Proton-Coupled Electron Transfer Reactions at a Heme-Propionate in an Iron-Protoporphyrin-IX Model Compound

A heme model system has been developed in which the heme-propionate is the only proton donating/accepting site, using protoporphyrin IX-monomethyl esters (PPIXMME) and N-methylimidazole (MeIm). Proton-coupled electron transfer (PCET) reactions of these model compounds have been examined in acetonitrile solvent (PPIXMME)Fe-III(MeIm)(2)-propionate (Fe-III similar to CO2) is readily reduced by the ascorbate derivative 5,6-isopropylidine ascorbate to give (PPIXMME)Fe-II(MeIm)(2)-propionic acid (Fe-II similar to CO2H). An excess of the hydroxylamine TEMPOH or of hydroquinone similarly reduces Fe-III similar to CO2, and TEMPO and benzoquinone oxidize Fe-II similar to CO2H to return to Fe-III similar to CO2. The measured equilibrium constants, and the determined pK(a) and E-1/2 values, indicate that Fe-II similar to CO2H has an effective bond dissociation free energy (BDFE) of 67.8 +/- 0.6 kcal mol(-1). In these PPIX models, electron transfer occurs at the iron center and proton transfer occurs at the remote heme propionate. According to thermochemical and other arguments, the TEMPOH reaction occurs by concerted proton electron transfer (CPET), and a similar pathway is indicated for the ascorbate derivative. Based on these results, heme propionates should be considered as potential key components of PCET/CPET active sites in heme proteins.