Suberin monomers

R. Ekman, The suberin monomers and triterpenoids from the outer bark of Betula verrucosa Ehrh, Holzforschung, 37, 205 211 (1983). 

It is important to emphasize that the exploitation of suberin monomers is not done to the detriment of the noble parts of cork, since these are extremely useful manifestations of renewable resources. What is used instead are the rejects of cork processing, like powder from stopper manufacturing and irregular morphologies. 

For instance an epoxy-functionalized polyester from the suberin monomer cis-9,10-epoxy-18-hydroxyoctadecanoic acid (see also Chapter 1) was synthesized by Olsson et al. [30]. The lipase-catalyzed polymerization was performed in toluene in the presence of 4A molecular sieves for 68 h and high molecular weight of epoxy-functionalized polyester was obtained (M , =20000 M JMn =2.2). 

Fig. 6. Structures of common cutin and suberin monomers, and ranges of typical composition values. Non-substituted fatty acids are not represented. There are overlaps in some classes of monomers (e.g. some monomers are epoxy hydroxy-fatty acids, of epoxy dicarboxylic acids).

Relative abundance of aliphatic suberin monomers from the extractive-free Quercus suber cork and Betulapendula outer bark (adapted from [14])... 

Alkanoic acids represent a smaller fraction of suberin monomers, viz. 2.5-14.9 per cent and 7.4-12.3 per cent in cork and birch bark suberin, respectively. In cork, this fraction is mainly composed of saturated even-numbered homologues, ranging from C16 to C26, with the C18 (including the 9,10-oxygenated derivatives) and C22 as the dominant components. 

As a general overview, crucial for tailoring applications of suberin monomers in polymer synthesis, it can be concluded that Cl8, followed by C22 monomers, are the dominant aliphatic compounds and that most of them are carboxylic acids, bearing at least one aliphatic-OH functionality. Additionally, the 9,10-epoxide functionality is also very common. 

In a preliminary study also involving model compounds [81], the kinetics of urethane formation was followed by FTIR spectroscopy using an aliphatic and an aromatic monoisocyanate and their homologous diisocyanates. Both the model reactions and the polymer synthesis gave clear cut second-order behaviour, indicating that the hydroxyl groups borne by the suberin monomers displayed conventional aliphatic-OH reactivity. 

Olsson A., Lindstrom M., Iversen T., Lipase-catalyzed synthesis of an epoxy-functionalized polyester fix>m the suberin monomer cw-9,10-epoxy-18-hydroxyoctadecanoic acid. Biomacromolecules, 8(2), 2007,757-760. 

Douliez J.R, Barrault J., Jerome F, Heredia A., NavaUles L., Nallet F., Glycerol derivatives of cutin and suberin monomers Synthesis and self-assembly. Biomacromolecules, 6(1), 2005, 30-34. 

A variety of oxygenated fatty acids with functional groups similar to those found in cutin and suberin monomers has been isolated from plants (Table V). Obviously they are generated by reactions similar to those discussed elsewhere in this chapter. The one case where the biosynthetic reaction has been elucidated is that of ricinoleic acid, and this case is described elsewhere (Kandler and Hopf, this Volume, Chapter 7). These oxygenated acids have been discussed in several reviews (Smith, 1970a,b Pohl and Wagner, 1972). 

Hofer R, Briesen I, Beck M, Pinot F, Schreiber L, Franke R (2008) The Arabidopsis cytochrome P450 CYP86A1 encodes a fatty acid m-hydroxylase involved in suberin monomer biosynthesis. J Exp Botany 59 2347-2360... 

The mid-chain oxygenated fatty acids that are found in some suberin polymers are generated by the biosynthetic pathways shown in Fig. 6.4.10. Although these pathways have been demonstrated by studies with cell-free preparations of cutin-synthesizing systems, similar reactions are most probably involved in the biosynthesis of suberin monomers. 

Such complexes include fatty acid synthases (FASes), elongases (ELSes) and polyketide synthases (PKSes) which can fimction individually or in concert. FAS synthesizes the 16 and 18 carbon acyl chains of membrane lipids as well as those of the plant cutin and suberin monomers. ELSes use these acyl chains as primers to synthesize longer ones for storage lipids in some seeds and for cuticular and epicuticular waxes. PKSes participate in a wide range of secondary metabolic pathways. In plants chalcone synthase contributes to the carbon skeleton of the flavonoids, in fungi and bacteria, especially... 

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