Structure of Suberin

Cork from the cork-oak (Quercus suber L.) differs chemically from wood, mostly by the presence of suberin as a major structural component (ca. 60% of extractive free cork) in addition to lignin and polysaccharides (6).The structure of suberin is not fully elucidated yet. It is a cross-linked polymer with a polyester linked aliphatic domain containing fatty acids, alcohols, hydroxyacids and diacids and a phenolic, probably lignin-like domain. 

The other macromolecule found only in certain wood species is suberin. This non-linear polyester contains very long aliphatic moieties which impart a characteristic hydrophobic feature to the natural material that contains it. Figure 1.7 shows a schematic structure of suberin. By far the most representative species containing this polymer in its very thick bark (the well-known cork) is Quercus suber, which grows in the Mediterranean area, but Nordic woods like birch, also have a thin film of suberin coating their trunks. The sources of suberin, as well as the corresponding structure and composition are described in Chapter 14, together with the use of its monomeric components for the synthesis of novel macromolecular materials. 

The present chapter will, therefore, first give a general overview of the properties and applications of cork, as well as of its utilization as a starting material for the synthesis of liquid polyols, before dealing with the macro-molecular structure of suberin, its depolymerization methods, and the composition and applications of the ensuing fragment mixtures. 

The structure of suberin is even less well understood than that of cutin. As indicated earlier, even the composition of the phenolic constituents of this polymer is unknown. The high content of phenolic materials in suberin and the finding that bark lignin usually contains fewer methoxyl groups than the... 

Fig. 10. Tentative models proposed for the structure of suberin (left) and cutin containing only the Cie family of acids (right).

Suberin is a natural biopolymer typically found in the cell walls of plants [46]. The structure of suberin in cork (i.e. its main chemical component) is not yet fully understood. It... 

The inability to solubilize aromatic components of suberin-enriched preparations by the methods used for lignin suggests that suberin structure is distinctly different from that of lignin, probably due to the aliphatic cross-linking and the higher degree of condensation present in suberin. 

Suberin is a composite of polymeric phenylpropanoids and ester-linked long chain fatty acids and alcohols and consists of a hydrophobic layer attached to the cell walls of roots, bark and the vascular system (8,10). The phenylpropanoid portion of suberin purportedly has a lignin-like structure to which both aliphatic domains and hydroxycinnamic acids are esterified. 

DC046 Kolattukudy, P. E., K. Kronman, and A. ]. Poulose. Determination of structure and composition of suberin from the roots of carrot, parsnip, rutabaga, turnip, red beet and sweet potato by combined gas-liquid chromatography and mass spectrometry. Plant Physiol 1975 55 567. 

Figure 9.16 Chemical structure of cutin, a biopolyester mainly composed of interester-ified hydroxy and epoxy-hydroxy fatty acids with a chain length of 16 and/or 18 carbons (Ci6 and C[s class). Also, the chemical strcuture of the aliphatic monomers of suberin, derived from the general fatty acid biosynthetic pathway, namely from palmitic (16 0), stearic (18 0), and oleic acids.

Recent work using the atmospheric dioxane extraction of cork has proved extremely difficult, yielding only small amounts of a lignin- enriched material (7). This was explained by the presence of suberin, a complex structure of phenolic and aliphatic domains and the interaction with lignin. Further work using a saponified cork stressed these arguments (8). 

Two recent studies (3 ,33) have partially elucidated the structure of cutin and suber1n Tn cotton. Cotton suberin is unusual in that it contains predominantly C-22 fatty acids (22-hydroxydocosanoic and 1,22-docosanedioic) in the polymer. Quantitation of these acids might be useful to determine changes in suberin content during NR responses. Cutin in cotton is composed mostly of C16 and some C18 fatty acid derivatives, and is very similar to that in other plants. 

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).

Waxes, in particular cutin and suberin, are polymerized and cross linked structures of hydroxy fatty acids that are resistant to oxidation and to microbial and enzymatic attack. Cutin is found on the outer surface of plant tissue while suberin is mainly associated with roots and bark of plants. Both contain an even number of carbons in the range from C, to C,(,. Cutan and suberan are also highly aliphatic polymers lacking ester cross linkages. They are linked by carbohydrate structures to form glycolipids that are integral parts of microbial cell walls (De Leeuw and Largeau, 1993). 

The detailed monomer distribution of suberin from several species has been recently reviewed [14]. Table 14.1 summarizes the composition of the two most relevant suberin sources in the present context i.e. Quercus suber cork and Betula pendula outer bark) and the structures of representative elements of each group are shown in Fig. 14.4. 

Figure 14.4 Structures of monomeric components of the most representative families of compounds formed in suberin...

Gil A.M., Lopes M., Rocha J., Neto C.P., A C-13 solid state nuclear magnetic resonance spectroscopic study of cork cell wall structure The effect of suberin removal, Int. J. Biol. MacromoL, 20(4), 1997,293-305. 

The advent of modem analytical techniques, particularly combined glc and mass spectrometry, was responsible for the great advances in our knowledge of the chemistry of waxes, cutin, and suberin. However, what is known constitutes only a beginning, as much more remains unknown. Even the monomer composition of the polymeric lipids of only a very limited number of plants has been determined, and hardly anything is known about the chemistry of the numerous internal layers of lipidlike staining materials veuiously designated suberin, cutin, suberin-cutin-like, etc. The phenolic components of suberin and the intermolecular structure of the polymerized lipids are very poorly understood. 

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