Structures of the Dextrans

Structure of the Dextran Synthesised from Sucrose by a New Strain of Betacoccus arabinosa-ceous" M. Stacey and G. Swift, J. Chem. Soc., (1948) 1555-1559. 

The discussion of the structural aspects of the polysaccharide will be divided into two Sections. The first deals with the chemical elucidation of the structure the second, with the physical measurements made on dextran. A survey of the literature in this area has indicated that the fine structure of the dextran obtained is closely related to the particular strain that pro-... 

Detailed studies of the enzyme(s) or of the structures of the dextrans produced have not, however, been published. 

The structures of the dextrans may be controlled by the type of enzyme preparation employed, by the type of initial receptor molecules, or by the addition of branched low-molecular-weight dextrans which produce products of low molecular weight 178), Some dextrans appear to have D-fruc-tose end groups which are introduced into the molecule by sucrose acting as a receptor to initiate dextran synthesis. 

Fig. 14.2. A dex-Eji 3 x 10 mmol estradiol/g of polymer B Chemical structure of the dextran-estradiol (dex-E ) conjugate...

Dextran is produced from sucrose by a number of bacteria the major ones being the nonpathogenic bacteria Leuconostoc mesenterodes and Leuconostoc dextranicum. As expected, the structure (and consequently the properties) of the dextran is determined by the particular strain that produces it. 

The chemical structures of five dextrans were partially determined by methylation, and found to be branched molecules having the following types of substitution (a) 6-0 and 3,6-di-O, (b) 6-0, 3-0, and 3,6-di-O, (c) 6-0,3,6-di-O, and 2,3-di-O, (d) 6-0, 4-0, and 3,4-di-O, and (e) 6-0 and 2,3-di-O. At 27° and pH 7 (external, Me4Si standard), the 13C shifts ofO-substituted, non-anomeric carbon atoms were C-2 (76.5), C-3 (81.6), and C-4 (79.5). The C-l resonances were also recorded, and may be used for reference purposes. Some variation of chemical shifts, relative to each other, was observed with changing temperature. (The work serves to emphasize the importance of accurately measuring the temperature of the solution when determining chemical shifts.102)... 

Figure 2. The structure of the polysaccharides cellulose, amylose, starch and dextran.

The affinity of the polymer-bound catalyst for water and for organic solvent also depends upon the structure of the polymer backbone. Polystyrene is nonpolar and attracts good organic solvents, but without ionic, polyether, or other polar sites, it is completely inactive for catalysis of nucleophilic reactions. The polar sites are necessary to attract reactive anions. If the polymer is hydrophilic, as a dextran, its surface must be made less polar by functionalization with lipophilic groups to permit catalytic activity for most nucleophilic displacement reactions. The % RS and the chemical nature of the polymer backbone affect the hydrophilic/lipophilic balance. The polymer must be able to attract both the reactive anion and the organic substrate into its matrix to catalyze reactions between the two mutually insoluble species. Most polymer-supported phase transfer catalysts are used under conditions where both intrinsic reactivity and intraparticle diffusion affect the observed rates of reaction. The structural variables in the catalyst which control the hydrophilic/lipophilic balance affect both activity and diffusion, and it is often not possible to distinguish clearly between these rate limiting phenomena by variation of active site structure, polymer backbone structure, or % RS. 

Figure 11. Alcohol oxidase. Interaction of polyelectrolytes/polyhydroxyls. The diagrams above represent the postulated interaction of alcohol oxidase with (a) DEAE—Dextran and (b) the same interaction in the presence of polyhydroxyl compounds. The structure of the enzyme was taken from Woodward 1990 (2).

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