Comparison of excitation energy transfer in cyanobacterial photosystem I in solution and immobilized on conducting glass

Abstract

Excitation energy transfer in monomeric and trimeric forms of photosystem I (PSI) from the cyanobacterium Synechocystis sp. PCC 6803 in solution or immobilized on FTO conducting glass was compared using time-resolved fluorescence. Deposition of PSI on glass preserves bi-exponential excitation decay of ~4–7 and ~21–25 ps lifetimes characteristic of PSI in solution. The faster phase was assigned in part to photochemical quenching (charge separation) of excited bulk chlorophylls and in part to energy transfer from bulk to low-energy (red) chlorophylls. The slower phase was assigned to photochemical quenching of the excitation equilibrated over bulk and red chlorophylls. The main differences between dissolved and immobilized PSI (iPSI) are: (1) the average excitation decay in iPSI is about 11 ps, which is faster by a few ps than for PSI in solution due to significantly faster excitation quenching of bulk chlorophylls by charge separation (~10 ps instead of ~15 ps) accompanied by slightly weaker coupling of bulk and red chlorophylls; (2) the number of red chlorophylls in monomeric PSI increases twice—from 3 in solution to 6 after immobilization—as a result of interaction with neighboring monomers and conducting glass; despite the increased number of red chlorophylls, the excitation decay accelerates in iPSI; (3) the number of red chlorophylls in trimeric PSI is 4 (per monomer) and remains unchanged after immobilization; (4) in all the samples under study, the free energy gap between mean red (emission at ~710 nm) and mean bulk (emission at ~686 nm) emitting states of chlorophylls was estimated at a similar level of 17–27 meV. All these observations indicate that despite slight modifications, dried PSI complexes adsorbed on the FTO surface remain fully functional in terms of excitation energy transfer and primary charge separation that is particularly important in the view of photovoltaic applications of this photosystem.

Photovoltaic activity of electrodes based on intact photosystem I electrodeposited on bare conducting glass

“…Cyanobacterial PSI immobilized directly on FTO conducting glass has recently been shown by ultrafast optical spectroscopy to preserve excitation quenching properties very similar to those of the native PSI in solution (Szewczyk et al 2017(Szewczyk et al , 2018. The detailed analysis has shown that the observed modest acceleration of the excitation decay in immobilized PSI (by a few ps) may be assigned to the crowding effect similar to that reported for aggregated LHCII (Ruban et al 2007).…”

“…In the following, we present photovoltaic data obtained from the system containing simple PSI-FTO photoelectrode identical to that one described in Szewczyk et al (2017Szewczyk et al ( , 2018 and propose a mechanism of the anodic and cathodic photocurrent generation in this system. The advantage of this system is the functionality of immobilized PSI proven to be highly intact at the level of excitation decay and charge separation.…”

“…Synechocystis sp. PCC 6803 cell cultures were grown and trimeric PSI particles were isolated and immobilized on the FTO glass as described in Szewczyk et al (2017). Shortly, immobilization was realized by drop-casting of PSI solution containing 10 mM Bis Tris pH 7.0 (typically, 30-40 µl of absorbance or optical density OD 679nm,1 cm ≈ 1) previously dialyzed to decrease the concentration of detergent used for isolation (n-dodecyl β-d-maltoside, β-DM).…”

“…Even more likely is that the electrons are injected to the FTO substrate from a fraction of Chls which are energetically uncoupled from the PSI reaction center. Such Chls are commonly observed in virtually all the PSI preparations (usually a few percent of all PSI Chls are uncoupled Chls; (Szewczyk et al 2020;Gobets et al 2001;Savikhin et al 1999;Holzwarth et al 2005;Quiniou et al 2015) and were also detected in the PSI-FTO electrodes (Szewczyk et al 2017(Szewczyk et al , 2018). The excited state lifetime of the uncoupled Chls is extended to ~ 0.5-5 ns-one to two orders of magnitude longer than the excited state lifetime of Chls in the fully intact PSI.…”

“…What differs various PSI-containing photoelectrodes described in the literature is: (1) the substrate on which PSI is deposited, (2) the method of PSI immobilization on the substrate, (3) the way by which the electric contact between PSI and the substrate as well as between PSI and the counter electrode is achieved, (4) the species from which the PSI is extracted and the type of the PSI preparation. The electrode substrates used for PSI immobilization are gold (Ciesielski et al 2008(Ciesielski et al , 2010aMukherjee et al 2010;Stieger et al 2016a), graphene (Gunther et al 2013;Feifel et al 2015), titanium dioxide (Mershin et al 2012;Gordiichuk et al 2014;Shah et al 2015;Yu et al 2015;Gizzie et al 2015), or a conducting glass, FTO or ITO (Indium-doped Tin Oxide) (Ocakoglu et al 2014;Stieger et al 2016b;Szewczyk et al 2017Szewczyk et al , 2018. The substrate may have a flat surface or a porous structure to increase the number of attached proteins and thus to maximize the photocurrent.…”