Holzforschung 2015; aop

Ferulates and lignin structural composition in cork Abstract: The structure of lignin and suberin, and feru-lic acid (FA) content in cork from Quercus suber L. were studied. Extractive-free cork (Cork), suberin, desuberized cork (Corksap), and milled-cork lignins (MCL) from Cork and Corksap were isolated. Suberin composition was deter-mined 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 mono-mers, 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, respec-tively. 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 MCLsap, respectively), fol-lowed by phenylcoumarans (18 % and 20 % in MCL and MCLsap, 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 impor-tant function in the chemical assembly of cork cell walls with a cross-linking role between suberin, lignin and carbohydrates