Lignin-carbohydrate Complexes from Coconut (Cocos nucifera) Coir: Fractionation, Structural Elucidation, and Potential Applications

Coconut coir, a major type of tropical lignocellulosic waste, has been restricted from higher-value applications due to its chemical complexity and inherent variability. To better disclose the chemical relationships between polysaccharides and lignin in coir and further exploit its high value-added bio-based materials, two lignin-carbohydrate complexes (LCCs) designated as glucan-lignin (GL) and xylan-lignin (XL) were successively isolated from coir via a simplified and quantitative fractionation method. The characterization of chemical composition, molecular weight distribution, and constituent substructures of the isolated LCC fractions were examined, and the lignin-carbohydrate (LC) chemical linkages were revealed as y-esters by a 2D heteronuclear singular quantum correlation (HSQC) NMR technique. Furthermore, XL was demonstrated as a more competitive candidate than GL for 2,2-diphenyl-1-picryl-hydrazyl (DPPH) removal in comparison with 2,6-di-tert-butyl-4-methylphenol (BHT, a commercial antioxidant), and was also featured as a more promising reinforcing agent for elevating the adsorption property of polyacrylamide-based hydrogels via the synergistic effect of physical and hydrogen bonding adsorptions.