Effect of cysteinyl caffeic acid, caffeic acid, and L-dopa on the oxidative cross-linking of feruloylated arabinoxylans by a fungal laccase

To study a way to covalently link arabinoxylans and proteins using a fungal laccase from the fungus Pycnoporus cinnabarinus, the effect of cysteinyl c affeic acid on the cross-linking of wheat arabinoxylans was investigated by means of capillary viscometry and RP-HPLC of alkali labile phenolic compou nds. Cysteinyl caffeic acid provoked a delay in gelation and in the consump tion of the esterified ferulic acid on arabinoxylans. When reacting free fe rulic acid and cysteinyl caffeic add with laccase, the ferulic acid consump tion and the dehydrodimers production were also diminished. These results s uggest that cysteinyl caffeic acid is oxidized while reducing the semiquino nes of ferulic acid produced by laccase. Thus, ferulic acid could not be ox idized into dimers until all cysteinyl caffeic acid was consumed, preventin g the cross-linking of feruloylated arabinoxylan chains. A similar mechanis m is proposed in the case of caffeic acid and of L-Dopa.