Aroxyl radical, stability

The reactions between free radicals and flavonoids (or polyphenols) are assumed to form aroxyl radicals (PO) (reaction 8). The stability of these secondary radical species is an important element to be considered in their antioxidant actions. Flavonoids with similar reduction potentials can originate radicals with very different reactivity toward other molecules present in biological systems. While a stable and relatively nonreactive PO is also nonreactive to propagate the chain reaction, a high reactive PO would propagate rather than interrupt a chain reaction. 

To compare the stability of aroxyl radicals derived from different flavonoids, the rate constants for the disproportionation reaction between two PO to render nonradical species (reaction 15) can be used ... 

An ortho-dihydroxy structure (catechol moiety) in the B-ring, which is the radical target site for flavonoids with a saturated C-2 and C-3 double bond, confers a high stability to the flavonoid aroxyl radical and participates in electron delocalization. 

The C2-C3 double bond enhances the activity by stabilization of the flavonoid aroxyl radical through electron delocalization across the molecule. Taxifolin (dihydroquercetin) had a TEAC value of 1.9 mM compared to 4.72 mM for quercetin. In the absence of a catechol moiety, the C2-C3 double bond has no significant influence on the antioxidant activity. The TEAC values of kaempferol (1.34 mM) and dihydrokaempferol (1.39 mM) were quite similar, because of the low hydrogen-donating capacity of a monophenolic B-ring. 

Similar to aroxyl-benzo[b]pyrylium systems (78ZOR1643), there is no detectable delocalization of the unpaired electron on the benzo[c]pyrylium ring in 301-303 (87RRC417). The stability of the radical cation is determined in this case by the position of the aroxyl substituent in the cation, and the most stable is 301. The formation of diradical species 303 together with the monoradical cation, was indicated on oxidation of the corresponding 2-benzopyrylium salt. 

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