Trans-coniferyl alcohol

Recent work by Atalla(H) supports the idea that lignin is at least a semi-ordered substance in wood with the plane of the aromatic ring parallel to the cell wall surface. Woody plants synthesize lignin from trans-coniferyl alcohol (pines), trans-sinapyl alcohol 2 (deciduous), and trans-4-coumaryl alcohol 3 by free radical crosslinking initiated by enzymatic dehydrogenation(l2). Structures of these alcohols are given in Figure 1. 

Figure 1. Structure of trans coniferyl alcohol and trans sinapyl alcohol (reproducedfrom reference 30. CAS Registry database records duplicated with permission of CAS, a division of the American Chemical Society).

The guaiacyl unit, derived from the trans-coniferyl alcohol, abundantly present in softwoods... 

In the purest sense, lignin is a complex racemic consisting from aromatic heteropolymers produced by free radical coupling reactions initiated by enzymatic dehydrogenation of the three primary precursors, the hydroxycinnamyl alcohol monomers differing in their degree of methoxylation trans-4-coumaryl alcohol [3-(4-hydroxyphenyl)-2-propenol)], trans-coniferyl alcohol [3-(4-hydroxy-3-methoxyphenyl)-2-propenol)] and trans-sinapyl alcohols [3-(4-hydroxy-3,5-dime-thoxyphenyl)-2-propenol)] (Fig. 8.4). 

Gymnosperms Mostly trans-coniferyl alcohol, but some irafis-p-coumaryl alcohol and trans-sinapyl alcohol... 

Monocotyledons trans-Coniferyl alcohol, trans-sinapyl alcohol and trans-p-coumaryl alcohol... 

The second type of data is derived from biochemical experiments related to the three cinnamyl alcohols—/>-coumaryl alcohol (I), coniferyl alcohol (II), and sinapyl alcohol (III) and their phenolic glucosides— >-glucocoumaryl alcohol (IV), coniferin (V), and syringin (VI). These and the following formulas do not differentiate between cis and trans isomers. 

Other monolignols formed during the dehydrogenation of coniferyl alcohol are conifer aldehyde (VII), trans- and m-ferulic acid (XV, XVI), vanillin (traces), and vanillic acid (traces). 

Ralph, J., Hehn, R. F., Quideau, S., and Hatfield, R. D. (1992) Lignin-feruloyl ester cross-links in grasses. Part 1. Incorporation of feruloyl esters into coniferyl alcohol dehydrogenation polymers. J. Chem. Soc., Perkin Trans. / (21), 2961-2969. 

G Leary, The Chemistry of Reactive Lignin Intermediates. Part 1. Transients in Coniferyl Alcohol Photolysis. J Chem Soc, Perkin Trans II, 640, 1972. 

Lignins, a major component of wood (15-30 wt%), are phenolic polymers based on three structural units, the content of which depends on the type of wood trans-p-coumaryl alcohol (I), trons-coniferyl alcohol (II), and trans-sinapyl alcohol (III) (see Chart 8.3). 

Monolignol units usually maintain their trans-double bond in the side chain. During heartwood formation in gymnosperms, however, this double bond is reduced by phenylpropenal double bond reductases (PPDBR). The enzyme from Pinus taeda accepted dehydrodiconiferyl and coniferyl aldehydes but not the respective alcohols and NADPH as substrates (Kasahara et at, 2006). The structure of a very similar alkenal double bond reductase from Arabidopsis was studied (Youn et at, 2006b). 

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