Formation from lignin

Guaiacylglycerol-/3-coniferyl ether is a dimeric decomposition product of coniferous lignin dehydrodivanillin may be also formed by dimerization of vanillin after formation from lignin. Among the lignin decomposition products are mono-, di- and triphenol derivatives which are derived from the different types of lignin of needle trees, deciduous trees, or graminees. 

Fig. 17 Mechanism of free radical formation from lignin degradation [93]...

Siegel S.M. (1957) Non-enzymic macromolecules as matrices in biological synthesis. The role of polysaccharides in peroxidase catalyzed lignin polymer formation from eugenol // J. Amer. Chem. Soc. V. 79. P. 1628-1632... 

Freudenberg, K. Lignin. Its constitution and formation from p-hydroxycinnamyl alcohols. Science 1965, 148, 595-600. 

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. 

There are 210 different isomeric possibilities, 75 of which are PCDDs and 135 are PCDFs. The toxicity of these isomers varies greatly, and only 15 exhibit extreme toxicity, the most toxic of which is 2,3,7,8-tetrachlorodibenzodioxin (2,3,7,8-TCDD). The toxicity of the other isomers is therefore expressed as a toxicity equivalent of 2,3,7,8-TCDD. The PCDDs and PCDFs are poorly water soluble but are fat soluble and are therefore able to accumulate in tissue fat, thus allowing them to bio-accumulate in living organisms. The origin of dioxins in the pulp and paper industry is not entirely clear. They may be produced from the chlorination of dibenzodioxin which may be present in recycled oils used to make defoamers, but they may also arise from wood chips which have been treated with polychlorophenol to prevent sap stain formation. It is also possible that they are derived from lignin by chlorination. Dioxins are also known to be formed naturally by combustion of material such as wood, and forest fires have been particularly identified as a likely major cause of dioxin emissions. 

Lignin its Constitution and Formation from />-Hydroxycinnamyl Alcohols. Science [Washington] 148, 595—600 (1964). 

Higmhi, T. Studies of Lignin using Isotopic Carbon X [cf. (2.5)]. Formation of Lignin from Phenylpropanoids in Tissue Culture of White Pine. Canad. J. Biochem. Physiol. 40, 31—34 (1962). 

Effect of pH on Lignin Peroxidase Catalysis. The oxidation of organic substrates by lignin peroxidase (Vmax) has a pH optimum equal to or possibly below 2. Detailed studies have been performed on the pH dependency of many of the individual reactions involved in catalysis. The effect of pH on the reaction rates between the isolated ferric enzyme, compounds I or II and their respective substrates has been studied. Rapid kinetic data indicate that compound I formation from ferric enzyme and H2O2 is not pH dependent from pH 2.5-7.5 (75,16). Similar results are obtained with Mn-dependent peroxidase (14). This is in contrast to other peroxidases where the pKa values for the reaction of ferric enzyme with H2O2 are usudly in the range of 3 to 6 (72). 

Some of the pathways of animal and bacterial metabolism of aromatic amino acids also are used in plants. However, quantitatively more important are the reactions of the phenylpropanoid pathway,173-1743 which is initiated by phenylalanine ammonia-lyase (Eq. 14-45).175 As is shown at the top of Fig. 25-8, the initial product from phenylalanine is trails-cinnam-ate. After hydroxylation to 4-hydroxycinnamate (p-coumarate) and conversion to a coenzyme A ester,1753 the resulting p-coumaryl-CoA is converted into mono-, di-, and trihydroxy derivatives including anthocyanins (Box 21-E) and other flavonoid compounds.176 The dihydroxy and trihydroxy methylated products are the starting materials for formation of lignins and for a large series of other plant products, many of which impart characteristic fragrances. Some of these are illustrated in Fig. 25-8. 

Figure 8. Synthesis of phenylcoumaran XXII) and its formation from a trimeric structure in lignin...

Phenolic Hydroxyl Group. An ethanolic solution of 3,4,5-tri-methoxybenzyl alcohol (4-0-methylsyringyl alcohol) and sodium hydroxide was prepared, and ultraviolet spectra of the solution were recorded immediately and 3 days after preparation. These spectra were compared with the spectrum of the model compound in neutral ethanol. The three spectra were identical with the absorption curve possessing a broad maximum in the 270-280 m/x region. Further visual observation of the alkaline solution for 2 weeks revealed no color formation. This suggests that phenoxide ion formation may be a necessary initial step in reactions leading to the development of chromophoric structures from lignin model compounds. 

Freudenberg (77, 15, 76, 17) studied the formation of formaldehyde from lignins under acidic and alkaline conditions and suggested that cohydroxymethyl groups may be the source of formaldehyde. Kratzl 24)... 

Figure 4-4. Reaction mechanism for the formation of 1,2,3- trithioethane phenylpropanoid monomers (4.25) from lignin (4.19). Ri is either an aryl group or a hydrogen atom. In H-residues both R3 and R5 are hydrogen atoms, in G-residues R3 is a methoxyl group and R5 is a hydrogen atom, and in S-residues both R3 and R5 are methoxyl groups. R4 is either a hydrogen atom or an alkyl group. The wavy bonds indicate that both the S- and R-stereo-isomers are present.

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