Excitation-Rate Determines Product Stoichiometry in Photochemical Ammonia Production by CdS Quantum Dot-Nitrogenase MoFe Protein Complexes

Abstract

The reduction of dinitrogen (N<sub>2</sub>) to ammonia (NH<sub>3</sub>) by nitrogenase MoFe protein is coupled to chemically driven electron transfer by nitrogenase Fe protein, where H<sub>2</sub> is an obligatory side product. Direct coupling of light-absorbing semiconductor nanocrystals to MoFe protein enables NH<sub>3</sub> production from photoexcited electron transfer, replacing Fe protein. Production of H<sub>2</sub> and NH<sub>3</sub> was measured for CdS quantum dot (QD) MoFe protein complexes illuminated under different excitation rates. <sup>15</sup>N-labeling of NH<sub>3</sub> production combined with background-corrected H<sub>2</sub> production enabled determination of MoFe protein catalysis products. The turnover rates of H<sub>2</sub> and NH<sub>3</sub> increased with excitation rate, with distinct kinetic responses that show the electron demand for NH<sub>3</sub> requires higher excitation rates to overcome the more favored H<sub>2</sub> production.