Theories Lignin

Figure 2.5. Schematic representation of the modified lignin theory of humus formation. Adapted with permission from Stevenson, F. J. (1994). Humus Chemistry Genesis, Composition, Reactions, 2nd ed. John Wiley and Sons, New York.

A modification of the lignin theory of the formation of humic substances is the polyphenol theory, which considers the key interactions to be those between quinones, derived either from polyphenols or lignin, and amino compounds. 

SCHEME 2.21 Two possible reaction mechanisms for the initiation step of the lignin polymerization, investigated at PCM-B3LYP/6-311G(2df,p)//B3LYP/6-31G(d,p) level of theory. 

To correlate these changes in absorption with theory, the ultraviolet spectroscopic behavior of model compounds closely related to the degradation products isolated from bagasse native lignin, i.e., p-hydr-oxybenzaldehyde, vanillin and syringaldehyde, was determined. The compounds used were p-hydroxypropiophenone, vanillin, acetovanillone and acetosyringone and their derivatives. 

That some of the lignin, undoubtedly, is present in a free state in wood, is concluded from the fact that up to 10% of the total native lignin in the wood can be extracted with ethyl alcohol at room temperature. Further support for the incrustation theory is given by Nord in that an additional 23% of the native lignin in white Scots pine wood... 

A great deal of work is reported in the literature concerning the formation of lignin directly from cellulose or pentosans. Phillips (92), (93), on the basis of his work on annual plants, opposed this theory. According to him, lignin is produced in the plant directly from glucose or sucrose. 

The term computational chemistry can refer in its broadest sense to a wide range of methods that have been developed to give insight into the fundamental behavior of chemical species. Such methods include, but are not necessarily limited to, those related to quantum mechanics (1), molecular mechanics (or force-field calculations) (2), perturbation theory (3), graph theory (4), or statistical thermodynamics (5). For the purposes of this chapter, comments will be restricted to force-field and quantum-based calculations, since these are the techniques that have been used in work on lignin. Furthermore, these methods have been reviewed in a very readable book by Clark (6). 

The utilization of Pearson s hard-soft acid-base theory to interpret the reactions of lignin has also been described in work from the Soviet Union by Zarubin and Kirysun (59). Beyond this specifically related paper, researchers in the Soviet Union have been quite active in the application of numerical methods to lignin-related problems (60,61). 

According to the present theory (e.g. Zherebov [1]) most of the hemicelluloses, pentosans and hexosans are removed during sulphitation, because calcium sulphite is capable of forming addition compounds with pentosans. Complete removal of hemicelluloses is only possible however after lignins have been removed, as lignin seems likely to be chemically combined with hemicellulose and thus with pentosans or hexosans. Only rupture of these bonds and the separation of lignins makes it possible to attack the non-cellulosic polysaccharides (Heuser and Haug [2]). 

Selective preservation theories have been questioned by several authors (O Brien and Stout, 1978 Nadelhoffer and Fry, 1988 Melillo et al., 1989) based on the fact that 813C values generally increase with depth in soils, whereas lignin and fatty acids... 

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