Siderophores as natural mediators in laccse-aided degradation of lignin

Hydroxamate siderophores, microbial metal chelating agents have structural similarities with synthetic mediators succesfully used in laccase-aided delignification processes. The iron-free forms of fungal hydroxamate siderophores; desferrioxamine B, desferritriacetylfusigen and nocardamin, were shown to have mediator characteristics in laccase catalyzed oxidative reactions. Ferricrosin, an iron-containing form of siderophore, did not react with laccase. It was concluded that the iron-binding structures, free hydroxyl groups in the hydroxamate siderophores, are the targets for laccase. GPC, 1H NMR and 13C NMR analysis indicated degradation and modification of siderophores during oxidation by laccase. The reaction mechanism differs from the corresponding reactions of synthetic mediators. When compared on molar basis, the ability of the oxidized siderophore to degrade lignin was better than that obtained by the synthetic mediator 1 hydroxybenzotriazole (HBT), due to the higher number of N-OH-groups in the siderophore molecule. The role of siderophores as natural mediators in delignification and as models for mediators is thus worth of further evaluation.