Lignin derivatives association

Lignin-derived compounds have been observed in pyrolysis products of low rank coals and associate woody tissue FI- 61. Mycke and Michaelis [2] isolated lignin-derived methoxyphenols from a Miocene coal by catalytic hydrogenolysis. Sigleo [S] reported the presence of phenolic compounds derived from iteration of lignin in pyrolysis products of silicified woody tissue as old as Triassic age. However,... 

Tannins will be associated with lignin derivatives in the NMR spectra of HS. There can be significant resonance in the 100- to 110-ppm resonances, which hence can overlap with the anomeric carbon resonance (centered at 105 ppm) of saccharides. However, the uses of chemical shift anisotropy (CSA) and dipolar dephasing (DD) NMR techniques (Mao and Schmidt-Rohr, 2004) allow the distinction to be made between tannin and anomeric carbon resonances. 

The molecular weight of lignin in the wood, ie, of protolignin, is unknown. In addition to difficulties of isolation and purification, the polymer exhibits strong solvent, ionic, and associative effects in solution. An unequivocal method of measurement has not been developed. The polymer properties of lignin and its derivatives have been discussed (10,16). 

Nowadays there is scientific evidence that, besides plant polysaccharides and lignin, other indigestible compounds such as resistant starch, oligosaccharides, Maillard compounds, and phytochemicals—mainly polyphenols—can be considered DF constituents (Saura-Calixto and others 2000). Of these substances, resistant starch is a major constituent in cereals, whereas phytochemicals are the most important such substance in fruits and vegetables. Here, we address mainly polyphenols and carotenoids associated with DF in fruits and vegetables because of the important biological properties derived from them. 

Proanthocyanidins and Procyanidins - In a classical study Bate-Smith ( ) used the patterns of distribution of the three principal classes of phenolic metabolites, which are found in the leaves of plants, as a basis for classification. The biosynthesis of these phenols - (i) proanthocyanidins (ii) glycosylated flavonols and (iii) hydroxycinnamoyl esters - is believed to be associated with the development in plants of the capacity to synthesise the structural polymer lignin by the diversion from protein synthesis of the amino-acids L-phenylalanine and L-tyro-sine. Vascular plants thus employ one or more of the p-hydroxy-cinnarayl alcohols (2,3, and 4), which are derived by enzymic reduction (NADH) of the coenzyme A esters of the corresponding hydroxycinnamic acids, as precursors to lignin. The same coenzyme A esters also form the points of biosynthetic departure for the three groups of phenolic metabolites (i, ii, iii), Figure 1. 

In this chapter chemical conversions of natural precursors resulting in flavour chemicals are discussed. The main groups of natural precursors are terpenes for all kinds of terpene derivatives, vanillin precursors like lignin and eugenol, sugars for Maillard-associated flavour chemicals, amino acids and molecules obtained by fermentation or available as residual streams of renewable resources. 

Recently Bogolytsin and co-authors (62) found that the associations of sulfur dioxide and other inorganic sulfur derivatives interact with the aromatic nuclei of lignins, and strongly influence their reaction and redox behavior. This association effect also forms a selective pre-association state in the sulfonation reaction in wood pulping. It is evident that a wide range of similar association effects may be present, but they remain to be detected and studied. 

In addition, the parenchymatous cells in the xylem appear to yield at least two lignitic materials. One is a dark red-brown material that may be derived from inclusions originally present in the cell. This material is common and may be observed in Figures 2, 3, 5, 6, 7, and 8. The other coalified product associated with this cell type is yellowish in color and derived from the wall of the cell (Figures 2 and 6). Ray parenchyma and longitudinal xylem parenchyma typically form only a primary wall, and usually the wall is devoid of lignin. It is of interest that this wall layer is preserved and that it is frequently possible to observe the simple pits that are characteristic of this cell type (Figure 2). 

The structural proposal for humic acids by Stevenson (1982) had significant logic, based on the state of information at that time. The structure (Figure 1.3) has compositional aspects of phenols derived from lignin and from tannins, and paper chemistry exercises would allow reactions and interactions needed to give structures of the type predicted. The structure shown is highly aromatic, and the proposed aliphatic moieties are saccharide- and peptide-derived. Modern analytical procedures invariably show peptide and saccharide components to be associated with... 

This process was developed for cellulose-rich raw materials since high concentration of the acid may cause degradation of the pentoses in hemicellulose to furfural derivatives. The ethanol yield is usually about 35%. However, corrosion problems and the need for higher capital investment and dangers associated with the recovery of the concentrated acid make this method less attractive. Furthermore, during combustion of lignin that is contaminated with hydrochloric acid there is some risk of dioxin emissions. 

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