EFFECTS OF CELLOBIOSE DEHYDROGENASE ON GUAIACYL AND GUAIACYL SYRINGYLLIGNINS IN THE PRESENCE OF IRON AND HYDROGEN-PEROXIDE/

Fe(III) reduction by cellobiose dehydrogenase (CDW) in the presence of hydrogen peroxide and guaiacyl or syringyl lignins was investigated. These two wood lignins behaved differently and reduction of Fe(III) by CDH was markedly decreased by the presence of synthetic lignin of the guaiacyl type or by milled wood lignin from spruce. with milled wood lignin from birch (guaiacyl/syringyl lignin) no decrease but rather a weak increase in Fe(III) reduction was obtained. Since it has been pro posed that Fe(II) plus hydrogen peroxide can form hydroxyl radicals an d/or iron(per)oxo complexes able to degrade lignin, it was investigate d whether lignin-related phenols and quinones affected Fe(III) reducti on. It was found I:hat vanillic and syringic acids, vanillin and syrin galdehyde al! increased Fe(III) reduction by CDH + cellobiose in the p resence of hydrogen peroxide. When Fe(III) and methoxyquinone were pre sent together, they were simultaneously reduced at a common wavelength . This reducing activity was mainly additive depending on the reductio n of the separate components. Since dimethoxyquinone was reduced more slowly it had little influence on CDH-catalyzed Fe(III) reduction. It is proposed that hydroxyl radicals and/or iron(per)oxo complexes prefe rentially attack hardwood lignin (guaiacyl/syringyl) thereby releasing phenols and quinones during consumption of hydrogen peroxide which ma y otherwise reoxidize Fe(II). With the guaiacyl lignins, hydrogen pero xide was probably not consumed and both velocity and initiation of Fe( III) reduction was strongly delayed in relation to the amount of added spruce lignin or coniferyl alcohol lignin (DHP).