A.V. Marques, H. Pereira, D. Meier, O. Faix
Structural Characterization of Cork Lignin by Thioacidolysis and Permanganate Oxidation
Summary
Quercus suber L. milled cork lignins obtained from extractive-free cork (MCL) and from saponified
cork (MCLsap) were characterized by thioacidolysis and KMnO4 oxidation. These techniques and the
previously used analytical pyrolysis revealed that cork contains a guaiacyl lignin (G lignin) with
94–96% guaiacyl-, ca. 3% syringyl-, and 2–3% of 4-hydroxyphenyl-propane units. The total yields
of degradation products in thioacidolysis and KMnO4 oxidation experiments were lower in comparison
to those of a spruce milled wood lignin (MWLspruce) suggesting a higher cross-linking in the G-lignin
of cork. The higher frequency of “condensed” structures (having C-C or C-O-C linkages to aromatic
rings) in cork lignin was also manifested in the relative abundance of various aromatic acids obtained
by KMnO4 oxidation. The cork lignin (MCLsap) contains only low amounts (ca. 2%) of covalently
bonded suberinic acids. Numerous free aliphatic suberinic acids were detected and identified in the ether
soluble part of MCLsap. Ferulic acid was not liberated by thioacidolysis, pointing at an in situ acylation.
We suggest that the major part of the aromatic domain in cork is a G-lignin but that it includes also
another structural moiety with higher H-unit content and linked by thioacidolysis resistant bonds.
Holzforschung, Walter de Gruyter
Print ISSN: 0018-3830
Volume: 53, 03/1999
Pages: 167 - 174
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