Lignin

From Palaeos

Wood. Lignin is formed by the removal of hydroxyl groups from sugars, creating phenolic compounds and short-chain alcohol ligands. Lignin polymers are heavily cross-linked. There is great variation in lignin, even within the same plant. Undoubtedly, it has evolved to have a somewhat random structure to foil enzymatic attack. Lignin is, in fact, extremely difficult to dissociate -- so much so that even as late as 1980 almost nothing was known of the detailed chemistry of lignin, the second most common biopolymer on the planet (next to cellulose). Fortunately, considerable progress has been made since then. The basic monomer of lignin can be thought of as 4-alkylcatechol. The alkyl ligand is a 1 to 3 carbon chain which may be substituted with hydroxy or keto ligands at any or all positions. Both phenol groups and the alkyl substitutions may cross-link to other monomers or their side chains, yielding a fairly light, but strongly cross-linked and randomly ordered mass, i.e., wood. What is more, the chemical structure virtually guarantees that any intermediate degradation product will be a highly toxic phenolic compound. Lignin was a rather early product of plant evolution. The necessary implication is that, contrary to most current thought on the subject, land plants were subject to biological attack (although not necessarily attack by herbivorous animals) by at least the Middle Devonian, and probably even earlier.

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