In recent years, redox active proteins have been investigated for multielectron reductions on dissolved gas to generate energy-abundant products such as methane or ammonia. Many redox enzymes function by the assembly of protein subunits by complex, multivalent interactions, with binding strengths that span from long-range and weak to short-range and near-covalent. Nitrogenase enzymes function through similar interactions by utilizing electrostatic-hydrogen bonding and conformational docking between the Fe protein (FeP) and MoFe protein (MoFeP) in order to initiate redox chemistries, which are driven by adenosine triphosphate (ATP). Substitution of ATP with alternative energy sources, including photocatalysts and electrochemistry in the absence of the FeP, have been shown to generate redox products. In this work, I demonstrate that native nitrogenase interactions can be mimicked between the MoFeP and CdS photocatalysts which leads to improvements in the photoreduction of CO2 by the enzyme. I will first demonstrate the result of reacting MoFeP with CdS nanoparticles that possess either negative, near-neutral, or positive surface charge densities. A strong correlation between the strength of CdS/MoFeP interactions and photodriven CO2 reduction was found. Additionally, the MoFeP studied in this work has two free, individually-functionalizable cysteines that can be tagged with different labels to control the protein orientation and docking at the CdS surface. One of these cysteines (Cys158) is in close proximity (~1.4 nm) to the electron accepting P-cluster of MoFeP and the other cysteine (Cys45) is native to the protein and exists almost twice the distance away from the P-cluster (~3.7 nm). We first show that when both cysteine sites are effectively blocked using iodoacetyl labels, a significant decrease in CO2 reduction was observed as compared to unmodified protein. However, the higher solvent accessibility of site Cys45 over Cys158 allowed us to preferentially modify only Cys45 to promote short-range interactions between Cys158 and CdS. In doing so, we found that as compared to unmodified MoFeP, a measurable gain in CO2 reduction was observed. Therefore, this work demonstrates for the first time the importance of using a combination of weak and strong interactions to ultimately position a key metallosite of the MoFeP within a few nanometers of the photoactive nanocrystal surface to enhance multi-electron transfer between the two for improved photocatalysis.

Mimicking protein interactions for photodriven nitrogenase activation

2019 Fall National Meeting

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