Abstract

The pyraninoxyl radical is readily formed from the MnO2-promoted oxidation of pyranine. The free radical can be formed in high concentrations (mM), and presents a characteristic EPR spectrum that indicates a high spin-density delocalization. It is relatively stable under nitrogen (half-life ca. 50 min) but readily decays in presence of O2. In spite of its high stability, the radical readily reacts with antioxidants (phenols and ascorbic acid) with a partial recovery of the parent pyranine. High concentrations of the pyraninoxyl radical (ca. 9 μM) are present when pyranine is exposed to a free radical source (10 mM 2,2′-azobis[2- amidinopropane], 37°). The fact that these radicals readily react with antioxidants (ascorbic acid and caffeic acid) supports the proposal that protection by antioxidants of peroxyl radical-promoted pyranine bleaching is mainly due to the occurrence of a repair mechanism. © 2007 Verlag Helvetica Chimica Acta AG.