Ferulates and lignin structural composition in cork

Abstract

The structure of lignin and suberin, and ferulic acid (FA) content in cork from Quercus suber L. were studied. Extractive-free cork (Cork), suberin, desuberized cork (Cork sap ), and milled-cork lignins (MCL) from Cork and Cork sap were isolated. Suberin composition was determined by GC-MS/FID, whereas the polymers structure in Cork, Corksap, and MCL was studied by Py-TMAH and 2D-HSQC-NMR. Suberin contained 94.4% of aliphatics and 3.2% of phenolics, with 90% of ω-hydroxyacids and α,ω-diacids. FA represented 2.7% of the suberin monomers, overwhelmingly esterified to the cork matrix. Py-TMAH revealed significant FA amounts in all samples, with about 3% and 6% in cork and cork lignins, respectively. Py-TMAH and 2D-HSQC-NMR demonstrated that cork lignin is a G-lignin ( > 96% G units), with a structure dominated by β-O-4′ alkyl-aryl ether linkages (80% and 77% of all linkages in MCL and MCL sap , respectively), followed by phenylcoumarans (18% and 20% in MCL and MCL sap , respectively), and smaller amounts of resinols (ca. 2%) and dibenzodioxocins (1%). HSQC also revealed that cork lignin is heavily acylated (ca. 50%) exclusively at the side-chain γ-position. Ferulates possibly have an important function in the chemical assembly of cork cell walls with a cross-linking role between suberin, lignin and carbohydrates.

Cork-Containing Barks—A Review

“…Therefore, it is to be expected that suberin spatial development may be more compact in Q. variabilis cork. Ferulic acid is also present in the suberin extracts (3.6-3.0%), enforcing the conclusions about its function in the cross-linking between suberin and lignin (Marques et al, 2016).…”

“…Suberin is a large biopolymer of lipid nature formed by the esterification of glycerol and long-chain fatty acids, α,ω-diacids and ω-hydroxyacids, either saturated or with an unsaturation, epoxy, or vicinal diol substitution at mid-chain (Graça and Pereira, 1997). Suberin also includes a few aromatic monomers in most cases ferulic acid (Graça and Pereira, 1998;Marques et al, 2016). The specific composition of suberin, i.e., the proportion of monomers varies between species, as detailed in the following sections.…”

“…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).…”

“…Cork lignin is a G-type lignin composed by 95% guaiacyl units, 3% syringyl units, and 2% 4-hydroxyphenyl units (Marques et al, 2006;Marques and Pereira, 2013) with 80% of the inter-unit linkages as β-O-4-alkyl-aryl ether bond (Marques et al, 2016).…”

Compositional Variability of Lignin in Biomass

“…The lignin monomeric composition of cork was assessed after isolation by the Bjӧrkman method by Marques [170][171][172][173] and Lourenço et al [125]. The S/G ratio of cork was 0.029 determined by Py-GC/FID and 0.1 by NMR, revealing a strong predominance of guaiacyl units that was also confirmed by FTIR analysis [170].…”