Reactivity of hydroxynaphthalenes towards peroxyl radicals

In the present work we report data bearing on the reaction of mono- and dihydroxynaphthalenes towards 2,2'-azo-bis(2-amidinopropane) dihydrochloride (AA.PH) derived peroxyl radicals. A comparison of the bleaching rates allows establishing their relative reactivity towards peroxyl radicals: 1,2-dihydroxynaphthalene (1,2-DHN) >> 2,3-dihydroxynaphthalene (2,3-DHN) > 1,3-dihydroxynaphthalene (1,3-DHN) > 2,7-dihydroxynaphthalene (2,7-DHN) > 1-hydroxynaphthalene (1-N) >= 2-hydroxynaphthalene (2-N). Bleaching rates measured under conditions of quantitative trapping of peroxyl radicals allow an estimation of the number of molecules consumed per each radical introduced into the system. The results obtained imply a chain consumption of 1,2-DHN and 2,3-DHN. In fact, a fast autoxidation of 1,2-DHN is observed even in absence of AAPH. The high reactivity of this compound suggests a fast hydrogen abstraction, due to its low O-H bond dissociation energy (BDE), and/or a very fast electron transfer from the deprotonated form. Oxygen radical absorbance capacity (ORAC) indexes of the naphthalene derivatives were evaluated from their effect upon the peroxyl radical promoted bleaching of c-phycocyanin (c-Pc). ORAC values are strongly influenced by the secondary reactions of the additive and do not correlate with the reactivity of the compound or the number of the naphthalene derivative molecules bleached per each radical. c-Pc bleaching rate in presence of an excess of hydroxyl-naphthalene derivative was taken as a measure of the damaging capacity of the corresponding naphthoxyl radicals. The results indicate that this damaging capacity is inversely proportional to the reactivity of the parent compound. In fact, addition of 2-naphthol, the less reactive of the tested compounds, increases the rate of c-Pc bleaching promoted by peroxyl radicals. Copyright (c) 2006 John Wiley & Sons, Ltd.