Journal
HOLZFORSCHUNG
Volume 70, Issue 4, Pages 275-289Publisher
WALTER DE GRUYTER GMBH
DOI: 10.1515/hf-2015-0014
Keywords
2D-NMR; ferulic acid; HSQC; Py-TMAH; Quercus suber; suberin
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Funding
- Instituto Politecnico de Lisboa, Portugal [IPL 19/2003]
- Fundacao para a Ciencia e Tecnologia, Portugal (FCT research Unit 702) [PEstOE/AGR/UI0239/2014]
- LIGNOCELL project from the Spanish MICINN - FEDER funds [AGL2011-25379]
- European Social Fund (ESF)
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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 omega-hydroxyacids and alpha,omega-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 beta-O-4' alkyl-aryl ether linkages (80% and 77% of all linkages in MCL and MCLsap, respectively), followed 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 gamma-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.
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