Water-Soluble Polyphosphazenes as Carrier Molecules for Iron(III) and Iron(II) Porphyrins

The water-soluble poly(aminophosphazenes) [NP(NHCH₃)₂]_n (1) and [NP(NHCH₃)_x(NHCH₂CH₂CH₂• C₃N₂H₃)_y]_n (2) (containing the 3-(1-imidazolyl)propylamino group) have been investigated as polymeric “carrier” ligands for iron(III) and iron(II) protoporphyrin IX. In the presence of aqueous polymer 1, hemin or heme exists as hematin-hemin hydroxide or bisaquoheme, respectively, with only weak (probably acid-base) binding between the polymer and the metallo-porphyrin. However, polymer 2 binds strongly to both heme and hemin via the imidazole residues. In aqueous solution or as solid films, the presence of the polymeric ligands did not prevent the irreversible oxidation of heme on contact with oxygen. Polymer-heme complexes of polymers 1 and 2, including the CO and oxidized complexes, were characterized by electronic absorption and Mossbauer spectroscopy. Although the Mössbauer data confirmed that Fe(II) oxidation occurred on contact with O₂, the electronic spectra were compatible with reversible dioxygen binding. The apparent “dioxygen cycling” in the solid state was attributed to oxidation of Fe(II) by dioxygen, followed by reduction of the Fe(III) by the polymer or, in some cases, by residual reducing agent.