Here we show that the active portion of a graphitic nanoparticle can be mimicked by a perylene diimide (PDI) to explain the otherwise elusive biological and electrocatalytic activity of the nanoparticle construct. Development of molecular analogues that mimic the antioxidant properties of oxidized graphenes, in this case the poly(ethylene glycolated) hydrophilic carbon clusters (PEG-HCCs), will afford important insights into the highly efficient activity of PEG-HCCs and their graphitic analogues. PEGylated perylene diimides (PEGn-PDI) serve as well-defined molecular analogues of PEG-HCCs and oxidized graphenes in general, and their antioxidant and superoxide dismutase-like (SOD-like) properties were studied. PEGn-PDIs have two reversible reduction peaks, which are more positive than the oxidation peak of superoxide (O2•-). This is similar to the reduction peak of the HCCs. Thus, as with PEG-HCCs, PEGn-PDIs are also strong single-electron oxidants of O2•-. Furthermore, reduced PEGn-PDI, PEGn-PDI•-, in the presence of protons, was shown to reduce O2•- to H2O2 to complete the catalytic cycle in this SOD analogue. The kinetics of the conversion of O2•- to O2 and H2O2 by PEG8-PDI was measured using freeze-trap EPR experiments to provide a turnover number of 133 s-1; the similarity in kinetics further supports that PEG8-PDI is a true SOD mimetic. Finally, PDIs can be used as catalysts in the electrochemical oxygen reduction reaction in water, which proceeds by a two-electron process with the production of H2O2, mimicking graphene oxide nanoparticles that are otherwise difficult to study spectroscopically.
Keywords: electron paramagnetic resonance; perylene diimide; radical anion; reactive oxygen species; superoxide dismutase.