Oxidative cross-linking of pentosans by a fungal laccase and horseradish peroxidase: Mechanism of linkage between feruloylated arabinoxylans

The potential of a laccase from the fungus Pycnoporus cinnabarinus to cross-link feruloylated soluble wheat arabinoxylans was investigated using capillary viscometry, size-exclusion HPLC, and reverse-phase HPLC of phenolic compounds. The laccase results were compared with those for a hydrogen peroxide/horseradish peroxidase system. The oxidants provoked an increase in viscosity of a 0.2% (w/v) arabinoxylan solution. A gel was formed after 30 min with laccase. Hydrogen peroxide was consumed rapidly before a gel could be formed. Free ferulic acid, methyl ferulate, and vanillic acid inhibited the gelation, whereas fumaric acid had no effect. This suggests that the aromatic ring, and not the propenoic chain of ferulic acid, was the initiating site for arabinoxylan cross-linking. Ferulic acid and its 8-O-4', 8-5', and 5-5' dehydrodimers were present in nonoxidized arabinoxylans. Upon oxidation, the 8-8' and 8-5' benzofuran dehydrodimers appeared and the 8-O-4' and 8-5' dimers increased. The production of dimers was proportional to the consumption of ester-bound ferulic acid. In cross-linked arabinoxylans, the major dimers were 8-5' benzofuran, 8-8', and 8-O-4', whereas the 5-5' dehydrodimer remained at the same level as in the nonoxidized solution.