Inactivation of [Fe-Fe]-Hydrogenase by O(2). Thermodynamics and Frontier Molecular Orbitals Analyses

The oxidation of H-cluster in gas phase, and in aqueous enzyme phase, has been investigated by means of quantum mechanics (QM) and combined quantum mechanics-molecular mechanics (QM/MM). Several potential reaction pathways (in the above mentioned chemical environments) have been studied, wherein only the aqueous enzyme phase has been found to lead to an inhibited hydroxylated cluster. Specifically, the inhibitory process occurs at the distal iron (Fe(d)) of the catalytic H-cluster (which is also the atom involved in H(2) synthesis). The processes involved in the H-cluster oxidative pathways are O(2) binding, e(-) transfer, protonation, and H(2)O removal.We found that oxygen binding is non-spontaneous in gas phase, and spontaneous for aqueous enzyme phase where both Fe atoms have oxidation state II; however, it is spontaneous for the partially oxidized and reduced clusters in both phases. Hence, in the protein environment the hydroxylated H-cluster is obtained by means of completely exergonic reaction pathway starting with proton transfer.A unifying endeavor has been carried out for the purpose of understanding the thermodynamic results vis-à-vis several other performed electronic structural methods, such as frontier molecular orbitals (FMO), natural bond orbital partial charges (NBO), and H-cluster geometrical analysis. An interesting result of the FMO examination (for gas phase) is that an e(-) is transferred to LUMO(alpha) rather than to SOMO(beta), which is unexpected because SOMO(beta) usually resides in a lower energy rather than LUMO(alpha) for open-shell clusters.