Structural Characterization of Cork Lignin by Thioacidolysis and Permanganate Oxidation

Abstract

Keywords Cork Quercus suber Suberin Ferulic acid Cork lignin Milled wood lignin (MWL) Thioacidolysis Permanganate oxidation SummaryQuercus suber L. milled cork lignins obtained from extractive-free cork (MCL) and from saponified cork (MCL sap ) were characterized by thioacidolysis and KMnO 4 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 KMnO 4 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 KMnO 4 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.

A Genomic Approach to Suberin Biosynthesis and Cork Differentiation

“…Two genes, both strongly up-regulated in phellem, code for HCBTs, a family of enzymes capable of catalyzing the synthesis of N-hydroxycinnamoyl amides such as feruloyltyramine (Yang et al, 1997). Feruloyltyramine is found in potato wound suberin (Negrel et al, 1996) and some evidence supports the hypothesis that it is also a component of the aromatic suberin in cork tree (Marques et al, 1999). HCBTs belong to the BAHD acyltransferase superfamily (D'Auria, 2006), discussed below.…”

“…Although the amounts of the different components can show significant variations (Pereira, 1988;Lopes et al, 2001), on average it contains 15% extractives (7.5% waxes and 7.5% tannins), 41% aliphatic suberin (referred to as suberin in cork tree literature), 22% aromatic suberin (referred to as lignin in cork tree literature), 20% polysaccharides, and 2% ashes (Pereira, 1988). The monomeric composition of the aliphatic (suberin) fraction has been analyzed by Holloway (1983) and by Bento et al (1998) and that of the aromatic (lignin) fraction by Marques et al (1999) and Lopes et al (2000), among others. Waxes have been analyzed by Castola et al (2005) and mainly consist of terpenes and sterols.…”

Compositional Variability of Lignin in Biomass

“…The reaction depolymerizes part of the lignin and the released monomers can be analyzed, e.g., by GC-MS allowing to estimate the amount and composition of uncondensed alkyl ether structures [43]. Most studies were focused on wood but thioacidolysis was also applied to other materials, e.g., cork [45]. Calculation of the S/G ratio is usually made.…”

Cork-Containing Barks—A Review

“…Lignin is usually defined as a polymer of phenylpropane units with three different aromatic units-p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S)-and the lignins are classified according to their H/G/S ratios. Lignin structural composition of barks, namely of corks, is largely unknown except for a few cases that showed that cork lignin is composed mainly of guaiacyl units with a low proportion of syringyl units (Marques et al, 1994(Marques et al, , 1996(Marques et al, , 1999(Marques et al, , 2006(Marques et al, , 2016Marques and Pereira, 2013).…”