Carboxylic Acids Produced Through Oxidative Cleavage of Aromatic Rings During Degradation of Lignin in Spruce Wood by Phanerochaete Chrysosporium

Abstract

Pre-extracted spruce wood chips were decayed by Phanerochaete chrysoaporium to a 20% loss in lignin (8% loss in total weight), and extracted successively with petroleum ether, chloroform, acetone, methanol, and aqueous dioxane. The low molecular weight fraction of the methanol extract was analyzed by gas chromatography/ high resolution mass spectrometry after acetylation and nethylation. Examination of the spectra resulted in structural assignments for 28 compounds, 10 of which were aromatic acids identified and reported previously (Holzforschung 36,3 (1982)). At least 13 of the remaining compounds were formed via aromatic ring cleavage. In addition to ring cleavages, the new structures revealed oxidation of α- and γ-hydroxyl groups, oxidative cleavage of C_α−C_β and C_β–C_γ bonds, and 3-O-demethylation. It is postulated that oxidative cleavage of aromatic rings in the lignin units with an ether linkage at C-4 of the guaiacyl group involves 3-O-demethylation, hydroxylation at C-2 and subsequent o-cleavage of the resulting catechol structures, whereas for lignin units with a phenolic hydroxyl group at C-4 of the guaiacyl group only 3-O-demethylation produces the catechol substrate for subsequent cleavage. In both cases, the resulting products are 2,4-hexadiene-l,6-dioic acid intermediates, differing merely in the nature, number, and position of substituent groups. These intermediates undergo further degradation via various pathways, depending on the nature of substituent groups, to produce the observed ring cleavage products. It is considered probable that the ring cleavages and subsequent reactions occurred at the macromolecular level.