Engineered polypeptide around nano-sized manganese–calcium oxide as an artificial water-oxidizing enzyme mimicking natural photosynthesis: Toward artificial enzymes with highly active site densities

The solar energy is intermittent, and thus, to be practical at a huge scale, will require a large capability for energy storage. One approach involves artificial photosynthesis to drive solar energy for water splitting into hydrogen or to reduce CO₂ to reduced carbon fuels. In such reactions, cheap electrons from water oxidation are critical. Herein we aim to design and synthesize an artificial water-oxidizing enzyme with highly active site densities, report on nano-sized Mn–Ca oxide in two engineered polypeptides (Arg-Arg-Glu-Glu-Glu-Glu-Arg-Arg and Tyr-Tyr-Tyr-Glu-Glu-Glu-Glu-His-Tyr-Tyr-Tyr) as structural models for biological water-oxidizing site in plants, algae, and cyanobacteria. The compounds were synthesized by a simple procedure and characterized with multiple methods. Using Nafion, electrochemical studies show the peptide has an important effect on the potential for Mn(III)/Mn(IV) oxidation on Mn–Ca oxide and it is decreased in the presence of the polypeptide. We also found that the peptide has an important role on morphologies of Mn–Ca oxide.