Chapter 24 Glycosides

Many of the plant secondary metabolites are found naturally attached to sugars (glycosides). Mostly, the sugars are monosaccharides (Figure 24.1), such as glucose, but they can be more complicated. The sugars link to a non-sugar part called an aglycone and can be attached by separate bonds or, more commonly, as di-, tri- or tetrasaccharides. This is done by one sugar attaching to the aglycone and the others linking onto that sugar.

Figure 24.1 The structure of glucose, a common component of glycosides.

The linkage between the sugar and the aglycone (glycosidic linkage) is difficult for human enzymes to break down. This is where having a healthy gut is important, as this job is done by the various microbes in the gut. When the two parts have been separated, the aglycone (the non-sugar part) is small enough to be transported through the gut wall into the bloodstream, where it is sent around the body.

The linkage between the two groups can vary. It can be:

Figure 24.2 The C-glycoside linkage. The sugars attached vary in composition and can consist of one or more sugars.

The well-established naming of glycosides using the termination ‘in’ (e.g. salicin, aloin) has persisted and results in some confusion, as some substances (e.g. pectin) are not glycosides. Similar examples of this, involving isoflavones, are discussed in Chapter 21 ‘Phenols’ (p. 161). The more modern termination ‘-oside’ can be used (e.g. sennoside) to prevent confusion.

Glycosides are generally inactive and are activated by a process of hydrolysis into: