Question

Amygdalin is a toxic component in the pits of bitter almonds, peaches, and apricots. (a) Name the two monosaccharide units in amygdalin and describe the glycosidic bond by which they are joined. (b) Account for the fact that hydrolysis of amygdalin in warm aqueous acid liberates benzaldehyde and HCN.

Short answer

Question: Identify the two monosaccharide units in amygdalin and explain why hydrolysis of amygdalin in warm aqueous acid liberates benzaldehyde and HCN. Answer: The two monosaccharide units in amygdalin are D-glucose and L-mandelonitrile glucoside, joined by a glycosidic bond. Hydrolysis of amygdalin in warm aqueous acid breaks the glycosidic bond and further hydrolyzes L-mandelonitrile glucoside into benzaldehyde and HCN due to its instability under acidic conditions.

Step-by-step solution

Identify the monosaccharide units in amygdalin and describe the glycosidic bond

Amygdalin is a cyanogenic glycoside and its structure contains two monosaccharide units: D-glucose and L-mandelonitrile glucoside. The two monosaccharide units are joined together by a glycosidic bond. This bond forms between the hydroxyl group (OH) of the D-glucose and the anomeric carbon of the L-mandelonitrile glucoside.

Hydrolysis of amygdalin

In warm aqueous acid, amygdalin undergoes hydrolysis, meaning it breaks down into smaller components as it reacts with water molecules. During this process, the glycosidic bond between D-glucose and L-mandelonitrile glucoside is cleaved. L-mandelonitrile glucoside then further hydrolyzes, producing benzaldehyde and hydrocyanic acid (HCN). The liberation of benzaldehyde and HCN during the hydrolysis of amygdalin can be explained by the instability of L-mandelonitrile glucoside under acidic conditions.

Key concepts

Glycosidic Bond

A glycosidic bond is a pivotal feature in the structure of many natural compounds, including amygdalin. In simple terms, it is the bond that holds two sugar molecules, or a sugar molecule and another compound, together. This type of bond is formed through a dehydration reaction, where a molecule of water is removed as the bond is created. Specifically, it connects the anomeric carbon atom of one sugar molecule (which is the carbon derived from the carbonyl group in the sugar's ring form) to an oxygen, nitrogen, or carbon atom of another molecule.

The glycosidic bond in amygdalin, for instance, is the critical link joining the two monosaccharide units within its structure. Through this bond, the sugar unit (D-glucose) and the L-mandelonitrile glucoside molecule are chemically married together. Breaking this bond, such as in hydrolysis reactions, is significant in metabolic processes as it releases the individual components, leading to the production of other substances like benzaldehyde and toxic HCN, observable in the case of amygdalin.

Monosaccharide Units

Monosaccharide units are the simplest form of carbohydrates and are often referred to as the building blocks of more complex sugars and polysaccharides. They consist of a single sugar molecule and can commonly be found in the form of hexoses, with six carbon atoms, or pentoses, with five. These units are key because they represent the fundamental structure and energy units of carbohydrates.

Amygdalin contains two such monosaccharide units, D-glucose and L-mandelonitrile glucoside, which play distinct roles within the structure of this cyanogenic glycoside. D-glucose is a well-known energy source, while the L-mandelonitrile glucoside unit includes a nitrile group that is part of the defense mechanism in some plants. Under enzymatic or chemical action, such as in hydrolysis, these monosaccharides can be released, showing the importance of understanding their structures and the bonds holding them in larger molecular frameworks.

Cyanogenic Glycosides

Cyanogenic glycosides, like amygdalin, belong to a class of compound that stores cyanide groups in a sugar-based framework. These substances can be found in various plants and serve primarily as a defense mechanism against herbivores and pathogens. Upon damage to the plant tissue or hydrolysis under certain conditions, these compounds can release cyanide, a potent toxin.

Understanding the behavior of cyanogenic glycosides is critical, particularly in amygdalin's case. During hydrolysis, enzymes or acids break the glycosidic bonds in amygdalin, leading to the release of benzaldehyde, a compound with an almond scent, and HCN (hydrocyanic acid). The production of these compounds explains why certain plant materials, despite their natural origins, must be treated with caution. Amygdalin's biochemistry, therefore, not only has implications for food safety but also provides insights into the intricate defense strategies of plants.