9.1 Basics of Carbohydrates and their Stereochemistry

The word “saccharide” is derived from the Greek word “sakcharon” meaning “sugar.”

All carbohydates are polyhydroxy aldehydes or ketones and can be classified into three groups:

Monosaccharides

Figure 9.3: Open Chain Representation of Glucose and Fructose

These include glucose and fructose and cannot be hydrolyzed into smaller molecules under mild conditions.

Nonetheless, monosaccharides are further classified based on their most highly oxidized functional group. If the most highly oxidized functional group is an aldehyde, then the monosaccharide is said to be an aldose (e.g., glucose). Otherwise, if the most highly oxidized functional group is a ketone, then the monosaccharide is termed a ketose (e.g., fructose).
Oligosaccharides

These are usually two to ten carbon atoms in length. The hydrolysis of an oligosaccharide can yield identical monosaccharides or two or more different monosaccharides.
Polysaccharides

These are made up of thousands of covalently linked monosaccharides. A homopolymer is a polysaccharide that is made up of one type of monosaccharide; a heteropolymer is a polysaccharide that is made up of more than one kind of monosaccharide.

9.1.1 Cyclic structures in solution

Figure 9.4: Hemiacetal and Hemiketal Formation

The interactions between an aldehyde or a ketone with an alcohol yields a hemiketal or a hemiacetal. This reaction also creates a new chiral center at the carbonyl carbon (see figure 9.4).

The substitution of a second alcohol molecule then produces an acetal or a ketal. Note that when the second alcohol is part of another sugar molecule, the resulting bond formed is then a glycosidic bond.

9.1.2 Anomeric forms of monosaccharides

Figure 9.5: Anomeric Forms of Glucose

The \(\alpha\) and \(\beta\) isomers of a monosaccharides are called anomers; in figure 9.5, the C1 atom is called the anomeric carbon atom.

The term mutarotation refers to the interconversion between the \(\alpha\) and the \(\beta\) forms of the monosaccharide. Mutarotases refer to enzymes that are responsible for catalyzing mutarotation.

9.1.3 Pyranose and furanoses

Figure 9.6: Pyranose and Furanose Structures

Pyranoses are six-membered rings; furanoses are five-membered rings.

Glucopyranose is a cyclic form of glucose; fructofuranose is the cycle form of fructose (see figure 9.6).

9.1.4 Oxidation and reduction reactions

When an organic molecule is oxidized, they can gain an oxygen atom or lose a hydrogen atom.

Figure 9.7: Reduction and Oxidation of an Aldehyde

When an aldehyde functional group is oxidized or reduced (see figure 9.7), it can turn into a carboxylic acid or an alcohol. Hence, when monosaccharides are oxidized, a sugar acid is produced; when monosaccharides are reduced, a sugar alcohol is produced.

Figure 9.8: Ribitol and Other Sugar Alcohols

Sugar alcohols are important components of lipids (e.g., glycerol and myo-Inositol). The sugar alcohol ribitol (see figure 9.8) is an important component of flavin nucleotides FMN (i.e., flavin nucleotide monophosphate) and FAD (i.e., flavin adenine dinucleotide).

9.1.5 Monosaccharides as reducing agents

Figure 9.9: Benedict’s Test and Reducing Sugars

Mild oxidizing agents such as Fe²⁺ and Cu²⁺ ions are capable of oxidizing monosaccharides (i.e., the carbonyl carbon is oxidized to a carboxyl group). Furthermore, a reducing sugar is a monosaccharide that is capable of reducing a Fe²⁺ or a Cu²⁺ ion (see figure 9.9). Hence, a nonreducing sugar is a monosaccharide that is involved in a glycosidic bond and is unable to assume a linear form.

However, do note that the oxidation of a monosaccharide’s anomeric carbon can only take place in the sugar’s linear form (which exist in equilibrium with its cyclic form(s)).

9.1.6 Monosaccharide derivatives

Figure 9.10: Monosaccharide Derivatives

There are many known derivatives of monosaccharides (e.g., oligosaccharides and polysaccharides) alongside various other classes of nonpolymerized compounds like the ones in figure 9.10 above.

Figure 9.11: Some More Monosaccharide Derivatives

The monosaccharide derivatives in figure 9.11 are intermediates in the metabolism of saccharides and constituents of nucleotides.