Homogeneous and heterogeneous methods for laccase-mediated functionalization of chitosan by tannic acid and quercetin

Homogeneous and heterogeneous methods for functionalization of chitosan with quercetin or tannic acid using laccase from Trametes versicolor is presented, yielding a bio-based product with synergistic antioxidant and antimicrobial properties. HPLC–SEC analysis and cyclic voltammetry kinetic studies showed that laccase catalyzes the oxidation of quercetin to electrophilic o-quinones, which undergo to an oligomer/polymer-forming structures. On the other hand, tannic acid was converted into gallic acid, its dimers, partially gallic acid esterified glucose and glucose, when exposed to laccases. ATR-FTIR spectroscopy provided evidence that quercetin o-quinones undergo coupling reactions with amino groups of chitosan via Schiff-base and Michael addition mechanisms under heterogeneous method, while oxidized tannic acid cross-linked with chitosan by hydrogen and electrostatic interactions under both methods. All polyphenols functionalized chitosans showed greatly improved ABTS+ cation radicals scavenging capacity, compared with the untreated chitosan, while antimicrobial activities significantly depended on the mode of functionalization and type of microorganism.