Ligand-bound S = 1/2 FeMo-Cofactor of Nitrogenase: Hyperfine Interaction Analysis and Implication for the Central Ligand X Identity

Broken symmetry density functional theory (BS-DFT) has been used to address the hyperfine parameters of the single atom ligand X, proposed to be coordinated by six iron ions in the center of the paramagnetic FeMo-cofactor (FeMoco) of nitrogenase. Using the X = N alternative, we recently found that any hyperfine signal from X would be small (calculated Aiso(X = 14N) = 0.3 MHz) due to both structural and electronic symmetry properties of the [Mo−7Fe−9S−X] FeMoco core in its resting S = 3/2 state. Here, we extend our BS-DFT approach to the 2e− reduced S = 1/2 FeMoco state. Alternative substrates coordinated to this FeMoco state effectively perturb the electronic and/or structural symmetry properties of the cofactor. Using an example of an allyl alcohol (H2C═CH−CH2−OH) product ligand, we consider three different binding modes at single iron site and three different BS-DFT spin state structures and show that this binding would enhance the key hyperfine signal Aiso(X) by at least 1 order of magnitude (3.8 ≤ Ais...