Wiring photosystem i for electron transfer to a tethered redox dye

We have recently reported the assembly of biological/organic hybrid nanoconstructs that generate H₂ in the light (Grimme et al., Dalton Trans., 2009, 10106, Lubner et al., Biochemistry, 2010, 49, 10264). In these constructs, electrons are transferred directly from a photochemical module, Photosystem I (PS I), to a catalytic module, either a Pt nanoparticle (NP) or an [FeFe]-hydrogenase (H₂ase), through the use of a covalently attached molecular wire. In neither case are any spectroscopic changes visible that would allow electron transfer to be monitored between the photochemical and catalytic modules. In this study, the catalytic module was replaced with an organic cofactor consisting of 1-(3-thiopropyl)-1′-(methyl)-4,4′- bipyridinium chloride that allowed electron transfer to be measured to a spectroscopically observable marker. EPR and optical spectroscopy showed that the tethered redox cofactor was attached to PS I through the F_B cluster of PsaC. Under steady-state illumination, the rate of reduction of the 4,4′-bipyridinium cofactor was comparable to the rate of H₂ evolution observed for the PS I - molecular wire - Pt-NP and PS I - molecular wire - [FeFe]-H₂ase nanoconstructs. These observations provide proof-of-concept for incorporating a redox cofactor in the molecular wire, thereby setting the stage for monitoring the rate and yield of electron transfer between PS I and the tethered [FeFe]-H₂ase.