Light-Mediated Hydrogen Generation in Photosystem I: Attachment of a Naphthoquinone-Molecular Wire-Pt Nanoparticle to the A1A and A1B Sites

The molecular wire-appended naphthoquinone 1-[15-(3-methyl-1,4-naphthoquinone-2-yl)]pentadecyl disulfide [(NQ(CH2)(15)S)(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 head-group 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(CH2)(15)S-Pt nanoconstruct evolves dihydrogen at a rate of 67.3 mu mol of H-2 (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)(15)S)(2) or the Pt nanoparticle is absent. Time-resolved optical studies of the PS I-NQ(CH2)(15)S-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)(16)S-Pt nanoconstruct depends strongly on the concentration the sacrificial electron donor cytochrome c(6). These observations can be explained if the iron-sulfur clusters are involved in stabilizing the electron; the similar to 50 ms residence time of the electron on F-A or F-B is sufficiently long to allow cytochrome c(6) to reduce P-700(+), thereby eliminating the recombination channel. In the absence of P-700(+), slow electron transfer through the molecular wire to the Pt catalyst can occur, and hence, H-2 evolution is observed.