Asymmetry in the Ligand Coordination Sphere of the [FeFe] Hydrogenase Active Site Is Reflected in the Magnetic Spin Interactions of the Aza-propanedithiolate Ligand

[FeFe]
hydrogenases are very active enzymes that catalyze the reversible
conversion of molecular hydrogen into protons and electrons. Their
active site, the H-cluster, contains a unique binuclear iron complex,
[2Fe]H, with CN– and CO ligands as well
as an aza-propane-dithiolate (ADT) moiety featuring a central amine
functionality that mediates proton transfer during catalysis. We present
a pulsed 13C-ENDOR investigation of the H-cluster in which
the two methylene carbons of ADT are isotope labeled with 13C. We observed that the corresponding two 13C hyperfine
interactions are of opposite sign and corroborated this finding using
density functional theory calculations. The spin polarization in the
ADT ligand is shown to be linked to the asymmetric coordination of
the distal iron site with its terminal CN– and CO
ligands. We propose that this asymmetry is relevant for the enzyme
reactivity and is related to the (optimal) stabilization of the iron-hydride
intermediate in the catalytic cycle.