Speciation and structure of ferriprotoporphyrin IX in aqueous solution: spectroscopic and diffusion measurements demonstrate dimerization, but not μ-oxo dimer formation

Abstract

Changes in ε ₃₉₃ (the Soret band) of aqueous ferriprotoporphyrin IX [Fe(III)PPIX] with concentration indicate that it dimerizes, but does not form higher aggregates. Diffusion measurements support this observation. The diffusion coefficient of aqueous Fe(III)PPIX is half that of the hydrated monomeric dicyano complex. Much of the apparent instability of aqueous Fe(III)PPIX solutions could be attributed to adsorption onto glass and plastic surfaces. However, ε ₃₄₇ was found to be independent of the aggregation state of the porphyrin and was used to correct for the effects of adsorption. The UV–vis spectrum of the aqueous dimer is not consistent with that expected for a μ-oxo dimer and the ¹H NMR spectrum is characteristic of five-coordinate, high-spin Fe(III)PPIX. Magnetic susceptibility measurements using the Evans method showed that there is no antiferromagnetic coupling in the dimer. By contrast, when the μ-oxo dimer is induced in 10% aqueous pyridine, characteristic UV–vis and ¹H NMR spectra of this species are observed and the magnetic moment is consistent with strong antiferromagnetic coupling. We propose a model in which the spontaneously formed aqueous Fe(III)PPIX dimer involves noncovalent interaction of the unligated faces of two five-coordinate H₂O/HO-Fe(III)PPIX molecules, with the axial H₂O/OH⁻ ligands directed outwards. This arrangement is consistent with the crystal structures of related five-coordinate iron(III) porphyrins and accounts for the observed pH dependence of the dimerization constant and the spectra of the monomer and dimer. Structures for the aqueous dimer are proposed on the basis of molecular dynamics/simulated annealing calculations using a force field previously developed for modeling metalloporphyrins.