Reaction of water-soluble fullerenes with O₂˙⁻ and other reactive radical species

In this work we focused on the reaction of water-soluble fullerene derivatives with various organic (i.e., methyl radical and t-butyl radical) and inorganic (i.e., superoxide radical and azide radical) radicals to mimic their superoxide dismutase (SOD) activity. Importantly, with the help of time-resolved pulse radiolysis and steady-state gamma radiolysis measurements all of the assays were conducted in aqueous solutions. Our fully-fledged spectroscopic and kinetic investigations leave no doubt about a diffusion-controlled addition mechanism (10¹⁰ M⁻¹ s⁻¹) by which superoxide radical reacts with fullerenes to yield (C₆₀-O₂)˙⁻. Notable is that the formations of (C₆₀-CH₃)˙, (C₆₀-(CH₂)(CH₃)₂COH)˙, and (C₆₀-N₃)˙ are much slower and proceed with activation-controlled rate constants less than 10⁹ M⁻¹ s⁻¹. The major deactivation path of (C₆₀-O₂)˙⁻ is a pH dependent protonation (10¹⁰ M⁻¹ s⁻¹). Despite lacking unambiguous evidence for the formation of C₆₀˙⁻ – via a direct impact or an indirect dissociation mechanism – (C₆₀-O₂)˙⁻ is still redoxactive. The latter has been confirmed by an activation-controlled reduction (10⁸ M⁻¹ s⁻¹) of a series of p-benzoquinones that display different electron affinities. In other words, (C₆₀-O₂)˙⁻ – but not C₆₀˙⁻ – is likely to emerge as a key intermediate in the SOD activity of fullerenes. Its slow protonation is beneficial toward an efficient dismutation to form H₂O₂.