Effects of Cellobiose Dehydrogenase on Guaiacyl and Guaiacyl/Syringyl Lignins in the Presence of Iron and Hydrogen Peroxide

Abstract

Fe(III) reduction by cellobiose dehydrogenase (CDH) 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 proposed that Fe(II) plus hydrogen peroxide can form hydroxyl radicals and/or iron(per)oxo complexes able to degrade lignin, it was investigated whether lignin-related phenols and quinones affected Fe(III) reduction. It was found that vanillic and syringic acids, vanillin and syringaldehyde all increased Fe(III) reduction by CDH + cellobiose in the presence of hydrogen peroxide. When Fe(III) and methoxyquinone were present together, they were simultaneously reduced at a common wavelength. This reducing activity was mainly additive depending on the reduction of the separate components. Since dimethoxy-quinone 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 preferentially attack hardwood lignin (guaiacyl/syringyl) thereby releasing phenols and quinones during consumption of hydrogen peroxide which may otherwise reoxidise Fe(II). With the guaiacyl lignins. hydrogen peroxide 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).