Dextran chain

FIGURE 7.25 Sephadex gels are formed from dextran chains cross-linked with 1 CH, 1 -... 

Homopolysaccharides are synthesised by relatively few spedfic enzymes and are not constructed from subunits. The commerdally important homo polymer dextran is synthesised extracellularly by the enzyme dextransucrase. In Leuconostoc mesenteroides the enzyme is induced by the substrate sucrose. This is deaved to release free fructose and link the glucose to the redudng end of the acceptor dextran chain, which is bound to the enzyme. The product from this bacterium is composed almost exdusively of... 

Dextran was chosen to study for the following reasons. First, it is water soluble allowing three dimensional modification employing aqueous solution and classical interfacial condensation routes. Second, it is readily available in industrial quantities. Third, it is available in a range of molecular weight allowing product modification to be studied as a function of dextran chain size. Fourth, it is generally considered to be an under-utilized natural feedstock. 

Naturally occurring dextran is basically amorphous. However, single crystals with lath-like shape can be grown in a mixture of water/polyethylene glycol at temperatures ranging from 120 to 200 °C [47], Combined electron and X-ray diffraction studies indicate that the unit cell contains two antiparallel dextran chains of two glucopyranosyl residues each [48],... 

The large hysteresis of contact angle, together with the noticeable influence of drying on the contact angle, suggests that the density of graft dextran chains at the EVAL surface... 

Bearing in mind technical application with scale-up, enzyme immobilization is a prerequisite. Since the large enzyme (ca. 180 kDa) is additionally linked with a dextran chain, it can be easily and economically immobilized by entrapment in calcium alginate [29, 94]. 

Strain NRRL B-512(F) produces large proportions of the extracellular enzyme dextransucrase,339 which is responsible for the synthesis of linear sequences ofa-D-(l —> 6)-linked D-glucosyl residues. The enzyme transfers the D-glucosyl group from a sucrose molecule to an enlarging dextran chain and liberates the D-fructose portion. As dextransucrase is an extracellular enzyme, production of dextran by cell-free, culture filtrates can result in enhanced yield and quality, and ease of purification of the product. By suitable adjustment of the conditions, products in a chosen molecular-weight range can be obtained. Formation of branches is not yet well understood, but the enzymes responsible will certainly be found. 

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