| Sumario: | 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, Ca, H, H, and O) 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., Hand CN) than π-acidic ligands (e.g., CO and O). 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 Ca, acyl, and pyridinol ligands present in the active site. In addition to normal d-,π backbonding, 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 Hligand (where 81% of the electron density of His 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, highlight the importance of a proposed structural reorganization during catalysis.
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