A recurring interest of our group is the cryogenic generation and spectroscopic characterization of small molecule heme-peroxo-copper adducts as ‘biomimetic’ models for a putative peroxidic species which may form transiently between the A to Pm intermediates during enzyme turnover in Cytochrome c Oxidase (CcO). One aspect of our ongoing studies has been to synthesize porphyrinoids with axial bases tethered to the porphyrin periphery to evaluate the role of tether properties, and base identity on heme-peroxo-copper adduct properties and reactivity. Three porphyrin systems have been used including an imidazole tethered with a benzylamide linker (PIm), pyridine attached via an alkylamide linker (PPy), and a porphyrinoid lacking any (tethered) axial base moiety (F8). Utilizing a well-studied tridentate cuprous chelate AN (N1-(3-(dimethylamino)propyl)-N3,N3-dimethylpropane-1,3-diamine), we have synthesized a series of heme-peroxo copper adducts, and can evaluate the role of axial base properties on adduct reactivity towards exogenous substrates. The spectroscopic and reactivity properties of these adducts can be compared to our published heme-peroxo-copper adducts with the F8 porphyrinoid system utilizing an exogenously added axial base, 1,5-dicyclohexylimidazole (1,5-DCHIm). Preliminary results have revealed intriguing differences in the reactivity of [(PIm)FeIII-(O2)2--CuII(AN)]+ (A) and [(1,5-DCHIm)(F8)FeIII(O2)2--CuII(AN)]+ (B) towards a phenolic acid (p-NO2-phenol), and reductant (Me10Fc). Namely, (A) undergoes a one-electron reduction reaction in the presence of Me10Fc to yield an end-on ferric heme-peroxo species, whereas (B) does not react with reductant alone. In the presence of excess phenolic acid, when reductant is added to (A), the product is still the end-on ferric heme peroxo species, whereas for (B), a two-electron reductive O-O bond cleavage reaction is observed. Spectroscopic (NMR), and computational studies help to rationalize this difference in reactivity for complexes with tethered vs free axial bases.

Axial base effects on heme-peroxo-copper adduct reactivity: Evaluating the role of axial base tether and type

254th National Meeting (2017)

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