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Monolignol Essay

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The monolignols are incorporated into lignin in the form of phenylpropanoids, such as guaiacyl (G) (Coniferyl alcohol), syringyl (S) (Sinapyl alcohol), and p-hydroxyphenyl (H) (p-Coumaryl alcohol). In general gymnosperm are composed mainly of G-units with little amount of H-units, and angiosperm dicot lignins of G- and S-units, but there are some exception (Uzal et al., 2009). Furthermore, lignin can be dissimilar among cell types and even at the level of individual cell wall layers, as revealed by Raman, IR and UV micro spectroscopy or by immunolabeling of the secondary wall with AB cross-reacting with specific lignin substructures (Shi et al., 2006, Gierlinger and Schwanninger, 2007, Gou et al., 2008, Ruel et al., 2009). Lignin polymerization starts with dehydrogenation of the monolignol phenol, generating a phenolic radical with the delocalization of the unpaired electron. …show more content…

Monolignol radicals favor coupling at their  position, resulting essentially in only the -, -O-4 and -5 dimers. This radical-radical polymerization process occurs in a chemical-combinatorial fashion; thus there is not a template to follow like it happens in DNA polymerization or protein synthesis but lignin polymerization is randomly driven by the chemical conditions of the cell and by the nature of the monomers (Ralph et al., 2004). Consequently the dimer needs to be dehydrogenated again becoming a phenolic radical in order to bind to another monomer radical. This mode of action, in which one only monomer at a time adds to a growing polymer, is termed endwise coupling (Figure

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