الفهرس 
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Abstract
Dextrans (C6H10O5) n are a class of homopolysaccharides produced by the lactic acid bacteria belonging to the genera Lactobacillus, Leuconostoc, Streptococcus and Weissella (Patel et al., 2011). Dextrans consist of D-glucose units polymerized predominantly in α-(1→6) linkage and α-(1→2), α-(1→3), α-(1→4) glycosidic linked branches (Kim et al., 2003). Dextransucrase is an extracellular enzyme
It is a glucosyltransferase (Barker 1991; Monsan 2001; Robyt 1995) that catalyzes the transfer of glucosyl residues from sucrose (S) to dextran polymer and liberates fructose (F) according to the following equation (Hehre 1951):
n S→n F +dextran (glucose)n
Dextransucrase production is affected by several factors, such as temperature, pH,aeration and substrate concentration (sucrose).Generally the fermentation temperatures are about 23-26ᵒC (Santos et al., 2000).
Dextran (C6H10O5)n is a high-molecular mass (107 to 108 Da) glucan (Robyt, 1995). It is consists of D- glucose units with predominantly α- (1→6) linkage in the main chain and a variable amount of α-(1→2), α-(1→3), α-(1→4) branched linkages (Monsan et al., 2001).
Dextrans have enormous industrial applications due to their non-ionic, inert, stable, porous, gelling and pseudoplasti attributes (Patel et al., 2011). These are used as food syrup stabilizers, matrix of chromatography columns, blood plasma substitutes, antithrombogenic agents, treatment for iron deficiency anaemia, drug carriers (Purama and Goyal, 2005; Patel et al., 2011).
Dextrans have also been reported to arrest the replication of human immunodeficiency virus (HIV-1), the causative agent of the dreaded AIDS. Dextran hydro gels are used in various pharmaceutical and biomedical applications such as contact lenses, cell encapsulation for drug delivery, burn wound dressing and in spinal cord regeneration. Dextrans act as protective coating against oxidation for metal nano particles and shield against biofouling in biomaterials .Use of dextrans have ramified into paper, metal-plating processes and enhanced oil recovery (Patel et al., 2011).
Dextrans from various microbial sources have been studied and characterized to evaluate their industrial potentials (Purama et al., 2009)
Dextran is commercially available, and it is used as drugs, especially as blood plasma volume expander. Dextran has also found industrial applications in food, pharmaceutical and chemical industries as adjuvant, Emulsifier, carrier and stabilizer (Goulas et al., 2004).Cross-linked dextran is known as Sephadex, which is widely used for the separation and purification of protein. In food industry dextran is currently used as thickener for jam and ice cream (Naessens et al., 2005). It prevents crystallization of sugar, improves moisture retention, and maintains flavor and appearance of various food items (Purama and Goyal, 2008).
1.1: Dextran history
Historically, dextrans had been long recognized as contaminants in sugar processing and other food production. The formation of dextran in wine was shown by Pasteur to be due to the activity of microbes. (Pasteur et al., 1861).
The name dextran was created by Scheibler in 1874, who demonstrated dextran was a carbohydrate with the formula (C6H10O6)n and a positive optical rotation.
1.2: Dextran structure
Dextrans are polysaccharides with molecular weights ≥1,000 Dalton, which have a linear backbone of α-linked d-glucopyranosyl repeating units. Three classes of dextrans can be differentiated by their structural features. The pyranose ring structure contains five carbon atoms and one oxygen atom. Class 1 dextrans contain the α (1→6)-linked d-glucopyranosyl backbone modified with small side chains of d-glucose branches with α (1→2), α (1→3), and α (1→4)-linkage (Figure 1.1). The class 1 dextrans vary in their molecular weight, spatial arrangement, type and degree of branching, and length of branch chains, depending on the microbial producing strains and cultivation conditions. (Cheetham, et al., 1990 ; Naessens, et al., 2005).
Isomaltose and isomaltotriose are oligosaccharides with the class 1 dextran backbone structure. Class 2 dextrans (alternans) contain a backbone structure of alternating α (1→3) and α(1→6)-linked d-glucopyranosyl units with α(1→3)-linked branches. Class 3 dextrans (mutans) have a backbone structure of consecutive α (1→3)-linked d-glucopyranosyl units with α (1→6)-linked branches. One and two-dimensional NMR spectroscopy techniques have been utilized for the structural analysis of dextran (Kim. et al., 2003).
The secretion of dextrans provides an opportunity for bacteria to modulate adhesion, e.g. in tooth decay, by having a softer or more rigid bacterial cell surface, depending on the polysaccharide itself and the pH and ionic strength. Low bacterial adhesion occurs at low salt conditions with more rigid polysaccharides and a softer surface, while high bacterial adhesion is obtained with more flexible polysaccharides and a rigid bacterial surface. Polymer elasticity is important for structural integrity. and the pyranose ring is the structural unit controlling the elasticity of the polysaccharide. This elasticity results from a force-induced elongation of the ring structure and a final transition from a chair-like to a boat-like conformation of the glucopyranose ring, which plays an important role in accommodating mechanical stress and modulating ligand binding in biological systems (Marszalek, et al., 1998).
Laboratory experiments have demonstrated that cleavage of the pyranose rings of dextran, amylose, and pullulan convert these different polysaccharide chains into similar structures where all the bonds of the polymer backbone can rotate and align under force. After ring cleavage, single molecules of dextran, amylose, and pullulan display identical elastic behavior as measured by atomic force microscopy.
Dextrans are found as bacterial extracellular polysaccharides. They are synthesized from sucrose by beneficial lactic acid bacteria, such as Leuconostoc mesenteroides and Lactobacillus brevis, but also by the dental plaque-forming species Streptococcus mutans. Bacteria employ dextran in biofilm formation10 or as protective coatings, e.g., to evade host phagocytes in the case of pathogenic bacteria (Meddens et al., 1984)
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