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

Abstract

A heme model system has been developed in which the heme-propionate is the only proton donating/accepting site, using protoporphyrin IX-monomethyl esters (PPIX_MME) and N-methylimidazole (MeIm). Proton-coupled electron transfer (PCET) reactions of these model compounds have been examined in acetonitrile solvent. (PPIX_MME)Fe^III(MeIm)₂-propionate (Fe^III∼CO₂) is readily reduced by the ascorbate derivative 5,6-isopropylidine ascorbate to give (PPIX_MME)Fe^II(MeIm)₂-propionic acid (Fe^II∼CO₂H). An excess of the hydroxylamine TEMPOH or of hydroquinone similarly reduces Fe^III∼CO₂, and TEMPO and benzoquinone oxidize Fe^II∼CO₂H to return to Fe^III∼CO₂. The measured equilibrium constants, and the determined pK_a and E_1/2 values, indicate that Fe^II∼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.