Defining Intermediates of Nitrogenase MoFe Protein during N2 Reduction under Photochemical Electron Delivery from CdS Quantum Dots

Coupling the nitrogenase MoFe protein to light-harvesting semiconductor nanomaterials replaces the natural electron transfer complex of Fe protein and ATP and provides low-potential photoexcited electrons for photocatalytic N-2 reduction. A central question is how direct photochemical electron delivery from nanocrystals to MoFe protein is able to support the multielectron ammonia production reaction. In this study, low photon flux conditions were used to identify the initial reaction intermediates of CdS quantum dot (QD):MoFe protein nitrogenase complexes under photochemical activation using EPR. Illumination of CdS QD:MoFe protein complexes led to redox changes in the MoFe protein active site FeMo-co observed as the gradual decline in the E(0 )resting state intensity that was accompanied by an increase in the intensity of a new "g(eff) = 4.5" EPR signal. The magnetic properties of the g(eff) = 4.5 signal support assignment as a reduced S = 3/2 state, and reaction modeling was used to define it as a two-electron-reduced "E-2" intermediate. Use of a MoFe protein variant, beta-188(Cy)(s), which poises the P cluster in the oxidized P+ state, demonstrated that the P cluster can function as a site of photoexcited electron delivery from CdS to MoFe protein. Overall, the results establish the initial steps for how photoexcited CdS delivers electrons into the MoFe protein during reduction of N-2 to ammonia and the role of electron flux in the photochemical reaction cycle.