Synthesis, Oxygen-Binding Behavior, and Mossbauer Spectroscopy of Covalently Bound Polyphosphazene-Heme Complexes

The water-soluble poly(aminophosphazene) [NPCNHCH3)x(NCH3CH2CH2NH2)y]n has been investigated as a polymeric carrier molecule for the covalent attachment of (a) a modified picket fence hemin (3) and (b) protohemin chloride 3-(l-imidazolyl)propylamide (4). Reduction of the polymer-bound species with dithionite in DMF/H2O 9:1 or H2O/ethylene glycol 1:1 yielded the corresponding heme complexes. The oxygenation behavior of these species was examined at -30 °C and at room temperature by electronic absorption spectroscopy. Polymer-bound 4 gave a stable O2 adduct at -30 °C in DMF/H2O but was oxidized in ethylene glycol/H2O. Polymer-bound 3 gave a stable O2 adduct in DMF/H2O or ethylene glycol/H2O only in the presence of excess axial base. Solid films of the two polymer-bound hemes were inert to oxygen for periods of weeks but did react with carbon monoxide to yield carbonmonoxy derivatives. The oxygenation behavior of the cross-linked films swollen with solvent showed a closer resemblance to that of the solution-state systems than to that of the solid state. Dc polarography was performed on the two polymeric hemins, and cathodic shifts relative to the free hemins of >100 mV were seen for both species. Iron-57-labeled derivatives of hemin 3 were prepared, and the oxygenation behavior of the polymer-bound iron-57-labeled species was examined by Mossbauer spectroscopy. The small molecule species N3P3(OPh)5[N(CH3)CH2CH2CH2NH2] was synthesized as a model for the high polymer, and the corresponding derivatives with 3 and 4 were prepared.