Density Functional Theory Calculations on the Mononuclear Non-Heme Iron Active Site of Hmd Hydrogenase: Role of the Internal Ligands in Tuning External Ligand Binding and Driving H2 Heterolysis

DFT calculations on active-site models of the non-heme Fe site of Hmd hydrogenase are reported. Binding of several biologically relevant ligands (e.g., CN-, CO, H-, H-2, and O-2) to the active site of Hmd was investigated using a method that reproduced the geometric and vibrational properties of the resting site. The results indicate that this neutral ferrous active site has higher affinity toward anionic ligands (e.g., H- and CN-) than pi-acidic ligands (e.g., CO and O-2). Natural population analysis and molecular orbital analysis revealed that this is due to extensive delocalization of electron density into the low-lying unoccupied orbitals of the CO, acyl, and pyridinol ligands present in the active site. In addition to normal d-pi back-bonding, metal 3d orbital-mediated charge transfer from occupied ligand orbitals to the unoccupied orbitals of the internal ligands was observed. This charge transfer leads to systematic variations in the experimentally observed C-O stretching frequencies. Protonation of the thiolate ligand present in the active site significantly enhances these anion ligand binding affinities. In fact, the calculated vibrational frequencies indicate that CN- binding is possibly associated with protonation of the thiolate ligand. The high affinity for binding of the anionic H- ligand (where 81% of the electron density of H- is delocalized into the active site) is calculated to play a dominating role in the H-H bond heterolysis step during catalysis. The binding energies of these ligands relative to the substrate, H-2, highlight the importance of a proposed structural reorganization during catalysis.