Protoporphyrin IX iron(II) revisited. overview of the Mössbauer spectroscopic parameters of low spin porphyrin iron(II) complexes Part II

Abstract

Mössbauer parameters of low spin six coordinate [Fe(II)(Por)L₂] and [Fe(II)(Por)LX] complexes (where Por is PPIX or a synthetic porphyrin; L is OH⁻, H₂O, a nitrogenous base, CCl₂, CO or CN⁻ and X is a different ligand to L) are discussed. Our point charge calculation approach is extended to investigate how the axial ligands and the four porphyrinato-N atoms generate the observed quadrupole splittings (ΔE_Q). Partial quadrupole splitting (p.q.s.) and partial centre shift (p.c.s.) values are derived and discussed for all the axial ligands studied herein. It is shown that when the D₄ₕ symmetry breaks down due to porphyrin ring ruffling, the point charge calculations no longer apply. This happens for many synthetic porphyrins but not for the uniquely important [Fe(II)(PPIX)] moiety, (which is ubiquitous in nature). Thus, synthetic porphyrins (like TPP) cannot always act as perfect proxies for natural haem proteins. In all the haem proteins containing [Fe(II)(PPIX)] moieties the haem is planar. Calculations indicate that if H₂O were bound to the [Fe(II)(PPIX))] present in deoxyhaemoglobin or deoxy myoglobin, then these Fe(II) atoms would be low spin. In contrast for synthetic and natural haem proteins containing haem b (where the haem is connected to the protein backbone via two cysteine residues) the haem plane is ruffled (probably because of the methionine linkages transferring force and controlling the stereochemistry of the haem plane, and thereby its chemistry). For the [Fe(II)(TPP)(CN)₂] and [Fe(II)(TPP)(1-MeIm)(CN)] the calculated CN⁻ ligand p.q.s values are suspect and probably show that the lattice charge is affecting them.