Structural Features of Cork Dioxane Lignin from Quercus suber L.

The dioxane lignin was isolated from extractives- and suberin-free cork (Quercus suber L.) by a modified acidolytic procedure and submitted to structural analysis by permanganate oxidation, analytical pyrolysis coupled with gas chromatography and mass spectrometry (Py-GC-MS), liquid- and solid-state nuclear magnetic resonance (NMR) spectroscopy, and Fourier transform infrared (FTIR) spectroscopy. The molecular weight (M_w = 2500 Da) was assessed by size exclusion chromatography (SEC). The results obtained show that the cork lignin is of syringyl (S)/guaiacyl (G) type with a small proportion of p-hydroxyphenyl (H) units (S:G:H molar ratio of 23:72:5). Among a dozen detected lignin structures, those linked by ether bonds, such as β-O-4′ (38 mol %) and 4-O-5′ (5 mol %), were the most abundant. The frequency of occurrence of β-5′, β-β′, 5-5′, tetrahydrofuran type, and structures arising from the condensation with concomitant procyanidins was assessed. Ferulates were the only cinnamic structure detected in the cork dioxane lignin.