems define the amount of photocurrent produced/photoactivity of PSI. In the previous work performed in our group, the photovoltaic efficiency turned out to be low (below 1% of absorbed photons to generated photoelectrons) when PSI from cyanobacterium Synechocystis sp. PCC 6803 was used as a light sensitive material in a semiartificial solar cell.[1] One of the hypothetical reasons for this low efficiency can be disturbed ET inside the PSI and, if so, an accurate understanding of the decay kinetics of P700+ (often caused by charge recombination reactions) is necessary. The ET in the cyanobacterial PSI suspended in a buffer solution was studied by transient absorption spectroscopy in the micro- to millisecond time domain with probing light of 700-nm wavelength (ground state absorption maximum of P700). The results of the preliminary measurements of charge recombination in PSI demonstrates that the electron from excited P700 reaches the final electron acceptor only in a small fraction of proteins (a few percent; a constant component resolved in a ~50-ms time window). The remaining faster phases (dominated by a ~1-ms component) observed were assigned to the charge recombination reactions (back ET to P700+) from the intermediate ET acceptors occurring in the disturbed proteins. Studies of P700+ decay by transient absorption at 700 nm were thus used as a fingerprint to reveal whether the electron localized on any of the intermediate ET cofactors recombines with the hole on P700⁺ or is transferred forward to the final electron acceptor, FA/FB, from which it can be transferred further outside PSI, and thus may contribute to the photocurrent in a photovoltaic system.
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