Suberin-enriched preparations

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. 

Initial studies, primarily done on suberin from the periderm of S. tuberosum give additional indication of the secondary structure of the polymer. Enriched preparations of suberin always contain high levels of carbohydrate (as much as 50 o). Obviously the suberin-enriched preparations contain covalently attached cell wall carbohydrates and the linkages between suberin and carbohydrate may be similar to those proposed for the attachment of lignin to carbohydrate (132, 268). Chemical studies on the polymer have shown that very few of the hydroxyl groups of cu-hydroxy acids are free and that the polymer has very few (< 5%) free aliphatic carboxyl moieties (230). Fractionation of S. tuberosum suberin by partial solubilization with 1 % HCl/dioxane has indicated that the aliphatic components may be in separate domains, for polymeric fractions that contained a larger proportion of fatty acids and fatty alcohols but a lower proportion of cu-hydroxy acids and dicarboxylic acids have been isolated. These fatty acids and fatty alco-... 

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. 

Suberin is tightly associated with the cell wall and cannot be removed in a pure form. The outer layer is usually removed physically and then treated with pectinase, cellulase and organic solvents to yield an enriched fraction. The crude suberin must then be subjected to chemical treatment in order to depolymerize it for chemical analysis (references in Kolattukudy, 1980). Crude suberin preparations have also been isolated from endodermis (Robards et al.y 1976) and hypodermis-epidermis (Clarkson et al.y 1978) but the amounts obtained from these sources are usually too small for meaningful analyses to be performed. 

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... 

---