Modeling low-pH hemoproteins

A tetracoordinate ferrous heme (iron-porphyrin) has been proposed as an intermediate at low pH (< 3.0) for respiratory hemoproteins, peroxidases, and model heme complexes. This intermediate is believed to arise via protonation of the N(≥) atom of the proximal histidine and consequent cleavage of the Fe-N(ϵ) bond. To establish a spectral signature for the proposed low-pH tetracoordinate species, we have obtained Soret excitation resonance Raman spectra on samples of crystallographically defined, tetracoordinate iron(II)-octaethylporphyrin (Fe.OEP; S = 1). The high-frequency (greater than or equal to 900 cm−1) resonance Raman spectral features of Fe.OEP are clearly distinct from those of high-spin pentacoordinate or low-spin hexacoordinate ferrous hemes. Rather, they are at frequencies more typically observed for low-spin hexacoordinate ferric porphyrins. Comparative spectral analysis of tetracoordinate Fe.OEP and other proposed tetracoordinate ferrous hemes (e.g. iron(II)-protoporphyrin IX) demonstrates little or no macrocycle effect on the resonance Raman frequencies above 900 cm−1. This work thus serves to provide a testable spectral signature by which the existence of the proposed tetracoordinate biological intermediate may be verified and by which its functional significance may be tested.