What is dextran?

Dextran is essentially a linear glucose chain linked by α-D-(1 – 6) linkages. The degree of branching is about 5 %. These branches are mostly 1 to 2 glucose units long but there is evidence that the largest fractions (>500 000) may possess some much longer branches. The branching decreases during the hydrolysis of the native dextran and the smallest fractions may only have 1-2%.

FIG. 1 Structural fragment of dextran chain with α(1-3) branch unit. Dextran is essentially a linear and neutral glucose chain. The degree of branching lies around 5 %.

The manufacture of dextran employs a fermentation process using a bacterium, Leuconostoc mesenteroides B512F and sucrose, which may be obtained from the sugar-beet or the sugar cane. The bacterium is one of many elaborating the enzyme dextransucrase, which can transfer glucose from sucrose to a growing glucose chain. The biosynthesis is also accompanied by the transfer of glucose also the other hydroxyls on the glucose chain giving rise to branching.

The native dextran formed may be broken down by partial acid hydrolysis to appropriate molecular sizes. These fractions are isolated after fractionation processes with ethanol or ultrafiltration, by spray-drying.

Dextran displays excellent biocompatibility and have been used clinically for over 50 years. It is well tolerated on parenteral infusion by most people but a very low incidence anaphylactoid reactions has been recorded. Dextran products for topical application, and ingestion have been approved. It should be remembered that dextran occurs naturally in sucrose, confectionary, jams and syrups.

Dextran fractions were originally developed for clinical purposes (blood plasma substitutes) by Pharmacia in Uppsala in the 1950s. It is still available for these purposes and is used for replacing lost blood as for example in accidents or war. A further application is for thrombosis prophylaxis where a dextran 40 is found optimal. It also may be found in cosmetics, eye drops, stabilizers for pharmaceuticals, and perfusion solutions for preservation of organs.

Dextran may also be regarded as a green product as it is prepared from solely from vegetable renewable resources (sucrose) and its recovery and purification require only ethanol and water.

Dextrans are stable polysaccharides. The optimal pH for storing in solution and as a powder is 6.5 ± 0.3. The stability is also affected by the temperature. As an example, the rate of hydrolysis of dextran at pH 1 decreases by 100-fold as the temperature falls from 100°C to 66°C. At a neutral pH and 100°C the rate of hydrolysis is very slow.

In alkali, no hydrolysis is to be expected. However, the reducing end-groups on each molecule will undergo keto-enol transitions and further rearrangements when subject to strong alkali. A strong discoloration will therefore be seen with fractions of lower molecular weight. MW fractions < 5000 Da are particularly sensitive to alkali owing to the high proportion of end-groups. This can be prevented by reducing the end-groups to alcohols with sodium borohydride.

Dextran powder or solution exposed to air and daylight for prolonged periods will undergo a general oxidative process throughout the chain. It also is hygroscopic and will slowly absorb moisture from the air. Thus, as with all polysaccharide it is advisable to store both the powder and solutions in airtight containers in the dark. Under these conditions, dextran has a shelf life of more than 6 years.

Dextran fractions are freely soluble in water. To hasten dissolution of dextran powders, the powder should be added portion wise to warm (hot) water with vigorous stirring to avoid lump formation. Some changes may however be noted when lower MW fractions (< 10 000 Da) are allowed to stand for longer periods permitting the molecules to aggregate which may result in turbidity and precipitation. These changes are however reversible under strong heating.
Dextran also is soluble in a limited number of organic solvents – the most commonly used are dimethylsulfoxide and formamide.

Dextran is insoluble in lower aliphatic -alcohols, ketones and esters – for example methanol, ethanol, butanol, acetone, ethyl acetate. Dextran is also insoluble in chloroform, ethylene chloride, pyridine, benzene and diethyl ether.

Dextran molecules in solution may be regarded as flexible expandable coils. However, this somewhat of an oversimplification since the smaller molecules < 5000 Da appear to behave as a more rigid rod-like structure – which would account for the tendency to aggregate on standing. The very large molecules with longer side chains display greater symmetry.

The sizes of dextran molecules in solution can be expressed either as the radius of gyration, √R2 , or as the hydrodynamic radius Rh (Stoke’s radius). These two parameters differ significantly as shown in table 1 below. Information on size and conformation is adduced from viscosity, sedimentation and diffusion studies.

The observed values are influenced not only by the molecular weight but also the branching, which may vary as the molecular weight increases. This is due to the cleavage of the branches on hydrolysis of the dextran and that the lower molecular weight fractions will require longer hydrolysis times than the higher fractions.

Table 1. The sizes of dextran molecules in solution. Radius of gyration or hydrodynamic radius (also called Stoke’s radius) are often used to express the size of dextran in solution.

The conformation of dextran and other polysaccharides is influenced significantly by substitution since repulsion of the charged groups will lead to an expansion of the coils. These effects will also depend on the degree of substitution and at very low levels the conformation is essentially unchanged. Thus, FITC-dextrans are identical on GPC to their parent dextran fractions. Dextran derivatives with hydrophobic substituents (e.g. phenyl- dextran) may form micelles but with GPC in pure water (to suppress hydrophobic interaction with gel), it does not appear to show anomalies. Many derivatives with charged substituents are eluted earlier, thus giving an apparently higher molecular weight.

Read our Dextran handbook