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

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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.