Structural features of dioxane lignin: A comparative study with milled wood and formaldehyde-stabilized lignins

Dioxane lignin (DL) is isolated from plant material under mild acidolysis conditions and is widely used in many studies as a representative sample of protolignin, an alternative to milled wood lignin (MWL). However, the structural changes caused by hydrolytic degradation reactions during DL extraction are still poorly understood. In this work, an integrated approach based on 2D NMR and high-resolution mass spectrometry was used to establish the features of the lignin structure on the example of pine lignin isolated using dioxane under various conditions: MWL, DL and "formaldehyde stabilized" lignin (LSF). In this context, we sought to gain a deeper understanding of the chemical structure of DL, focusing on the least studied fragments. Cross-peaks related to the degradation products of 1,2-diarylpropane were uniquely identified for the first time by a combination of HSQC-HMBC spectra. In addition, the 1,3-dioxane structure of β-aryl ether, the presence of which in DL has not been previously observed and originates from elements of the native structure and formaldehyde (product of lignin destruction), was unambiguously shown and confirmed by NMR and mass spectrometry analysis. Additionally, signals related to the 1,3-dioxane structure of 1,2-diarylpropane were also detected for the first time in the HSQC spectrum of LSF.