Structural characterization of cork lignin by thioacidolysis and permanganate oxidation

Quercus suber L. milled cork lignins obtained from extractive-free cork (MC L) and from saponified cork (MCLsap) were characterized by thioacidolysis a nd KMnO4 oxidation. These techniques and the previously used analytical pyr olysis revealed that cork contains a guaiacyl lignin (G lignin) with 94-96% guaiacyl-, ca. 3% syringyl-, and 2-3% of 4-hydroxyphenyl propane units. Th e total yields of degradation products in thioacidolysis and KMnO4 oxidatio n experiments were lower in comparison to those of a spruce milled wood lig nin (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 linkage s to aromatic rings) in cork lignin was also manifested in the relative abu ndance of various aromatic acids obtained by KMnO4 oxidation. The cork lign in (MCLsap) contains only low amounts (ca. 2%) of covalently bonded suberin ic acids. Numerous free aliphatic suberinic acids were detected and identif ied 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 als o another structural moiety with higher H-unit content and linked by thioac idolysis resistant bonds.