![Figure 2. (A). The three canonical monolignols (left: p-coumaryl, coniferyl and synapyl alcohol) are structurally very related to other lignols (right) produced through the same phenylpropanoid pathway (see Figure 3). Of note, the catechol groups in the caffeyl alcohol (producing C units in lignin) and 5-hydroxyconiferyl alcohol allow for a different radical/oxidative reactivity. (B). A few flavonoids (based on a polyphenolic α,β-unsaturated cyclic ketone structure (chromone); left) and hydroxystilbenes and their glycolides (1,3-diphenolic ring linked to a phenolic, catecholic or 2-methoxyphenolic ring through an ethylene residue; right) have been found to be monolignols too and are produced through the acetate/malonate polyketide pathway. Taxonomically, tricin is a flavon; dihydrotricin and naringenin are flavanones, and naringenin chalcone is-as suggested by the name-a chalcone. (C). Two hydroxycinammamides behave as monolignols. They are ferulic acid derivatives, which derive from the amino acid metabolic pathway. (D). Flavonolignans (left of the vertical dashed line) and stilbenolignans (right of the dashed line) are low MW products of reaction between a flavonoid (e.g., tricin) or a hydroxystilbene (e.g., piceatannol) and a phenylpropanoid lignol, whose sub-structures are separated by a red dashed line in the panel. While typical mechanisms for such reactions are listed in Figure 4. The reader is addressed elsewhere for more comprehensive lists of flavono-[63] and stilbenolignan [64] structures.](https://www.researchgate.net/publication/372484023/figure/fig1/AS:11431281176109470@1690036467741/A-The-three-canonical-monolignols-left-p-coumaryl-coniferyl-and-synapyl-alcohol_Q320.jpg)
![Figure 3. The phenylpropanoid pathway leads to the biosynthesis of both canonical (no background) and non-canonical monolignols (light yellow background), while other non-canonical monolignols are produced through different but connected pathways (pink background). Phenylalanine (Phe) is sequentially converted to p-coumaryl alcohol (H unit) by phenylalanine ammonia-lyase (PAL), cinnamate 4-hydroxylase (C4H), 4-coumarate:CoA ligase (4CL), cinnamoyl-CoA reductase (CCR), and cinnamyl alcohol dehydrogenase (CAD). p-coumaric acid (red circle) is the 'hinge' of all these biosynthetic paths, since not only it is the precursor of p-coumaryl alcohol, but directly (via p-coumarate 3-hydroxylase, C3H) or through p-coumaryl-CoA (the so-called shikimate shunt (blurred red arrow). Enzymes involved: p-hydroxycinnamoyl-CoA:quinate/shikimate (HCT), p-coumaroyl shikimate 3-hydrolase (C3 H), and caffeoyl shikimate esterase (CSE)) also lead to the production of caffeic acid and then to that of all non-canonical phenylpropanoid monolignols. There, multiple and redundant pathways lead to coniferyl (G unit) and sinapyl alcohols (S unit) and involve caffeic acid O-methyltransferase (COMT), caffeoyl-CoA O-methyltransferase (CCoAOMT), and ferulate 5-hydroxylase (F5H). Please note that among the non-canonical, phenylpropanoid-derived monolignols here we consider also compounds absent in Figure 2, such as ferulic acid (used in the formation of polysaccharide-lignin complexes, see later) or dihydroconiferyl alcohol, which is present in the lignin of CAD-deficient trees [65]. PMT: p-coumaroyl-CoA monolignol transferase (PMT). FMT: feruloylCoA with feruloyl-CoA monolignol transferase. For the non-phenylpropanoid pathway, hydroxystilbenes are produced through stilbene synthase (STS), and flavonoids are produced through chalcone synthase (CHS), whereas hydroxycinnamoyl-CoA:tyramine N-hydroxycinnamoyltransferase (THT) and hydroxycinnamoyl-CoA:putrescine hydroxycinnamoyltransferase (PHT), respectively, mediate the biosynthesis of diferuloylputrescine and of feruloyltyramine.](https://www.researchgate.net/publication/372484023/figure/fig2/AS:11431281176128406@1690036467899/The-phenylpropanoid-pathway-leads-to-the-biosynthesis-of-both-canonical-no-background_Q320.jpg)
![Figure 8. Schematic illustration of the synthesis of lignin-incorporated nanogels for wound healing application. (A). Extraction of lignin from coconut husk. (B). Synthesis of the thermoresponsive nanogel based on poly(ethylene glycol) (PEG, brown), poly(propylene glycol) (PPG, grey), and poly(dimethysiloxane) (PDMS-diol, blue). (C). Preparation of temperature-sensitive lignin-incorporated nanogel. (D). Wound healing application of the nanogel. Reprinted with permission from ref. [238] Copyright 2021, American Chemical Society.](https://www.researchgate.net/publication/372484023/figure/fig4/AS:11431281176118922@1690036468361/Schematic-illustration-of-the-synthesis-of-lignin-incorporated-nanogels-for-wound-healing_Q320.jpg)
July 2023
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14 Citations
International Journal of Molecular Sciences
At a time when environmental considerations are increasingly pushing for the application of circular economy concepts in materials science, lignin stands out as an under-used but promising and environmentally benign building block. This review focuses (A) on understanding what we mean with lignin, i.e., where it can be found and how it is produced in plants, devoting particular attention to the identity of lignols (including ferulates that are instrumental for integrating lignin with cell wall polysaccharides) and to the details of their coupling reactions and (B) on providing an overview how lignin can actually be employed as a component of materials in healthcare and energy applications, finally paying specific attention to the use of lignin in the development of organic shape-memory materials.