Differences in the content, composition and structure of the native-like lignins in leucaena and tagasaste shrubs for biorefinery applications

Leucaena and tagasaste are fast-growing leguminous shrubs recognized for producing substantial amounts of lignocellulosic biomass, making them promising candidates as alternative feedstocks for biorefineries. However, despite extensive research into their chemical composition, a detailed understanding of their native lignin structure remains limited, hindering their optimal use in lignocellulosic biorefineries. In this study, native-like lignins were isolated and thoroughly analyzed using a variety of analytical techniques, including gel permeation chromatography (GPC), analytical pyrolysis coupled to gas chromatography/mass spectrometry (Py-GC/MS), two-dimensional Nuclear Magnetic Resonance (2D NMR), and ³¹P NMR. The data revealed significant differences in the lignin composition of the two species. Leucaena lignin showed a strong predominance of guaiacyl (G) units, with minor amounts of syringyl (S), and p-hydroxyphenyl (H) units (H:G:S ratio of 1:83:16; S/G 0.19). In contrast, tagasaste lignin exhibited a slight predominance of S units (H:G:S ratio of 2:46:52; S/G 1.13). These compositional variations significantly influenced the distribution of the different lignin interunit linkages. Hence, leucaena lignin was characterized by fewer of β–O–4′ alkyl-aryl ether linkages (66 % of all linkages) and a higher proportion of condensed linkages, making it more resistant to delignification. In contrast, tagasaste lignin presented higher levels of β–O–4′ alkyl-aryl ether linkages (74 %) and fewer condensed linkages, rendering it more susceptible to delignification. The insights gained from this study are expected to enhance the valorization of these lignocellulosic materials as feedstocks in future lignocellulosic biorefineries.