Light-mediated hydrogen generation in photosystem I: Attachment of a naphthoquinone-molecular wire-Pt nanoparticle to the A_1A and A _1B sites

The molecular wire-appended naphthoquinone 1-[15-(3-methyl-1,4- naphthoquinone-2-yl)]pentadecyl disulfide [(NQ(CH₂) ₁₅S)₂] has been incorporated into the A_1A and A_1B sites of Photosystem I (PS I) in the menB variant of Synechocystis sp. PCC 6803. Transient electron paramagnetic resonance studies show that the naphthoquinone headgroup displaces plastoquinone-9 from the A _1A (and likely A_1B) sites to a large extent. When a Pt nanoparticle is attached to the molecular wire by reductive cleavage of the disulfide and reaction with the resulting thiol, the PS I-NQ(CH ₂)₁₅S-Pt nanoconstruct evolves dihydrogen at a rate of 67.3 μmol of H₂ (mg of Chl)^-1 h^-1 [3.4 e^- (PS I)^-1 s^-1] after illumination for 1 h at pH 6.4. No dihydrogen is detected if wild-type PS I, which does not incorporate the quinone, is used or if either (NQ(CH₂)₁₅S)₂ or the Pt nanoparticle is absent. Time-resolved optical studies of the PS I-NQ(CH₂)₁₅S-Pt nanoconstruct show that the lifetimes of the forward electron transfer to and reverse electron transfer from the iron-sulfur clusters are the same as in native PS I. Thus, electrons are not shuttled directly from the quinone to the Pt nanoparticle during either forward or reverse electron transfer. It is found that the rate of dihydrogen evolution in the PS I-NQ(CH₂)₁₅S-Pt nanoconstruct depends strongly on the concentration the sacrificial electron donor cytochrome c₆. These observations can be explained if the iron-sulfur clusters are involved in stabilizing the electron; the ∼50 ms residence time of the electron on F_A or F_B is sufficiently long to allow cytochrome c ₆ to reduce P₇₀₀⁺, thereby eliminating the recombination channel. In the absence of P₇₀₀⁺, slow electron transfer through the molecular wire to the Pt catalyst can occur, and hence, H₂ evolution is observed.