Suberin polymers from

It has been reported that the major aliphatic monomers of suberin polymers from several tree barks are fatty acids with both a>- and mid-chain oxygenation (192). These monomers include 9,10-epoxy-18-hydroxyoctadecanoic acid and 9,10,18-trihydroxyoctadecanoic acid, both of which are common components of cutin polymers (232). For example, 9,10-epoxy-18-hydroxyoctadecanoic acid was reported to be the major monomer (31%) of Quercus ilex bark suberin and... 

Table 6.4.6. Major components released by the depolymerization of suberin polymers from the bark of various trees and shrubs. 

A fraction analogous to BjOrkman lignin (39) was obtained when a portion of finely-powdered suberin polymer from the periderm of 5. tuberosum was solubilized with dioxane. This soluble fraction, however, was not enriched in either aromatic or aliphatic components over the insoluble residue (unpublished results). Other procedures from lignin chemistry - including refluxing in HCl/dioxane or HCl/dimethylformamide, and dioxane treatment at 160 C and high pressure (268, 396) - resulted in 20 o to 50% solubilization of the suberin preparation, but with each method the insoluble material contained the majority... 

It therefore follows that when isolated lignins (and suberins) are examined and subsequent structural representations are proposed, critical information on native structure has already been lost, e.g., as regards the extent of polymer modification during removal from the cell wall, and the effect of mixing polymers from the various cell wall layers from which they originated. For these reasons, all current representations of native lignin (and suberin) structure should be viewed with caution until such questions are satisfactorily resolved. 

The phenolic domain of the suberin polymer has not been well characterized. Early chemical studies on suberin from tree bark indicated that the polymer contains phenolics. Suberin from the cork of Pseudotsuga menziesii bark released phenolic acids (43% of extractive-free cork) and aliphatic acids (35%) upon saponification (180). Suberin, which was subjected to a limited saponification (2% KOH at room temperature) and successive solvent extraction with hexane. 

The insoluble nature of suberin prevents its analysis by conventional NMR spectroscopy. The development of solid sample C-NMR spectroscopy with cross polarization/magic-angle spinning provides an opportunity to obtain spectra of the intact polymer. The NMR spectrum of the cutin polymer from the fruit of Malus pumila indicates, as expected, that the majority of carbons are methylenes (CH2, 20-30 ppm). The spectrum of suberin from 5. tuberosum showed the presence of a high proportion of aliphatic CH2 but also had a large amount of CHOH (60 to 80 ppm) carbon, probably from contaminating cell wall carbohydrates. This presumed carbohydrate material was still present in the residue left after LiAlH4 reduction, whereas the majority of aliphatic CH2 had been removed by this treatment (Fig. 6.4.8). This result shows that not much of the... 

Trihydroxyoctadecanoic acid was generated from 9,10-epoxy-18-hy-droxyoctadecanoic acid by a 3000-g particulate fraction obtained from the skin of the young fruit of Malus pumila (91). This enzyme activity did not require any cofactors and showed fairly stringent substrate specificity. It catalyzed the hydration of the m-epoxide to the /ireo-product and this stereospecificity is consistent with the natural occurrence of the /Areo-product in cutin. The trihydroxy Cig acid thus generated probably is the biosynthetic precursor of 9,10-dihy-droxyoctadecanedioic acid, which has been found in several suberin polymers (244). 

The time course of deposition of aromatic monomers into the phenolic portion of suberin in wound-healing slices of Solanum tuberosum tubers was determined using the amount of / -hydroxybenzaldehyde and vanillin generated by alkaline nitrobenzene oxidation as a measure of aromatic deposition (85). The deposition of such phenolics into the polymer exhibited a lag period of about three days after wounding followed by several days when the deposition of phenolics increased rapidly, and subsequently the process ceased. Exogenous L-[U- C]-phenylalanine and [U- Clcinnamic acid were incorporated into the insoluble polymeric material by wound-healing slices of S. tuberosum (85). Nitrobenzene oxidation of the polymeric material derived from labeled cinnamic acid released labeled /7-hydroxybenzaldehyde and vanillin. The time course of incorporation of phenolics into the suberin polymer correlated with the time course for the deposition of aliphatic monomers into the polymer, the deposition of suberin-associated waxes into the periderm layer and the development of diffusion resistance (Fig. 6.4.12) (86). 

Suberin, being an adcrustation on the cell wall, cannot be separated from cell walls. Instead, suberin-enriched wall preparations can be obtained by digesting away as much carbohydrate polymers as possible using pectinases and cellu-lases [3,7]. Depending on the source of the suberized cell wall preparation, the polyester part may constitute a few percent to 30% of the total mass. 

The second group of phenylpropanoids, which is the main emphasis of this chapter, consists of those components which are integrated into the cell wall framework. This group can be subdivided into three categories monomers, such as hydroxycinnamic acids, dimers, such as didehydrofer-ulic and 4,4 -dihydroxytruxillic acids, and polymers, such as lignins and suberins. It is important to emphasize, at this juncture, that the dimers (4,5) and polymers (8,9) discussed in this chapter are considered to be formed within the cell walls from their corresponding monomers. 

Vascular plant cell walls contain a wide variety of phenylpropanoids, such as monomers, dimers and polymers. Of these, the polymers (i.e., lignins and suberins) are the most abundant. According to our current knowledge, all cell-wall phenylpropanoids are derived from monomers synthesized in the cytoplasm. Following their excretion into the plant cell wall, these monomers can then be either photochemically or biochemically modified within the cell wall. 

Suberized Cell Walls. An analogous set of CPMAS experiments is presented for suberin in Figure 6. Because this polymer is an integral part of the plant cell wall, the 13C NMR spectrum had contributions from both polysaccharide and polyester components. Chemical-shift assignments, summarized in Table IV, demonstrated the feasibility of identifying major polyester and sugar moieties despite serious spectral overlap. Semiquantitative estimates for the various carbon types indicated that, as compared with cutin, the suberin polyester had dramatically fewer aliphatic and more aromatic residues. A similar observation was made previously for the soluble depolymerization products of these plant polymers (1,8,11). 

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. 

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