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

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.