Imidazole-Ligated Compound I Intermediates:  The Effects of Hydrogen Bonding

An unresolved issue of peroxidase activity is the role played by the hydrogen bond between the proximal histidine ligand and a highly conserved aspartate group. It has been postulated that this hydrogen bond imparts imidazolate character to the axial ligand, increasing its donating ability and stabilizing the high valent compound I intermediate. A general feature of biological compound I intermediates is that they display weaker spin couplings than model systems. To understand how the Asp-His hydrogen bond affects the electronic structure of the compound I intermediate and to establish a connection between it and the decreased coupling strengths observed in peroxidases, we have perfomed density functional calculations on imidazole- and imidazolate-ligated perferryl porphyrin species. It was found that the imidazolate compound I intermediate possesses an imidazolate-based radical, while the imidazole compound I intermediate possesses a porphyrin-based radical. Delocalizaton of the radical onto the axial ligand was found to reduce the strength of the spin coupling.