Differences in the content, composition and structure of the lignins from rind and pith of papyrus (Cyperus papyrus L.) culms

Papyrus (Cyperus papyrus L.) is a plant with high productivity rate that is considered an interesting raw material for obtaining biofuels and biomaterials. In the present work, the composition and structural features of the lignins present in the rind and pith of papyrus culms have been studied. The analyses revealed that the lignins from both parts of papyrus culms differed greatly not only in their contents but also in composition and structure. In the rind, the lignin represented up to 27.0% (dry-matter) and was composed mainly of G-units (H:G:S composition of 4:63:33), while the lignin in the pith (15.7% dry-matter) presented a slightly higher amount of S-units (H:G:S composition of 8:43:49). These lignins also presented considerable amounts of cinnamates (p-coumarates and ferulates) incorporated. The differences in composition influenced the content of the lignin inter-unit linkages. Thus, the rind lignin presented lower amounts of β–O–4′ ether linkages (77% of all identified linkages) and higher content of condensed structures, such as phenylcoumarans (15%), resinols (2%), dibenzodioxocins (3%), and spirodienones (2%), while the pith lignin contained more β–O–4′ ethers (85% of all identified linkages) and lower amounts of phenylcoumarans (11%), tetrahydrofurans (2%), and spirodienones (2%). Moreover, 2D NMR analyses revealed that the lignins from papyrus were highly acylated at the γ-OH (40% acylation in the rind, and 60% acylation in the pith). DFRC analyses proved that p-coumarates were the main acylating groups, and that they were predominantly acylating the S-lignin units. Finally, the analyses also showed the presence of important amounts of the flavanone tricin in the rind lignin and, at lower amounts, in the pith lignin.