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Carbohydrates can contain either hydroxy and aldehyde function or hydroxy and ketone functions. Therefore carbohydrates can be defined as polyhydroxy aldehydes/ketones and their simple derivatives [1]. Monosaccharides are the simplest carbohydrates with the general formula C_nH_2nO_n, (n ≥3). In this structure, one of the carbon atoms is double-bonded to an oxygen atom forming a carbonyl (C=O) group and there is one hydroxyl (OH) group on each of the other carbon atoms [2]. Polysaccharides contain 1 to 5 different aldose and/ or ketose monosaccharide units. Structurally, the most frequent monosaccharide units of polysaccharides are found in the six-membered ring structure, although pentoses are not rare, but eight- and nine-carbon-atom sugars are less frequently found [3, 4]. The monosaccharide units are attached by covalent bonds called “glycosidic linkages”. The linkage links the carbon atom in position 1 (C(1)) to a carbon atom of the following unit through an oxygen bridge [4]. Most polysaccharides contain more than 20 monosaccharide units [2]. As specified by the International Union of Pure and Applied Chemistry, the terms glycan (glyc-for sweet or sugar; -an for polymer) and polysaccharide are used as synonyms to describe “compounds consisting of a large number of monosaccharides linked glycosidically” [5]. The number of monosaccharide units in a polysaccharide polymer is called as Degree of Polymerization (DP) and this value varies with polysaccharide type. Most of the naturally occurring polysaccharides have DPs less than 100. However, larger polysaccharides also exist; like cellulose (7,000–15,000 DPs) and the amylopectin component of starch (DPs of more than 100,000) [2]. Depolymerization of polysaccharides can be achieved by specific enzymes, heating, acid-catalyzed hydrolysis, oxidation in an alkaline system, and ultrasonic treatments [3, 6].

In polysaccharides, hydroxyl groups can be acylated (esterified), alkylated (etherified) or oxidized, amino groups can be acylated or deacylated, and carboxyl groups can be converted into amides, amines, and esters. Therefore, structural modifications of polysaccharides can result in even more complex structures [3]. Furthermore, a protein (or a peptide) and a polysaccharide molecule can be linked to each other, like in the structure of proteoglycans which glycosaminoglycan side-chains are found to be attached a core protein [7]. On the other hand, in the lipopolysaccharide structure, the polysaccharide is found to be linked with a lipid molecule along with O-antigen [8]. The diversified structural properties contribute significantly to the functional properties of polysaccharides [9]. In the functional aspect, polysaccharides are ideal natural resources for a broad range of applications, but their activities are not always satisfactory. Thus, based on structure-activity relationships, researchers have modified the structures of natural polysaccharides to obtain functionally improved biopolysaccharides. These molecular modification methods can be classified as chemical, physical, and biological methods. Among these approaches, chemical modification is the most widely used one [10]. Chemical techniques employed to modify polysaccharides include derivative formation, grafting, cross-linking, and polymer-polymer blending. The derivative formation is based on the inclusion of functional groups to polysaccharides by acetylation, carboxymethylation, cyanoethylation, carbamoylethylation, deacetylation, sulfation, phosphorylation, and esterification reactions [11]. In blending, a physical mixture of two or more than two polymers is prepared to obtain the required characters. In the grafting method, different monomers are covalently linked to the polymer. By grafting method, the desired properties of polymer changes, but initial characteristics of the polymer are not affected. In the curing method, via physical forces, an oligomer mixture adheres to a substrate surface to form a coating [12]. Through chemical modifications, spatial structures and physical and chemical features of polysaccharides change and therefore, biological activities of polysaccharides, such as immune regulation, anti-tumor, anti-oxidation properties, can be modified [10]. The mechanism of the physical modification method is based on the truncation of the polysaccharide backbone by using ultrasonic disruption, radiation-induced reaction, or microwave exposure, to obtain lower molecular weight fragments. Finally, in most cases, biological modification of polysaccharides refers to the enzymatic degradation of polysaccharides. Enzymatic modification can be applied to only certain kinds of polysaccharides; however, investigation of different types of enzymes, such as transferase and synthase, will increase the use of the enzymatic modification of polysaccharides [10].