Polymers formed from lignins

Lignin, which represents some 20% to 30% of wood, can be regarded as a polymer formed from phenylpropane skeletons, such as 3-methoxy-4-hy-... 

Several simple phenylpropanoids (C6-C3) are produced from cinnamate via a series of hydroxylation, methylation, and dehydration reactions these include p-coumaric, caffeic, ferulic and sinapic acids and simple coumarins. The free acids rarely accumulate to any great extent within plant cells but are usually conjugated to sugars, cell wall carbohydrates, or organic acids (quinic, malic, tartaric, etc.). Lignin and suberin are complex polymers formed from a mixture of simple phenylpropanoids, and will not be considered in the present review. 

Polysaccharides are carbohydrate polymers, formed from either mono- or disaccharides through glycosidic bonds. The most significant polysaccharides of relevance here are cellulose, starch, lignin, pectin and chitin. The structures of some of these polymers are shown in Figure 7.2. 

The second group of phenylpropanoids, which is the main emphasis of this chapter, consists of those components which are integrated into the cell wall framework. This group can be subdivided into three categories monomers, such as hydroxycinnamic acids, dimers, such as didehydrofer-ulic and 4,4 -dihydroxytruxillic acids, and polymers, such as lignins and suberins. It is important to emphasize, at this juncture, that the dimers (4,5) and polymers (8,9) discussed in this chapter are considered to be formed within the cell walls from their corresponding monomers. 

The production of humic substances by microorganisms is an extracellular process, because the enzymes are secreted into the external solution that contains the phenolic compounds derived from lignin and tannic acid degradation and microbial and plant metabolites. These phenolic compounds can then be enzymatically oxidized to quinones, which can undergo further polymerization or polycondensation reactions with other biomolecules (e.g., amino acids) to form humic polymers (Stevenson, 1994 Bollag et al., 1998 Burton, 2003). 

Catechol melanin, a black pigment of plants, is a polymeric product formed by the oxidative polymerization of catechol. The formation route of catechol melanin (Eq. 5) is described as follows [33-37] At first, 3-(3, 4 -dihydroxyphe-nyl)-L-alanine (DOPA) is derived from tyrosine. It is oxidized to dopaquinone and forms dopachrome. 5,6-Dihydroxyindole is formed, accompanied by the elimination of C02. The oxidative coupling polymerization produces a melanin polymer whose primary structure contains 4,7-conjugated indole units, which exist as a three-dimensional irregular polymer similar to lignin. Multistep oxidation reactions and coupling reactions in the formation of catechol melanin are catalyzed by a copper enzyme such as tyrosinase. Tyrosinase is an oxidase con-... 

The alcohols formed from some cinnamic acid derivatives, namely /7-coumaryl alcohol, coniferyl alcohol (LI), and sinapyl alcohol (L2), commonly known as monolignols, undergo dimerization reactions that yield lignans such as (-l-)-pinoresinol (L3), (-l-)-sesamin (L4), (-)-matairesinol (L5), and podophyllotoxin (L6) (Fig. 13). Several thousand lignans are found to occur in nature. Lignins, the structural components of plant cell walls, are polymers of monolignols and/or lignans. 

A term more general than polymer is that of macromolecule. Macromolecules are chemical compounds formed from at least one thousand atoms linked by covalent bonds. They are common as natural substances like cellulose, proteins, lignin, etc., and also as synthetic compounds including plastics, fibers, elastomers, coatings, and adhesives. Many synthetic and some natural macromolecules have repetitive structures and are known as polymers. For example, cellulose is made from p-D-glucose residues interconnected by p-glucoside (1->4) links, polystyrene is made from 1-phenylethylidene units, etc. 

Lignin, one of the major polymeric constituents of wood, is a highly crosslinked phenolic polymer derived from the free-radical coupling of substituted phenylpropa-noids (Figure 5.9). It reacts with aqueous chlorine to form oxidized and chlorinated species. This is a key reaction in the bleaching of wood pulp by the kraft process... 

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