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 A_iso(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 A_iso(X) by at least 1 order of magnitude (3.8 ≤ A_iso(X = ¹⁴N) ≤ 14.7 MHz), and this result should not depend strongly on the exact identity of X (nitrogen, carbon, or oxygen). The interstitial atom, when the nucleus has a nonzero magnetic moment, should therefore be observable by ESR methods for some ligand-bound FeMoco states. In addition, our results illustrate structural details and likely spin-coupling patterns for models for early intermediates in the catalytic cycle.