Characterization of some African tropical heartwood lignins by 1D 13C and 1H-NMR: molecular structure and hydroxyl groups’ distribution

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In order to understand the potential of some tropical hardwood wastes as source of utilizable lignins, the chemical structure of acetylated dioxane acid lignins from Testulea gabonensis Pellegr (T. gabonensis), Julbernardia pellegriniana (J. pellegriniana), Aucoumea klaineana Pierre (A. klaineana) and Tieghemella africana Pierre (T. africana) widely used in the tropical timber industry of Congo basin was investigated by 1D 1H and 13C-NMR. T. gabonensis and J. pellegriniana were the richest in guaiacyl (G) units, while A. klaineana and T. africana ones exhibited the highest syringyl (S) content. 1H-NMR exhibited an S/G 1. 13C-NMR aliphatic to phenolic hydroxyl group ratios were 1.42; 2.10; 2.0 and 1.41 for T. gabonensis, J. pellegriniana, A. klaineana and T. africana, respectively. The guaiacyl hydroxyl groups of T. gabonensis and J. pellegriniana dioxane acid lignins were more acetylated than A. klaineana and T. africana ones. The lowest acetylation of A. klaineana dioxane acid lignin was reinforced by the weak esterification of its guaiacyl-type hydroxyl groups at the Cα and Cγ position. The same trend was observed for acetylated syringyl-type hydroxyl groups at the Cα position of A. klaineana. Nevertheless, T. gabonensis was the richest in β-O-4 and 5-5′ condensed structures. Those lignins showed a low content on β-5 links for which the weakest signal strength was for T. africana which exhibited, however, the highest content on β–β structures of pinoresinol moieties. The occurrence of guaiacyl-type β-O-4 with Cα=O or β-O-4 threo and erythro forms within the studied hardwood lignins was discussed.