Studies on the binding of nitrogenous bases to protoporphyrin IX iron(II) in aqueous solution at high pH values. Part I. Pyridine and imidazole ligands

Abstract

The binding constants between protoporphyrin IX iron(II) and a series of pyridine ligands and imidazole have been determined by titration of the metal complex with these ligands. The adduct formation was monitored through optical spectroscopy. Parallel Mössbauer spectroscopic experiments were conducted to monitor the electronic environment around the central iron atom in these adducts. Both spectroscopic methods indicated that, with the exception of pyridine- N-oxide, all nitrogen bases yielded low spin octahedral complexes. The magnitude of the overall binding constants (β 2 values) are discussed and related to (i) the p K a values of the free ligands and (ii) to the quadrupole splitting, Δ E o, of the haem iron. The β 2 and Δ E o values are also discussed in terms of the structure of the ligand.