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Paper
J. Mater. Chem., 2010, 20, 83 - 89, DOI: 10.1039/b917346n
Reaction of water-soluble fullerenes with O2
- and other reactive radical speciesAxel Kahnt, Dirk M. Guldi, Michael Brettreich, Uwe Hartnagel and Andreas Hirsch
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.
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