Lignin skeleton

An alternative approach to measurement of lignin distribution is to assess variation in the porosity of wood which has been hydrolyzed to form a lignin skeleton (Donaldson 1987). Hydrolysis with sulfuric acid is particularly useful because of the swelling that occurs. This magnifies areas of the cell wall such as the S3 layer or the compound middle lamella which would otherwise be too small to measure. 

Figure 2. The technical nature of wood tissue. (A) Scanning electron micrograph (SEM) of the surface of a softwood resulting from a cut directed perpendicular to the tree axis. This view is known tecnnicallu as the transverse or cross-sectional plane. (B) Schematic showing the location of the major constituents of wood chemistry. (C) Transmission electron micrograph (TEM) of the transverse section of a white spruce wood lignin skeleton that was prepared by removing cellulose and hemicelluloses with hydrofluoric add. Key L, lumen W, wall and ML, middle lamella. (Reproduced with permission from Ref. 38. Copyright 1974, Forest Products...

Figure 18. TEM of normal layering (SfSJS ) in the fiber secondary wall ofloblolly pine transverse sections. (Reproducedfrom Ref 41. Copyright 1971, Springer-Verlag.) Key A, portion of two earlywood fibers (ML = middle lamella) and B, lignin skeleton of region simitar to that...

Figure 9, Oak, heavily degraded dissolving lignin skeletons of S 2 in system of ML S3 layer still intact (arrow) RC, ray cells scanning electron micrograph.

Along the degradation front, there is a substantial breakdown and dissolution of carbohydrates of both Sg and S1. About 90% of the carbohydrates still present in the slightly degraded tissue are dissolved here. The residual lignin skeletons of the secondary cell walls appear granular and are detached from the compound middle lamellae. In SEM preparations they often appear as a very flimsy network (Figure 20). 

In this transition zone we find secondary cell walls that take up blue stain, which indicates that they are strongly swollen. But evidently this state does not last very long. Breakdown of carbohydrates seems to set in immediately, with a detached granular lignin skeleton left behind (Figure 27). This thorough degradation starts at the lumen and comprises the whole secondary cell wall (S2 and Sj). The residual system of compound middle lamellae is very frail and brittle and is easily ruptured. 

Then the cellulose in the inner layers of the secondary cell walls, the S2 and S3, is attacked either from the lumen side or from the Sj-Sg border. Its crystalline structure is broken down, and the chain macromolecules are degraded and dissolved. The residual lignin skeleton degenerates to a granular debris that might shrink and come loose from the S, which is still intact and adheres to the middle lamellae. The thin Si is then degraded, until finally only the system of compound middle lamellae remains. 

The view that the lignin molecule was made up of building stones with a guaiacyl-syringyl propane skeleton was supported by the brilliant work of Hibbert on ethanolysis and that of Adkins and Harris on the pressure hydrogenation of wood and isolated lignin preparations. 

To our knowledge, the coumarone ketol (XXII) is the first crystalline dimeric lignin degradation product with two complete phenylpropane skeletons which has been reported in the literature 26). On the basis of the model experiments discussed above, the presence of the ketol side chain as well as the phenylcoumarone system in (XXII) indicates that the origin of the product is a sequence of three phenylpropane monomers (XXVIII) in lignin, involving a phenylcoumaran system carrying a gly-... 

Wood is built up of parallel columns of cells. Around these cells, cellulose embedded in lignin is wrapped. If wood is heated to temperatures above 600 °C (the exact temperature depends on the type of wood) in an inert atmosphere, then the polyaromatic biopolymers are broken down and what remains is a carbon skeleton with the anatomy of wood, both at a microscopic and macroscopic level. This skeleton facilitates the infiltration of, for instance, silicon and the reaction to silicon carbide. 

The wood cell consists mainly of cellulose, hemicelluloses, and lignin (see Appendix). A simplified picture is that cellulose forms a skeleton which is surrounded by other substances functioning as matrix (hemicelluloses) and encrusting (lignin) materials. 

The structural elements comprising lignin are linked by carbon-carbon and ether bonds. Units that are trifunctionally linked to adjacent units represent branching sites which give rise to the network structure characteristic of lignin (see Fig. 1.1, units 3, 8, and 15). As the interunit carbon-carbon linkages are difficult to rupture without excessively fragmenting the carbon skeleton of the... 

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