Studies on the binding of nitrogenous bases to protoporphyrin IX iron(II) in aqueous solution at high pH values part II. Aliphatic nitrogen ligands

Abstract

The first complimentary studies using Mössbauer spectroscopy and ligand titrations on protoporphyrin IX iron(II) with a series of aliphatic nitrogenous ligands are reported. Binding constants 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 all the aliphatic nitrogen bases yielded low spin octahedral complexes. The magnitude of the overall binding constants (logβ 2 values which varied from 1.7 for ethanolamine to 4.26 for n-octylamine) are discussed and related to (i) the p K a values of the free ligands and (ii) to the quadrupole splitting, Δ E Q, of the haem iron. The aliphatic nitrogenous ligands studied here exhibited cooperative binding (except ethanolamine) with Hill coefficients close to 2 for protoporphyrin IX iron(II), but with significantly lower (∼ 1000-fold) affinity than pyridine and imidazole ligands (which we have studied previously). However, these aliphatic and aromatic ligands displayed comparable Δ EQ values (∼ 1.00 to 1.20 mm s −1). The β and Δ E Q values are also discussed in terms of the structure of the ligand.