Native lignin, structure

More recently, Brauns and Seiler (12), (13) extracted native lignin from spruce wood and then homogenized the wood in a colloidal mill. A suspension of approximately one gram of colloidal wood per liter of water was obtained. After the colloidal wood was changed to a solid form, it was found that its cell structure was completely destroyed. However, extraction of this wood with ethyl alcohol at room temperature did not yield further quantities of native lignin. These investigators have also reported the isolation of a lignosulfonic acid-carbohydrate compound. 

It therefore follows that when isolated lignins (and suberins) are examined and subsequent structural representations are proposed, critical information on native structure has already been lost, e.g., as regards the extent of polymer modification during removal from the cell wall, and the effect of mixing polymers from the various cell wall layers from which they originated. For these reasons, all current representations of native lignin (and suberin) structure should be viewed with caution until such questions are satisfactorily resolved. 

Figure 1. Structure of native lignin according to Adler (3).

The most likely lignin condensation reaction during the kraft cook is the formation of diphenylmethane structures 29, 30). These structures are also proposed 14) as being already present in native lignin. Phenolic diphenylmethanes do not exhibit any color, but they can easily be dehydrogenated to quinonemethides 20, 23) or quinonemethide radicals (P). 

The native Bjorkman milled wood lignin has a very slight yellow color and a low absorption in the visible part of the spectrum (Figure 2). If quinonemethides are present in the native lignin, as suggested by Harkin (20), the amount of such structures must be exceedingly small. 

Biphenyl structures and a-carbonyl-p-aryl ether structures, which are both assumed to be present in native lignin with a higher abundance than coniferyl alcohol structures (22,23) and both considered to be important leucochromophores, were not observed among die products, presumably because they are not present in spruce lignin as end groups. Both these types of structures are very stable and unlikely to be structurally changed during mild acid hydrolysis (24). 

The absorption spectra of (14) and wood slices treated according to the above sequence are similar, thereby providing evidence that structures (11) and (12) are present in native lignin. Quinone monochlorimide (15) reacts only with p-hydroxybenzyl alcohols (il), forming indophenol (blue) as shown below (Gierer 1954, 1956). 

It is known [21], that hydrolysis lignin differs from both native lignin and cellulose by its chemical composition and structure. In hydrolysis lignin there are many catbon-... 

Another case in point is the solvent extraction of lignins (Nimz, 1966). Are the propylbenzene oligomers found in the extract degradation products of the original lignin polymer, or are they precursors co-occurring with the polymer In any case, their identities have helped establish the structure of native lignin. 

Figure 2.1 is probably an oversimplified caricature of the complex lignin structure due to structural refinements of the outlined basic patterns and to the occurrence of other coupling modes between aromatic units, such as the p-6 one. There is a universal consensus for the predominance of the labile p-0-4 ether linkages (structure A, Figure 2.1) in native lignins, as well as the occurrence of the more resistant p-5,... 

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