Maximizing the efficiency of ferritin as a photocatalyst for applications in an artificial photosynthesis system

Alternate fuel sources are becoming increasingly important as the reserve of fossil fuels decrease. We describe a photosynthesis mimic that is capable of extracting electrons from sacrificial electron donors. This model is based on the bio-photo-catalyst ferritin. Ferritin is an iron storage protein that naturally sequesters ferrihydrite inside a spherical 12 nm protein shell. Ferrihydrite is a semi-conductor that functions as a photo-catalyst in aqueous solvents. Ferritin has been shown to photoreduce Au³⁺ to form Au(0) nanoparticles. Citrate acts as a sacrificial electron donor to supply electrons for the photoreduction. We describe studies designed to understand the mechanism of this catalyst in order to improve the efficiency of the reaction. We have developed a spectrophotometric assay to simultaneously illuminate the sample and kinetically monitor the formation of products of Au³⁺ reduction. We report that buffers containing sulfur significantly increase the rate of the reactions. Control reactions with colloidal ferrihydrite nanoparticles do not catalyze the photochemical reaction, but produce a black precipitate indicating that the protein shell has an important function in nanoparticle formation.