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Polysaccharides, or glycans, may be classified in a number of ways; the following scheme is frequently used. Homopolysaccharides are defined as polysaccharides formed from only one type of monosaccharide. Homopolysaccharides may be further subdivided into straight-chain and branched-chain representatives, depending upon the arrangement of the monosaccharide units. Heteropolysaccharides are defined as polysaccharides containing two or more…

Lactose is one of the sugars (sucrose is another) found most commonly in human diets throughout the world; it constitutes about 7 percent of human milk and about 4–5 percent of the milk of mammals such as cows, goats, and sheep. Lactose consists of two aldohexoses—β-D-galactose and glucose—linked so that the aldehyde group at the anomeric carbon of glucose is…

Disaccharides are a specialized type of glycoside in which the anomeric hydroxyl group of one sugar has combined with the hydroxyl group of a second sugar with the elimination of the elements of water. Although an enormous number of disaccharide structures are possible, only a limited number are of commercial or biological significance. Sucrose and trehalose Sucrose, or common table sugar, is a major commodity…

Treatment of a carbohydrate with methyl iodide or similar agents under appropriate conditions results in the formation of compounds in which the hydroxyl groups are converted to methyl groups (―CH3). Called methyl ethers, these compounds are employed in structural studies of oligosaccharides and polysaccharides because their formation does not break the bonds, called glycosidic bonds, that link adjacent monosaccharide units. An…

The hydroxyl group that is attached to the anomeric carbon atom (i.e., the carbon containing the aldehyde or keto group) of carbohydrates in solution has unusual reactivity, and derivatives, called glycosides, can be formed; glycosides formed from glucose are called glucosides. It is not possible for equilibration between the α- and β-anomers of a glycoside in solution (i.e., mutarotation) to occur. The reaction…

The reactions of the monosaccharides can be conveniently subdivided into those associated with the aldehyde or keto group and those associated with the hydroxyl groups. The relative ease with which sugars containing a free or potentially free aldehyde or keto group can be oxidized to form products has been known for a considerable time and once was the basis for the detection of…

Monosaccharides Sources The most common naturally occurring monosaccharides are D-glucose, D-mannose, D-fructose, and D-galactose among the hexoses and D-xylose and L-arabinose among the pentoses. In a special sense, D-ribose and 2-deoxy-D-ribose are ubiquitous because they form the carbohydrate component of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), respectively; these sugars are present in all cells as components of nucleic acids. Some naturally occurring monosaccharides sugar sources L-arabinose mesquite gum, wheat bran…

Although optical rotation has been one of the most frequently determined characteristics of carbohydrates since its recognition in the late 19th century, the rotational behaviour of freshly prepared solutions of many sugars differs from that of solutions that have been allowed to stand. This phenomenon, known as mutarotation, is demonstrable even with apparently identical sugars and is caused…

Molecules, such as the isomers of glyceraldehyde—the atoms of which can have different structural arrangements—are known as asymmetrical molecules. The number of possible structural arrangements for an asymmetrical molecule depends on the number of centres of asymmetry; i.e., for n (any given number of) centres of asymmetry, 2n different isomers of a molecule are possible. An asymmetrical centre…

Stereoisomerism Studies by German chemist Emil Fischer in the late 19th century showed that carbohydrates, such as fructose and glucose, with the same molecular formulas but with different structural arrangements and properties (i.e., isomers) can be formed by relatively simple variations of their spatial, or geometric, arrangements. This type of isomerism, which is called stereoisomerism, exists in all biological systems. Among carbohydrates,…