Classification and chemical structure of oligosaccharides in plant organisms

Oligosaccharides are oligomers of monosaccharides linked together by glycosidic bonds. This category includes carbohydrates containing from two to ten monomeric units. A glycosidic bond is an ether bond, the formation of which is accompanied by the release of a water molecule.

Fig. 22. Formation of oligo- and polysaccharides

There are various ways of forming glycosidic bonds. In linear oligo- and polysaccharides, it is created between the C1 atom of one sugar residue and either the third or fourth carbon atom of the next residue; in this case, both sugars, as a rule, are in the C1 (equatorial) conformation, i.e., the bulky OH-groups are parallel to the plane of the sugar ring and directed to the sides. Glycosidic bonds are denoted as follows: β (1→n) is a bond between the oxygen atom at the C 1 atom in the β-position and the carbon n of the next sugar; the α (1→n) bond involves the same carbon atoms, but now the oxygen atom at C 1 occupies the α-position. The most common glycosidic bonds are β (1→4) and β (1→3), for example, cellulose and hemicellulose of plants, as well as α (1→4) and α (1→6), for example, plant starch and animal glycogen. Bonds of the 2→1 and 2→6 types are quite rare; they have been discovered in compounds such as fructans found in some plants.

Depending on the number of monosaccharide molecules that make up the oligosaccharide, they are distinguished into disaccharides, tri-, tetra-, penta-, hexo-, hepto-, octo-, and nono-saccharides. By the composition of monosaccharide residues, they are divided into: 1) homooligosaccharides – containing identical monomers and 2) heterooligosaccharides – containing different monosaccharide units. In terms of molecular structure, oligosaccharides can be branched or linear.

There is no unified approach to constructing oligosaccharide nomenclature. For reducing oligosaccharides, the name is based on the monosaccharide residue with a free hemiacetal hydroxyl, and all units linked to it are considered substituents. The substitution sites and the configuration of the linked monosaccharides are indicated. For non-reducing sugars, all compounds are considered as glycosides. There is another nomenclature according to which the names of the units are written in sequence. An arrow and numbers are placed between the units, indicating exactly which atoms of adjacent residues are connected. In both cases, the -ose ending in the names of monosaccharides is changed to -yl for substituent monosaccharides, and to -ide for monosaccharides taken as the basis of the name, if the residue does not contain a free hemiacetal hydroxyl. If one exists, the -ose ending remains. There is also a short system for denoting the structure of oligosaccharides. Each monosaccharide residue is denoted by two or three Latin letters. The configuration (D or L) and the nature of the ring are also indicated by letters (p-pyranose, f-furanose). Numbers indicate the atoms through which the bond is formed, and an arrow indicates the direction of this bond. In practice, trivial names of oligosaccharides are more often used, indicating the origin and properties of the given oligosaccharide, with the -ose ending.

Below are the trivial names of some oligosaccharides and their designations according to the three nomenclatures listed: