Oxidized Activated Charcoal Nanoparticles as Catalytic Superoxide Dismutase Mimetics: Evidence for Direct Participation of an Intrinsic Radical

The mechanism for carbon-based superoxide dismutase (SOD) nanomimetics is not known, hindering the optimization of this potentially clinically useful feature. Here we studied oxidized activated charcoal (OAC) prepared by fuming nitric acid oxidation of activated charcoal and characterized its properties. The OAC nanoparticles have sizes 5-30 nm and are highly water-soluble. The OACs are strong SOD mimetics with a kcat = 2.1 × 105 s-1 at pH 12.7 and 108 M-1 s-1 rate constants at pH 8.5, having great potential in therapeutic application for disorders with pathological superoxide levels. Electron paramagnetic resonance (EPR) indicates that resting OACs are fully oxidized, exhibiting a stoichiometric level of an intrinsic radical. The OACs can be reduced with superoxide, leading to a decreased level of the intrinsic radical; however, the reduction is incomplete even at high superoxide levels. This outcome was predicted by a simple two-step SOD reaction mechanism using the species containing the intrinsic radical as the fully oxidized state. Pretreatment of the OACs with superoxide causes little change to its SOD activity, indicating that a stoichiometric amount of the intrinsic radical is not mandatory for full activity. This study indicates the direct participation of the intrinsic radical in the catalytic turnover of a highly active SOD-like nanozyme.