Comprehensive analysis of the chemical structure of lignin from raspberry stalks (Rubus idaeus L.)

Lignins of non-woody fast-growing plants cause high interest in recent time as a potential source of raw materials for biorefining. Studying of the lignin structure in shrubs will allow obtaining the information about processes of biosynthesis of these plants and developing methods of their delignification for bioprocessing industry. We studied the structure of raspberry dioxane lignin (Rubus idaeus L.) using various spectroscopic methods, including FT-IR and NMR (1D and 2D) for various nuclei. Also, we used gel permeation chromatography and elemental analysis. It is obtained that raspberry dioxane lignin has relatively wide polydispersity, with Mw/Mn = 2.6, and a relatively lower molecular weight (Mw = 5111 g/mol) due to low molecular weight oligolignols. ³¹P-NMR spectroscopy allowed to obtain that raspberry lignin belongs to GS-type. Due to ¹³C-NMR we calculated hypothetical empirical formula of the studied lignin and determined the substructures and ether bonds content. S/G ratio of lignin is important parameter for delignification process, and it is equal to 1.19 indicating that lignin rich in S-units will easily degrade even under mild alkaline conditions.