Distance and Potential Dependence of Charge Transport Through the Reaction Center of Individual Photosynthetic Complexes

Abstract

Charge separation and transport through the reaction center of photosystem I (PSI) is an essential part of the photosynthetic electron transport chain. A strategy is developed to immobilize and orient PSI complexes on gold electrodes allowing to probe the complex's electron acceptor side, the chlorophyll special pair P700. Electrochemical scanning tunneling microscopy (ECSTM) imaging and current–distance spectroscopy of single protein complex shows lateral size in agreement with its known dimensions, and a PSI apparent height that depends on the probe potential revealing a gating effect in protein conductance. In current–distance spectroscopy, it is observed that the distance‐decay constant of the current between PSI and the ECSTM probe depends on the sample and probe electrode potentials. The longest charge exchange distance (lowest distance‐decay constant β) is observed at sample potential 0 mV/SSC (SSC: reference electrode silver/silver chloride) and probe potential 400 mV/SSC. These potentials correspond to hole injection into an electronic state that is available in the absence of illumination. It is proposed that a pair of tryptophan residues located at the interface between P700 and the solution and known to support the hydrophobic recognition of the PSI redox partner plastocyanin, may have an additional role as hole exchange mediator in charge transport through PSI.

Light- and Redox-Dependent Force Spectroscopy Reveals that the Interaction between Plastocyanin and Plant Photosystem I Is Favored when One Partner Is Ready for Electron Transfer

“…Plant PSI was immobilized on a thin transparent gold surface via interaction with the synthetic peptide pIQA-Cys designed recently . As mentioned, pIQA-Cys allows to orient the plant PSI unidirectionally, exposing the Pc binding site toward the solution (Figure A).…”

“…Pc-cysteine mutant (Pc-SH) was immobilized onto the AFM probe via a heterobifunctional PEG 27 linker with a length of approximately 8 nm (monomer length ≈ 0.3 nm), , thereby exposing the PSI-binding site toward the sample surface (Figure A). The redox states of PSI and Pc were respectively controlled by illuminating PSI (λ = 690 nm, power = 60 mW·cm –2 ) and using a reducing agent for Pc (sodium ascorbate, 5 mM) , (Figure B). Note that sodium ascorbate can also reduce P700 + but it does so in ∼30 s, − which is orders of magnitude longer than the typical times for Pc Ox reduction (∼3 ms), for P700 photo-oxidation (200 μs, see below), for PSI-Pc ET (10 μs), and also longer than the PSI-Pc (sample-probe) contact time in our experiments (∼25 ms).…”

“…Plant PSI was immobilized on a thin transparent gold surface via interaction with the synthetic peptide pIQA-Cys designed recently. 15 As mentioned, pIQA-Cys allows to orient the plant PSI unidirectionally, exposing the Pc binding site toward the solution (Figure 1A). Pc-cysteine mutant (Pc-SH) 14 was immobilized onto the AFM probe via a heterobifunctional PEG 27 linker with a length of approximately 8 nm (monomer length ≈ 0.3 nm), 16,17 thereby exposing the PSI-binding site toward the sample surface (Figure 1A).…”

“…14 PSI was unidirectionally immobilized to the sample surface, via a cysteine-modified peptide that recognizes the stromal side and exposes the Pc binding site of PSI to the solution. 15 Besides, since the activity of PSI is photoregulated, sufficient photon flux per PSI complex must be provided in its absorption band to ensure that PSI is oxidized before complex formation. To this end, we employed an optically transparent thin gold electrode to anchor PSI and allow the passage of light through the substrate.…”

“…Therefore, the AFM probe was functionalized with unidirectionally immobilized Pc, via an introduced cysteine in the C-terminal region, to expose the ET active site to the solution . PSI was unidirectionally immobilized to the sample surface, via a cysteine-modified peptide that recognizes the stromal side and exposes the Pc binding site of PSI to the solution . Besides, since the activity of PSI is photoregulated, sufficient photon flux per PSI complex must be provided in its absorption band to ensure that PSI is oxidized before complex formation.…”