Antioxidant functionalization of pullulan with ferulic acid using enzymatic catalysis

Antioxidant polysaccharides were synthesized using an innovative, green, enzymatic reaction to catalyze the grafting of ferulic acid (FA) onto pullulan (Pull). Several grades of Pull-FA with verified purity were successfully synthesized, with maximum experimental grafting rates (GR) up to 8.8%, as determined by the Folin Ciocalteu method. For the first time, the chemical structure of the grafts was proposed using ¹³C solid-state NMR and MALDI-TOF MS analyses, which proposed the putative grafting of a negatively charged, monodecarboxylated FA dimer (β-5). SEC/MALS/DRI/UV/visco analyses confirmed the grafting with a tendency towards intra- and intermolecular hydrophobic self-association, which depends on the grafting rate. The aggregative power of Pull-FAs in dilute medium as a function of the grafting rate and the pH of the medium was highlighted using the Nile red fluorescent probe and light scattering. In semi-dilute/concentrated media, intermolecular self-association was revealed by an increase in texture and a reversible physical gel behavior under favorable pH conditions. DPPH^• analyses showed the antioxidant activity of Pull-FAs and confirmed the linear decrease in EC₅₀ as a function of grafting rate.