Interfacing Photosystem I Reaction Centers with a Porous Antimony-Doped Tin Oxide Electrode to Perform Light-Driven Redox Chemistry

A biohybrid electronic system incorporating photosystem I (PSI) on a porous antimony-doped tin oxide electrode has been developed. The resulting light-powered reduced nicotinamide adenine dinucleotide phosphate (NADPH) was coupled to a dehydrogenase system for the enzymatic reduction of a reporter molecule. Cytochrome c was used both to orient the PSI reaction centers and to act as a conduit for transporting electrons from the electrode to PSI. Photocurrent was generated upon illumination of the PSI/electrode system itself at microamp/cm2 levels, with reduced oxygen as the primary carrier. When the PSI-cytochrome c-coated electrode system was coupled with ferredoxin, ferredoxin–NADP+ reductase (FNR), and NADP+, NADPH was generated, and resazurin was reduced to fluorescent resorufin. However, even though diaphorase was added to catalyze the dye reduction by NADPH, dye reduction took place in the absence of diaphorase. Indeed, significant dye reduction occurred in the absence of FNR as well. Subsequent analysis revealed that direct, light-dependent reduction of NADPH took place and that resazurin could be reduced by NADPH when resazurin was excited by light. Based on the low overall quantum yield of photoreduction, it is likely that the vast majority of NADPH produced near the electrode was rapidly reoxidized.