Heme ferrous–hydroperoxo complexes: some theoretical considerations

We report density functional calculations on complexes of ferrous hemes with hydroperoxide, where the axial ligand trans to OOH− is imidazole, thiolate, or phenoxide. The geometrical parameters and charge distributions within the Fe–O–O–H moiety are identical between the ferrous complexes reported here and their ferric counterparts previously described, even though the latter contain one unpaired electron on iron as opposed to the former, which are diamagnetic. The extra negative charge upon going from a formally ferric state to formally ferrous appears to be distributed essentially on the porphyrin. These findings support recent experimental data showing that the ferrous state of certain hemoproteins can interact with peroxides in a catalytically competent fashion, cleaving the O–O bond heterolytically in a manner reminiscent of the “canonical” ferric–peroxo complexes, and contrary to any expectations based on the Fenton concept commonly invoked in non-heme chemistry.