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 poly peptide. We also found that the peptide has an important role on morphologies of Mn-Ca oxide. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.