Isolation and Structural Characterization of Lignin from Cardoon (Cynara cardunculus L.) Stalks

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The lignin from Cynara cardunculus stalks was isolated by the classical Björkman method and characterized by pyrolysis coupled with gas chromatography and mass spectrometry (Py-GC/MS), two-dimensional nuclear magnetic resonance spectroscopy (2D-NMR), and derivatization followed by reductive cleavage (DFRC). The milled Cynara lignin (MCyL) was constituted mainly by guaiacyl (G) and syringyl-units(S) (S/G molar ratio of 0.7), with the complete absence of p-hydroxyphenyl (H) units. The 2D-NMR analysis indicated a predominance of alkyl-aryl ether linkages (70 % of all inter unit linkages are β–O–4′) and significant amounts of condensed structures such as phenylcoumarans (β-5′, 14 %), resinols (β-β′, 7 %), spirodienones (β-1′, 5 %), and dibenzodioxocins (5-5′, 4 %). Furthermore, the analyses indicated that the lignin is partially acylated at the γ-OH (12 % acylation) by acetate groups and that acetylation occurs preferentially on syringyl-units. As in other plants, acetylation occurs at the monomer stage, and sinapyl acetate behaves as a real lignin monomer participating in lignification in cardoon stalks. The detailed structural characterization of cardoon lignin reported here will foster the industrial use of this biomass for the production of biofuels and other bio-based chemicals under the lignocellulosic biorefinery. © 2015, Springer Science+Business Media New York.

We thank Duarte M. Neiva and Solange Araujo for their contribution during the chemical analysis and Alejandro Rico for his technical support during MCyL isolation. We also thank Dr. Manuel Angulo (CITIUS, University of Seville) for performing the NMR analyses.The research was financed by FEDER through the Operational Program for Competitive Factors of COMPETE, and by the Portuguese Science Foundation (FCT), through a R&D project PTDC/AGR-FOR/3872/2012, and the base funding to the Forest Research Center (CEF) under the PEst-OE/AGR/UI0239/2014 and UID/AGR/00239/2013. This study has also been partially funded by the Spanish project AGL2011-25379 (co-financed by FEDER funds), the CSIC project 2014-40E-097, and the EU-project INDOX (KBBE-2013-7-613549). The first author was funded by FCT through a post-doctoral grant (SFRH/BPD/95385/2013). Jorge Rencoret thanks the CSIC for a JAE-DOC contract of the program "Junta para la Ampliación de Estudios" co-financed by Fondo Social Europeo (FSE).