Magnetic properties of [FeFe]-hydrogenases: A theoretical investigation based on extended QM and QM/MM models of the H-cluster and its surroundings

In the present contribution, we report a theoretical investigation of the magnetic properties of the dihydrogen-evolving enzyme [FeFe]-hydrogenase, based on both DFT models of the active site (the H-cluster, a Fe₆S ₆ assembly including a binuclear portion directly involved in substrates binding), and QM/MM models of the whole enzyme. Antiferromagnetic coupling within the H-cluster has been treated using the broken-symmetry approach, along with the use of different density functionals. Results of g value calculations turned out to vary as a function of the level of theory and of the extension of the model. The choice of the broken-symmetry coupling scheme also had a significant influence on the calculated g values, for both the active-ready (H_ox) and the CO-inhibited (H_ox-CO) enzyme forms. However, hyperfine coupling-constant calculations were found to provide more consistent results. This allowed us to show that the experimentally detected delocalization of an unpaired electron at the binuclear subcluster in Desulfovibrio desulfuricans H_ox is compatible with a weak interaction between the catalytic centre and a low-weight exogenous ligand like a water molecule.