The Structure of Lignin

Hardwood lignins contain both guaiacylpropane (G) and syringylpropane (S) units, with the G/S ratio from 4 1 to 1 2, and smaller amounts of p-hydroxyphenylpropane (H). Hardwood lignins are of lower moleeular weight, perhaps because the syringylpropane units are unable to cross link at the Cs position as it is blocked by the additional methoxyl group. 

In summary, lignin polymerization is carefully controlled and occurs in different parts of the cell wall at different times utilizing specific monolignols with the assembly template for seeding polymerization being the hemicelluloses (or proteins) already adsorbed on the microfibrils - and the lignin molecule takes its stractural form from that template. 

Further, in the eellulose phase the surface chains of the microfibrils are subtlely different in their hydrogen bonding to those in the crystalline interior. 

Clearly there are commercial interests in developing clonal lines of trees that have modified or reduced amounts of lignin for fast-growing, short-rotation crops 

---

Although many suggestions have been advanced for the structure of lignin (15), establishment of a valid constitutional formula for this substance is still far from realized. It was considered (67) that the building stones of lignin are linked together by a methylene dioxv bridge... 

These complications proved to be the greatest deterrents to identification of the structure of lignin. Without knowledge of the make-up of lignin, it is clear that little promise could be expected from empirical studies directed at finding useful applications for it. In view of this situation, there has been a great intensification in fundamental research on the structure and properties of lignin in the hope that a better scientific foundation could be laid for future applied research. 

Present views on the structure of lignin are still largely speculative and in particular little is known of the non-aromatic parts of the molecules. 

We now wish to report our results on isolating lignin-derived dimers by alkaline hydrogenation and to propose mechanisms for their formation. In addition, we will present some information on the (NMR) spectra of these and related compounds this technique has been useful for analyzing the structure of lignin degradation products and related synthetic materials. 

Figure 13.3 The structure of lignin, and the phenolic moieties that may be derived from it.

The formation of lignin proceeds by the uncontrolled coupling of the radical species, which leads to an irregular structure (Figure 2). The structure of lignin can be represented by the statistic ratio of each monomer, the frequency of each coupling mode, and the steric hindrance of substituents. This reaction mechanism has been successfully simulated [22],... 

Lignin polymers are often found in most plant structures in association with cellulose. The structure of lignin is not well defined, but lignin appears to be made up of polymers of propylbenzene with hydroxy and methoxy groups attached. Lignin is primarily hydrocarbon in nature and makes up a major portion of insoluble dietary fiber. 

Lignin is an amorphous non-polar macromolecule, constituted of phenylpropane units. The structure of lignin depends on the source. Moreover, the extraction method modifies the structure of the lignin prior to analysis. Recent studies (Banoub et al., 2007 Forss and Fremer, 2000) have shown fundamental differences with the known structures of lignin. 

Fiserova M, Suty L (1980) Comparative studies on milled wood lignins and cellulolytic enzyme lignin of beech wood (Fagus silvatica L ) Cellul Chem Technol 14 243-252 Glasser WG, Barnett CA (1979) The structure of lignins in pulps II A comparative evaluation of isolation methods Holzforschung 33 78-86... 

Hemmmgson JA (1983) The structure of lignin from Pinus radiata exploded wood J Wood Chem Technol 3 289-312... 

Glasser WG, Morohoshi N (1979) The structure of lignins in pulps The distribution of interunit linkages Tappi 62(12) 101-105... 

Despite extensive research, the commercial utilization of lignin in phenolic resins is negligible. The major problem is that the structure of lignin restricts its re-... 

---