Excitation-Rate Determines Product Stoichiometry in Photochemical Ammonia Production by CdS Quantum Dot-Nitrogenase MoFe Protein Complexes
Katherine A. Brown, Jesse L. Ruzicka, Hayden Kallas, Bryant Chica, David W. Mulder, John W. Peters, Lance C. Seefeldt, Gordana Duković, Paul W. King
Abstract
The reduction of dinitrogen (N2) to ammonia (NH3) by nitrogenase MoFe protein is coupled to chemically driven electron transfer by nitrogenase Fe protein, where H2 is an obligatory side product. Direct coupling of light-absorbing semiconductor nanocrystals to MoFe protein enables NH3 production from photoexcited electron transfer, replacing Fe protein. Production of H2 and NH3 was measured for CdS quantum dot (QD) MoFe protein complexes illuminated under different excitation rates. 15N-labeling of NH3 production combined with background-corrected H2 production enabled determination of MoFe protein catalysis products. The turnover rates of H2 and NH3 increased with excitation rate, with distinct kinetic responses that show the electron demand for NH3 requires higher excitation rates to overcome the more favored H2 production.