Mössbauer spectroscopic evidence for molecular complex formation between histidine and μ-oxo-bis(protoporphyrinato IX)iron(III)

Abstract

Compelling Mössbauer spectroscopic evidence is presented here for the formation of molecular complexes of μ-oxo-bis(protoporphyrinato IX)iron(III) ([(PPIXFe) 2O]) with histidine. The quadrupole splitting of [(PPIXFe) 2O] (Δ E Q=0.56±0.01 mm s −1) is modified considerably in the molecular complexes to give Δ E Q=0.43±0.01 mm s −1 in the histidine complex. Histamine and N-α-acetyl histidine also form similar molecular complexes. Mössbauer studies and a molecular scale model suggest that the donor-acceptor complex is formed via π-π charge transfer interaction between the imidazole of histidine with one of the pyrrole rings of the porphyrin. The role of the amino group in the histidine side chain is to form a NH··· −OOCR hydrogen bond with the propionic carboxylate group of protoporphyrin IX. This aligns the imidazole ring of histidine parallel to the pyrrole ring of the porphyrin. It is suggested that the apoprotein conformation in a haem protein may provide such a favourable alignment of an aromatic amino acid residue in the protein pocket so as to favour such a π-π interaction. This is to our knowledge the first Mössbauer spectroscopic study on a molecular π complex involving a porphyrin moiety.