Overview
Dextran is a complex, branched polysaccharide composed of glucose units linked primarily by α-(1→6) glycosidic bonds, with α-(1→3) side branches. It is synthesized by certain lactic acid bacteria, notably species of the genus Leuconostoc, during the fermentation of sucrose. Dextran exhibits a wide range of molecular weights, from approximately 4,000 to over 2,000,000 daltons, depending on the microbial strain and fermentation conditions. Its high solubility in water, viscosity, and biocompatibility make it valuable in pharmaceutical, medical, and industrial applications.
Chemical Structure and Properties
Dextran is a linear glucose polymer with extensive branching. The primary chain consists of glucose residues connected by α-(1→6) glycosidic bonds, while side chains are attached via α-(1→3) linkages. The degree of branching varies between bacterial strains, influencing the polymer’s physical properties. For example, Leuconostoc mesenteroides produces dextrans with 3–7% branching. The polymer’s hydroxyl groups contribute to its hydrophilicity and ability to form viscous aqueous solutions.
Dextran is thermally stable and resistant to enzymatic degradation by most mammalian enzymes, though it can be hydrolyzed by specific bacterial glycosidases. It is non-toxic and biodegradable, with degradation in the environment mediated by microbial action. Its osmotic pressure and colloidal properties make it effective as a plasma volume expander in medical contexts.
Production and Synthesis
Dextran is produced industrially via microbial fermentation using Leuconostoc species, which secrete the enzyme dextran sucrase (glucosyltransferase). The enzyme catalyzes the polymerization of glucose from sucrose, following the reaction: Sucrose + dextran → dextran + fructose. Fermentation occurs in nutrient-rich media containing sucrose, with pH and temperature optimized for bacterial growth. Post-fermentation, dextran is purified through precipitation with ethanol or isopropanol, followed by filtration, dialysis, and drying.
The molecular weight and branching pattern of the final product are controlled by adjusting fermentation parameters, such as sucrose concentration, bacterial strain selection, and reaction time. Industrial-scale production requires sterile conditions to prevent contamination by competing microorganisms.
Applications
Dextran has diverse applications across multiple sectors:
- Medical Uses:
- Plasma Expanders: Dextran 70 (average molecular weight ~70,000) is used as a colloid to maintain blood volume during hypovolemia.
- Drug Delivery: Dextrans serve as carriers for controlled drug release, particularly in formulations requiring prolonged circulation times.
- Diagnostic Tools: Dextran-coated particles are employed in immunoassays and as matrices for enzyme immobilization.
- Pharmaceutical Industry:
- Acts as a binding agent in tablet formulations and a stabilizer in injectable solutions.
- Industrial Applications:
- Used as a thickening agent in food products, a stabilizer