Structural characterization of cork lignin by thioacidolysis and permanganate oxidation

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.