Suberin depolymerization methods

TLC is commonly used for the separation of different classes of wax components or for analysis of monomers from cutin and suberin depolymerization. A typical separation is shown in Fig. 6.12. By such methods, it is possible to separate hydrocarbons, wax esters, primary alcohols, secondary alcohols and /8-diketones from plant waxes (von Wettstein-Knowles, 1979). Products of hydrogenolysis from cutin can be separated by TLC into alkan-l-ols, alkane-a,ft>-diols, Cis triols, Ci6 triols and Cis tetrols (Kolattukudy, 1980). Unsaturated components can be resolved by argentation-TLC (Tulloch, 1976) and threo or erythro diastereoisomers separated by boric acid/silica gel TLC (Eglinton and Hunneman, 1968). Straight-chain compounds can be preferentially removed from branched compounds as their urea complexes (Kolattukudy, 1980). 

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 suberin polymeric structure cannot be defined in terms of a monomer repeating unit, since the spatial arrangement of these moieties cannot be accurately defined, even when their relative abundance is known. Additionally, the latter aspect depends on the depolymerization methods used to isolate the aliphatic fraction and, moreover, as far as the aromatic domain is concerned, its identification/quantification is also quite complex because of its macromolecular nature and structural similarity with lignin [1, 42-45]. Notwithstanding these difficulties, several models have been proposed to illustrate the suberin macromolecular structure in suberized cell walls [46, 47]. In 2002, Bernards proposed a model for suberin from potato periderm, which summarized the state of the art on the structural data related to this macromolecular component [1]. 

High-resolution 13C NMR studies have been conducted on intact cuticles from limes, suberized cell walls from potatoes, and insoluble residues that remain after chemical depolymerization treatments of these materials. Identification and quantitation of the major functional moieties in cutin and suberin have been accomplished with cross-polarization magic-angle spinning as well as direct polarization methods. Evidence for polyester crosslinks and details of the interactions among polyester, wax, and cell-wall components have come from a variety of spin-relaxation measurements. Structural models for these protective plant biopolymers have been evaluated in light of the NMR results. 

Fig. 6.4.5. Methods used to analyze the aliphatic components of suberin. Top left chemical methods used to depolymerize suberin. Top right gas-liquid chromatogram of the mixture of monomers generated by LiAlD4 treatment of suberin from the chalazal region of the inner seed coat of Citrus paradisi the components are trimethylsilyl ethers of 1,16-dihydroxyhexadecane (1), 1,18-dihydroxy-octadecene (2), 1,9,18-trihydroxyoctadecene (3), 1,9,18-trihydroxyoctadecane (4), 1,9,10,18-tetrahy-droxyoctadecane (5), 1,22-dihydroxydocosane (6), 1,24-dihydroxytetracosane (7). Bottom mass spectrum of component 2 from gas chromatogram (111)...

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