Suberin composition

Conde E., Garcia-Vallejo M.C., Cadahia E., Variability of suberin composition of reproduction cork from Quercus suber throughout industrial processing, Holrforschung, 53(1), 1999, 56-62. 

Holloway P J 1972 The suberin composition of the cork layers from some Ribes species. Chem Phys Lipids 9 171-179... 

Certain compositional differences between coals of differing origins can be inferred from available data. Differing anatomical distributions of cellulose, lignin and suberin, with implications for the origins of vitrinites, and differing distribution of phenolic substances in plants of different orders and families, have been referred to above. Some biochemical investigations of modern representatives of ancient plants have been made (e.g., refs. 14,... 

Table 2. Compositional difference between cutin and suberin ...

P.J. Holloway, Composition of suberin from the corks of Quercus suber and Betula pendula, Chemistry and Physics of Lipids, 9, 158 170 (1972). 

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. 

Solid-state 13C NMR was employed to characterize intact samples of cutin and suberin biopolyesters. Although a considerable degree of structural heterogeneity was observed for both materials, it was possible nonetheless to resolve and assign many NMR peaks, even when the polyesters were accompanied by waxes or cell walls. Quantitative estimates for the various aliphatic, aromatic, and carbonyl carbon types indicated that cutin was primarily aliphatic in composition, whereas suberin had more aromatic and olefinic moieties. Additional analysis should be facilitated by the biosynthetic incorporation of selectively 13C-enriched precursors (26,27). 

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. 

Espelie, K. E., Sadek, N. Z., Kolattukudy, P. E. (1980). Composition of suberin-assoeiated waxes from the subterranean storage organs of seven plants. Parsnip, earrot, rutabaga, turnip, red beet, sweet potato and potato. Planta, 148,468 76. 

The chemical composition of bark is complicated, varies among the different tree species and also depends on the morphological elements involved. Many of the constituents present in wood also occur in bark, although their proportions are different. Typical of bark is its high content of certain soluble constituents (extractives) such as pectin and phenolic compounds as well as suberins. The mineral content of bark is also much higher than that in wood. 

The chemical composition of cork is made up by about 43% suberin (composed of fatty acids and alcohols), 28% lignin, 13% cellulose, 6% tannins, 5% waxes, and 5% ash. About 90% of the tissue is gas, resulting in a density of 0.12 to 0.20 kg/L. Cork has a unique capability as a bottle seal because of its excellent resilience after insertion into a bottle. This is due to its structure consisting of polygonal cells (30 to 42 million/cm ) separated by spaces filled with gas (atmospheric air without COj) which slows oxygen diffusion without completely eliminating it. 

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).

Cutin and suberin are lipid biopolymers of variable composition which are part of the protective outer coatings of all higher plants. Chemically, cutin and suberin are closely related polyesters composed of long-chain fatty and hydroxy fatty acid monomers. Both types of biopolymers represent labile, easily metabolizable terrigenous organic matter because they are sensitive to hydrolysis. After sedimentation, they have only a moderate preservation potential. 

The surface layers of plants are covered in cutin or suberin (Section 2.11). Both of these polymeric structures are lipid-derived. Furthermore, the lipid nature of the surface covering is enriched by the presence of wax (Section 2.10), especially in the cuticle. There are two problems which should be borne in mind when considering the structural determination of these compounds. Firstly, cutin and suberin are polymers whose extraction from tissues and analysis thereafter pose all the problems usually associated with such materials. Secondly, wax, cutin and suberin have variable compositions depending on environmental and developmental conditions and, especially, on the nature of the tissue being analysed. 

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. 

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