5.5. Glycosidic Bonds

Recall that polymers made of biomolecules (biopolymers) are very common in nature (see Section 4.4.1). There are multiple ways of forming polymers using monosaccharide subunits. By a wide margin, the most common way is to form an acetal by combining the hemiacetal functional group of one monosaccharide with an OH group from another (Figure 5.22). This type of connectivity is often called a glycosidic bond.

Figure 5.22 – Example of Two Monosaccharides Combining to form a Disaccharide via a β-1,4 Glycosidic Bond.

Glycosidic bonds are very common. Technically, they can occur between a saccharide’s hemiacetal and any OH group. This results in many of the more complex biomolecules (e.g. glycoproteins, glycolipids, nucleosides, etc.) and more complex connectivity between monomers (e.g. the hemiacetal of one combining with the OH of the hemiacetal of another). These compounds are typically explored in advanced courses in biochemistry and/or bioorganic chemistry. This text will focus on glycosidic bonds between the hemiacetal of one saccharide and the alcohol OH of another. However, it is important to be aware that other more complex glycosidic bonds are possible (see Section 5.6.2.2 for an example) and may be encountered in other sources.

Monosaccharides have one hemiacetal functional group but typically have several alcohol functional groups. As a result, there are theoretically many different possible types of glycosidic bonds between monosaccharide monomers. Each of these types has a distinct three-dimensional shape (Figure 5.23).

Figure 5.23 – Examples of Chair Representations of D-Glucopyranose Disaccharides Connected by Varying Types of Glycosidic Bonds Attempting to Convey Three-Dimensional Shape.

This has a variety of consequences for the di-/poly-saccharide’s overall shape, what enzymes may interact with it, etc. However, it is often very challenging to draw the three-dimensional shapes for many of these structures. At an introductory level students are typically required to interpret/label complex glycosidic connections but draw only the simpler three-dimensional connections. The “real” conformations and three-dimensional shapes are often not accurately conveyed even using these drawings (Figure 5.24).

Figure 5.24 – Chair and Ball-and-Stick Representations of a Disaccharide of β-D-Glucopyranose Attempting to Convey Three-Dimensional Shape.

5.5.1. Nomenclature of Glycosidic Bonds

Monosaccharides have one hemiacetal functional group but typically have several alcohol functional groups. As a result, there are theoretically many different possible types of glycosidic bonds between monosaccharide monomers (Figure 5.25). To accurately describe the connectivity two pieces of information are needed: the stereochemistry of the acetal (α/β) and the regioselectivity of the bond (which alcohol of the next monosaccharide it connects to).

Figure 5.25 – Examples of D-Glucopyranose Disaccharides Connected by Varying Types of Glycosidic Bonds.

The most common format for naming/describing a glycosidic bond follows the pattern: [α/β]-[carbon number of the acetal],[carbon number of the alcohol] (Figure 5.26). Some sources use a slight variation on this, with an arrow in place of the comma. Use of either style is acceptable. Occasionally the term “linkage” may be used with the description to indicate that this bond “links” the saccharides.

Figure 5.26 – Examples of Naming Glycosidic Bonds.

Only the α/β stereochemistry of the acetal saccharide is included in the name of the bond; the stereochemistry of the other anomeric position is included in the compound’s name (see Section 5.6.1) but not in the name of the glycosidic bond.