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 u sing capillary viscometry, size-exclusion HPLC, and reverse-phase HPLC of phenolic compounds. The laccase results were compared with those f or a hydrogen peroxide/horseradish peroxidase system. The oxidants pro voked an increase in viscosity of a 0.2% (w/v) arabinoxylan solution. A gel was formed after 30 min with laccase. Hydrogen peroxide was cons umed rapidly before a gel could be formed. Free ferulic acid, methyl f erulate, and vanillic acid inhibited the gelation, whereas fumaric aci d had no effect. This suggests that the aromatic ring, and not the pro penoic chain of ferulic acid, was the initiating site for arabinoxylan cross-linking. Ferulic acid and its 8-O-4', 8-5', and 5-5' dehydrodim ers were present in nonoxidized arabinoxylans. Upon oxidation, the 8-8 ' and 8-5' benzofuran dehydrodimers appeared and the 8-O-4' and 8-5' d imers increased. The production of dimers was proportional to the cons umption of ester-bound ferulic acid. In cross-linked arabinoxylans, th e 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 soluti on.