Abstract
Rice (Oryza sativa L.) is a major cereal crop used for human nutrition worldwide. Harvesting and processing of rice generates huge amounts of lignocellulosic by-products such as rice husks and straw, which present important lignin contents that can be used to produce chemicals and materials. In this work, the structural characteristics of the lignins from rice husks and straw have been studied in detail. For this, whole cell walls of rice husks and straw and their isolated lignin preparations were thoroughly analyzed by an array of analytical techniques, including pyrolysis coupled to gas chromatography-mass spectrometry (Py-GC/MS), nuclear magnetic resonance (NMR), and derivatization followed by reductive cleavage (DFRC). The analyses revealed that both lignins, particularly the lignin from rice husks, were highly enriched in guaiacyl (G) units, and depleted in p-hydroxyphenyl (H) and syringyl (S) units, with H:G:S compositions of 7:81:12 (for rice husks) and 5:71:24 (for rice straw). These compositions were reflected in the relative abundances of the different interunit linkages. Hence, the lignin from rice husks were depleted in β–O–4′ alkyl-aryl ether units (representing 65% of all inter-unit linkages), but presented important amounts of β–5′ (phenylcoumarans, 23%) and other condensed units. On the other hand, the lignin from rice straw presented higher levels of β–O–4′ alkyl-aryl ethers (78%) but lower levels of phenylcoumarans (β–5′, 12%) and other condensed linkages, consistent with a lignin with a slightly higher S/G ratio. In addition, both lignins were partially acylated at the γ-OH of the side-chain (ca. 10–12% acylation degree) with p-coumarates, which overwhelmingly occurred over S-units. Finally, important amounts of the flavone tricin were also found incorporated into these lignins, being particularly abundant in the lignin of rice straw.
Highlights
Lignin is a complex aromatic heteropolymer present in the cell-walls of vascular plants where it provides structural support, waterproofs the cell wall enabling transport of water and solutes through the vascular system, and acts as a barrier against pathogens
Lignin is formed by the combinatorial oxidative radical coupling of three main monolignols, p-coumaryl, coniferyl, and sinapyl alcohols, that form the respective p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) lignin units when incorporated into the polymer, and this mechanism generates a series of substructures with a considerable variety of inter-unit linkages (β–O–4, β–5, β– β, β–1, 5–5, 4–O–5, etc.) within the polymer (Ralph et al, 2004; Vanholme et al, 2010, 2019)
We report the comprehensive structural characterization of the lignins of rice husks and straw by the use of different analytical techniques, including analytical pyrolysis coupled to gas chromatography and mass spectrometry (Py-GC/MS), two-dimensional nuclear magnetic resonance (2D-NMR), and the so-called derivatization followed by reductive cleavage (DFRC) degradation method
Summary
Lignin is a complex aromatic heteropolymer present in the cell-walls of vascular plants where it provides structural support, waterproofs the cell wall enabling transport of water and solutes through the vascular system, and acts as a barrier against pathogens. During the last few years, other phenolic compounds derived from beyond the canonical monolignol biosynthetic pathway have been identified to behave as true lignin monomers participating in coupling and cross-coupling reactions with monolignols and being integrally incorporated into the lignin polymer (del Río et al, 2020) This is the case of the flavone tricin, that was found incorporated into the lignin structure in grasses and other monocots (del Río et al, 2012b; Rencoret et al, 2013; Lan et al, 2015, 2016a,b), or the hydroxystilbenes, piceatannol, that were found incorporated into the lignins of palm fruit endocarps (del Río et al, 2017; Rencoret et al, 2018). The discoveries of these “novel” lignin monomers widely expanded our understanding of the lignin structure and revealed the structural complexity, heterogeneity, and variability of the lignin polymer
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