Fig. 5

Laccase-mediated oxidation of hydroxycinnamic acids. The O• radical (a) is formed by the action of laccase, resulting in the intermediate radicals β (b) and 5 (c), while only the radicals that contribute to bond-forming events are shown. The R could be either a hydrogen for ferulic acid, or a methoxy group for sinapic acid. The coupling of these intermediates results in the formation of β-β, β-5 β-Ο-4, 5–5, 5-O-4, and O–O dimers. The O–O coupled dimer is unstable, and thus, it is not found among the products, while for sinapic acid oxidation, C5 is occupied by a methoxy group, and therefore does not participate in bond forming [47, 48]