X-ray absorption spectroscopic studies of high valent iron porphyrins. Horseradish peroxidase compounds I and II and synthetic models

X-ray absorption edge and extended x-ray absorption fine structure studies have been undertaken on resting (ferric) horseradish peroxidase, HRP compound I (HRP-I), HRP compound II (HRP-II), and several highly oxidized synthetic iron porphyrins that may have relevance as models for the iron site in horseradish peroxidase. The energies of the absorption edges are consistent with an Fe(IV) formulation for the highly oxidized species. The shapes of the absorption edges further support the assignment of HRP-I and one of the model compounds as Fe(IV)-porphyrin pi-cations. The edge shapes also demonstrate that the iron sites in the model porphyrins are not identical to the iron sites in the enzyme. Preliminary curve fitting analysis of the extended x-ray absorption fine structure clearly indicates the presence of two different nearest neighbor distances for the iron, both in HRP-I and HRP-II, as well as in two of the highly oxidized model porphyrins. These distances are interpreted as an iron-porphyrin nitrogen distance and as a short (approximately 1.6 A) iron-oxygen distance.