Catecholate LMCT Bands as Probes for the Active Sites of Nonheme Iron Oxygenases¹

A series of [Fe^IIIL(catecholate)] complexes has been synthesized to correlate the energies of catecholate-to-Fe(III) charge-transfer transitions with the nature of the iron coordination environment. L is a tetradentate tripodal ligand, N(CH₂X)₃, where X can be phenolate, carboxylate, pyridine, or benzimidazole, or a combination thereof, and the resulting ternary complexes are high-spin ferric based on their EPR spectra. The complexes exhibit two ligand-to-metal charge-transfer (LMCT) bands in the 400-900-nm spectral region; these are shown to be catecholate-to-Fe(III) charge transfer in nature by resonance Raman studies. The LMCT bands systematically shift to lower energy as oxyanionic ligands on the tripod are replaced by neutral nitrogen ligands, which is consistent with the increased Lewis acidity of the metal center. These observations have been used to gain insight into the iron coordination environment of rat liver phenylalanine hydroxylase and soybean lipoxygenase, both of which form catechol complexes in their Fe(III) forms. The catechol complexes of these enzymes exhibit spectral features that are similar to those of the synthetic catecholate complexes. Based on the energies of the observed catecholate LMCT bands, it is proposed that the iron sites in phenylalanine hydroxylase and lipoxygenase resemble that of the tripod with two pendant pyridines and one carboxylate. These observations should complement other approaches for deducing the metal coordination environment in oxygenases which lack a visible chromophore, i.e., ligand-field spectral studies which provide information on coordination number and geometry, and EPR studies with NO and ¹⁷O-labeled ligands which provide information on the binding of exogenous ligands.