Unraveling the structural diversity of lignin, a wood-derived raw material that holds the key to replacing fossil resources
Updated by Shinya Kajita on July 25, 2025, 3:17 PM JST
Shinya KAJITA
Graduate School of Tokyo University of Agriculture and Technology
Professor, Department of Biological System Science, Graduate School of Agriculture, Tokyo University of Agriculture and Technology / Specializes in plant molecular biology and biomass chemistry. His research aims to elucidate the mechanism of lignin production by trees and to create tailor-made lignin suitable for industrial use using biotechnology.
The structure of wood, with cellulose, hemicellulose, and lignin as its main chemical components, is often compared to reinforced concrete. The polysaccharides cellulose and hemicellulose correspond to thick steel beams and thin rebar, respectively, and lignin to concrete. Like reinforced concrete, wood has long been used as a material for building materials, furniture, and various household utensils because of its physical strength. In addition, by separating some or most of the lignin, the cell walls in wood can be easily loosened and used as a raw material for pulp and paper.
Like many synthetic polymers made from petroleum, polymers made by living organisms often have repeating structures. Cellulose is a polymer of glucose linked by glucosidic bonds, and starch, which we eat as food, is also linked by α-1,4 or α-1,6 type glycosidic bonds, although there are branches. DNA, the blueprint of our body, is a polymer of four types of nucleotides consisting of sugar, phosphate, and base, linked by phosphate diester bonds. Proteins, which make up about 20% of the human body, are also polymers consisting of 20 types of amino acids linked by peptide bonds.
The reason why the same bonds exist repeatedly in biopolymers as in synthetic polymers is due to the presence of enzymes that catalyze the elongation of molecules. Cellulose, starch, DNA, and proteins are synthesized in vivo under the control of enzyme-catalyzed reactions, and their molecular structures are strictly defined. In other words, if these molecules are not produced as predetermined, they are not useful for the maintenance of living organisms, be they plants or animals.
Of the first three main components of wood, lignin is made in a somewhat unusual way and does not have a specific repeating unit like glucose in cellulose. Lignin is mainly composed of three siliceous alcohols, p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, polymerized (linked) via mutual radical reactions as monomers. Each of these lignin monomers has three to four reaction points (linkage sites), and the manner of linkage differs depending on which of them is used.
In the case of cellulose, glucose residues are enzyme-mediated, so neighboring residues are linked only between carbons at positions 1 and 4. However, the linkage of the siliceous alcohols that make up lignin proceeds by radical reactions that do not involve any enzymes, so the linkage is not predetermined.
For this reason, there are more than 10 different bonding modes in the lignin molecule, and the order in which they are arranged depends on the conditions of the reaction field. In other words, the molecular structure of lignin is like a Lego piece that varies in shape and color depending on the creator. In other words, the structure of lignin is quite "fuzzy" for a biopolymer, in contrast to molecules such as cellulose, DNA, and proteins, which require a strict sequence.
How does this structural ambiguity found in lignin benefit the formation of wood? As mentioned earlier, if we compare wood to reinforced concrete, lignin is the concrete that is poured into the gaps between the polysaccharide framework that serves as the steel frame and rebar. If the structure of the lignin were strictly defined, it would be quite difficult to fill and consolidate the gaps between the differently shaped voids of various sizes that exist between the framework. Lignin is required to change its own shape to fit the gaps in the cell walls, just like highly fluid concrete.
I think you understand that for wood to be strong, it is better if the structure of the lignin is not strictly defined. However, it is tricky when we use wood, and lignin in particular, as a raw material for certain low molecular weight compounds as an alternative to fossil resources. For cellulose, which has a repeating structure, glucose can be obtained in a relatively high yield by acid treatment. Lignin, on the other hand, does not have a strict repeating structure, so it is unlikely to efficiently recover a degraded product with the same structure even if it is degraded in a specific way.
For these reasons, lignin has been utilized as a heat source mainly by burning, but its application as a material is also expected to expand greatly in the future. For specific applications of lignin, please refer to the article that has already been posted on this site or will be posted in the future. (Shinya Kajita, Professor, Department of Biological System Science, Graduate School of Agriculture, Tokyo University of Agriculture and Technology)
