Physical properties of dextran
Dextrans come in a wide range of molecular weights and sizes. They are soluble in water and solutions, can be filtered, have stable viscosity, and maintain a favourable osmotic environment.
Molecular weight
Dextran fractions are supplied in molecular weights from 1,000 Daltons to 2,000,000 Daltons
The molecular weight distribution curve for each fraction obtained by size-exclusion chromatography offers a unique method for characterising the dextran fraction.
The molecular weight distribution of Dextran 10 is illustrated here:
Molecular size
The designation 5, 10, etc. represents the weight-average molecular weight divided by 1,000 Daltons. Thus Dextran 10 corresponds to a weight-average molecular weight of 10,000 Daltons.
Dextran fractions behave as very flexible and extended polymers, and in solutions dextran exists as an expandable coil.
The molecular dimensions of some dextran fractions are shown on the side.
Solubility of dextran fractions
Dextran fractions are readily soluble in water and electrolytes to form clear, stable solutions. The pH does not affect solubility significantly, and concentrated
solutions (> 50% w/v) may be prepared.
Dextran fractions are also soluble in some organic solvents, notably, Dimethyl sulfoxide, dimethyl formadide, N-methylpyrrolidone, pyridine, ethylene glycol, and glycerol. Dextran fractions are insoluble in monohydric alcohols, for example, methanol, ethanol and isopropanol, and also most ketones, such as acetone.
Although dextran fractions will form clear solutions, it should be noted that the lowest molecular weight fractions 5 and 10 may, when standing, form
turbid solutions, particularly when concentrated solutions are used. This effect may be delayed by boiling the solutions immediately after preparation.
Filtration of dextran solutions
Dextran fraction solutions can be easily filtered, although more concentrated solutions require larger filters or filter series and higher pressures to increase
the rate of filtration.
Further increases in filtration rates may be achieved by raising the temperature. The dimensions of the filter system must be related to the volume and
concentration of the dextran solution used.
Viscosity of dextran solutions
Dextran fraction solutions exhibit Newtonian flow characteristics, that is, the flow rate is independent of shear stress. As dextran is a neutral polysaccharide, the viscosity is not significantly influenced by changes in pH or salt concentration.
The below figure shows the dependence of viscosity on the concentration of dextran fractions at 25° C.
Colloid osmotic pressure of dextran solutions
Colloid osmotic pressure is important for many applications using dextran. When comparing osmotic pressures, it is important that the molecules do not pass through the membrane with which they are in contact. For similar solute concentrations, osmotic pressure will be largely dependent on the molecular weight of the solute.
Since dextran is a neutral polymer with large dimensions, it will not easily permeate many human tissues and will therefore maintain a favourable osmotic environment (unlike, for example, saline, which readily diffuses into cells and tissues).
A comparison of colloid osmotic pressures for dextran fractions 40 and 70 is shown here.
Specific optical rotation
The calculation is [α]D = +195 – +201 (at 25 °C) where [α] is the specific optical rotation measured in the sodium D line.
Below approximately 20,000 Daltons, the specific optical rotation decreases with decreasing molecular weight.
1. Da: When nothing is indicated molecular weight units are assumed to be Daltons = g/mol.
2. Mw: Weight average molecular weight of Dextran fractions.
3. Mn: Number average molecular weight of Dextran fractions.
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