Physicochemical Properties of Conifer Lignins Using Juniperus communis as an Example

Abstract

The physicochemical properties of slightly altered dioxane lignins isolated from Juniperus communis wood of different ages were studied. It was shown that the functional nature and reactivity of the lignins changed during lignification of the wood. The composition, structure, and properties of lignocarbohydrate composites are formed at the nano-level with direct involvement of multi-enzyme complexes in wood biosynthesis. Then, they are responsible for the behavior of plant polymers to chemical and physical factors. Therefore, issues with the biosynthesis of cell-wall components and their self-organization are becoming more and more important. Wood is viewed as a bionanocomposite according to current concepts (1). It consists of polysaccharide components (cellulose, hemicelluloses, and pectinic substances) in addition to lignin, an aromatic polyfunctional polymer of irregular structure. The functions of lignin in wood include its participation in biosynthesis and formation of the wood supramolecular structure, provision of rigidity to the cell-wall carbohydrate composition, and protection of the plant from pathogenic microorganisms. In many respects, this is determined by its functional nature and physicochemical properties. Much information has now been accumulated on issues of lignin biosynthesis. However, changes of the lignin reactivity at separate stages of the biochemical process remain unclear. We chose Juniperus communis, an ancient conifer species, as the biological subject for isolating native lignin preparations. It has a long life cycle and broad geographic distribution, and is of interest for the production of valuable biologically active substances (2). Herein we present analytical results for the change of physicochemical properties of lignin that determine its biosynthesis in plants. The quantitative ratio of principal wood components changed during cell-wall biosynthesis (3). Thus, the content of cellulose was 36.4-47.0%; of lignin, 28.6-34.2%. This was characteristic of conifer species. Figure 1 shows the change of lignin content as a function of wood age. The wood lignocarbohydrate composition depends on the biosynthetic features of the separate cell-wall components and the course of the lignification. According to current thinking, lignin biosynthesis occurs through a free-radical mechanism by addition of separate phenoxyl radicals to the growing polymer (4). Monolignols in different redox states are involved in the biochemical reactions. Plant peroxidases play a significant role in the formation of the phenoxyl radicals and the initiation of lignification. Considering this, lignin biosynthesis can be represented by the following general scheme: H2O2