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 (MCL_sap) 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 (MWL_spruce) 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 (MCL_sap) 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 MCL_sap. 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.