25.8 Disaccharides
3 min read•may 7, 2024
are sugar pairs with unique structures and properties. , , , and each have distinct glycosidic bonds and compositions, affecting their reducing abilities and roles in nature.
Understanding these differences is key to grasping carbohydrate chemistry. From maltose in grains to lactose in milk, disaccharides showcase how small structural changes can lead to diverse functions in biological systems.
Disaccharide Structures and Properties
Maltose vs cellobiose structure
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- Maltose and cellobiose are both disaccharides composed of two glucose units
- Maltose is formed from two -D-glucose units connected by an (found in malt and germinating cereal grains)
- Cellobiose is formed from two -D-glucose units connected by a glycosidic bond (structural unit of cellulose)
- The glycosidic linkages between the glucose units differ in their stereochemistry and anomeric configuration
- Maltose has an glycosidic bond with both anomeric carbons in the configuration
- Cellobiose has a glycosidic bond with both anomeric carbons in the configuration
- Reducing properties are determined by the presence of a free group at the C1 position of the non-reducing end glucose unit
- Maltose is a due to the free hemiacetal group on the non-reducing end glucose (can be oxidized by Fehling's or Benedict's reagent)
- Cellobiose is also a reducing sugar for the same reason as maltose (can be detected by reducing sugar tests)
- The of the reducing end glucose unit in both maltose and cellobiose can undergo in solution
Composition of lactose
- Lactose is a disaccharide composed of and glucose units connected by a glycosidic bond
- The galactose unit is -D-galactose (a C-4 of glucose)
- The glucose unit is -D-glucose
- Comparison to other common disaccharides highlights similarities and differences in structure and composition
- Like maltose and cellobiose, lactose has a glycosidic bond connecting its monosaccharide units (common structural feature)
- Lactose differs from maltose and cellobiose in its monosaccharide composition, as it contains galactose instead of two glucose units (unique sugar combination)
- Lactose is a reducing sugar, similar to maltose and cellobiose, due to the free hemiacetal group at the C1 position of the glucose unit (can reduce copper(II) ions in Fehling's solution)
Properties of sucrose
- Sucrose is a non-reducing disaccharide composed of glucose and units connected by an glycosidic bond
- The glucose unit is -D-glucose ( form)
- The fructose unit is -D-fructose ( form)
- The glycosidic bond in sucrose is unique among common disaccharides
- Formed between the C1 of glucose and the C2 of fructose (only disaccharide with a linkage)
- Connects the anomeric carbons of both , resulting in a non-reducing disaccharide (no free hemiacetal group)
- of sucrose yields equimolar amounts of glucose and fructose (monosaccharides)
- The hydrolysis products are referred to as due to the change in optical rotation (from to )
- Invert sugar is sweeter than sucrose and is used in confectionery products (candy, syrups)
- Sucrose is a non-reducing sugar because it lacks a free hemiacetal group
- The glycosidic bond formation between C1 of glucose and C2 of fructose eliminates the reducing ends of both monosaccharides (no open-chain form with free aldehyde or ketone group)
- Sucrose does not react with Benedict's or Fehling's reagent (negative reducing sugar test)
Key Terms to Review (29)
1→4 Link: A 1→4 link in the context of disaccharides is a specific type of covalent bond between two sugar molecules where the carbon atom at position 1 of one sugar molecule is bonded to the carbon atom at position 4 of another. This linkage determines the structure and properties of many disaccharides, such as lactose and maltose.
Anomeric Carbon: The anomeric carbon is a unique carbon atom found in carbohydrates that is bonded to two oxygen atoms, one of which is part of a hydroxyl group. This carbon atom exhibits special reactivity and plays a crucial role in the formation of acetals, the cyclic structures of monosaccharides, and the linkages between monosaccharides in disaccharides and polysaccharides.
Cellobiose: Cellobiose is a disaccharide composed of two glucose molecules linked by a β-1,4-glycosidic bond. It is a key intermediate in the breakdown of cellulose, the most abundant polysaccharide in nature.
Dextrorotatory: Dextrorotatory, also known as dextrorotation or (+)-rotation, refers to the ability of certain chiral molecules to rotate the plane of polarized light in a clockwise direction when viewed from the direction of the light source. This property is closely linked to the concept of optical activity and enantiomers, and has important implications in various fields, including organic chemistry, biochemistry, and pharmaceutical sciences.
Disaccharides: Disaccharides are a class of carbohydrates composed of two monosaccharide units joined together through a glycosidic bond. They are an important part of the classification of carbohydrates and play a role in the reactions of monosaccharides as well as the formation of disaccharides.
Epimer: An epimer is one of two or more stereoisomers that differ in configuration at only one stereogenic center. Epimers are closely related and have similar chemical properties, but they can be distinguished by their differing biological activities.
Fructose: Fructose is a monosaccharide, or the simplest form of carbohydrate, that is naturally found in fruits, honey, and some vegetables. It is one of the three dietary sugars, along with glucose and galactose, and is known for its unique properties and role in various metabolic processes.
Furanose: Furanose is a cyclic structure of monosaccharides, specifically five-membered ring structures, that are commonly found in carbohydrates. This structural feature is crucial in understanding the properties and behavior of monosaccharides, disaccharides, and their role in various biological processes.
Galactose: Galactose is a monosaccharide, or simple sugar, that is a C-4 epimer of glucose. It is an important component of lactose, the primary sugar found in mammalian milk, and is also produced in the body during the metabolism of lactose.
Glycosidic Bond: A glycosidic bond is a covalent bond that connects a carbohydrate (sugar) molecule to another molecule, such as another carbohydrate, a lipid, or a protein. This bond is formed when the hydroxyl group of one molecule reacts with the anomeric carbon of a monosaccharide, creating a new compound with unique properties and functions.
Hemiacetal: A hemiacetal is a type of functional group formed by the addition of an alcohol to the carbonyl carbon of an aldehyde or ketone, resulting in a cyclic structure with an ether and a hydroxyl group. This term is particularly relevant in the contexts of nucleophilic addition reactions, the cyclic structures of monosaccharides, reactions of monosaccharides, and the formation of disaccharides.
Hydrolysis: Hydrolysis is a chemical reaction in which a compound is cleaved into smaller molecules by the addition of water. This process involves the breaking of chemical bonds through the insertion of water molecules, often resulting in the formation of new functional groups or the decomposition of larger molecules.
Invert Sugar: Invert sugar is a mixture of glucose and fructose that is produced by the hydrolysis of sucrose. It is called 'invert' because the optical rotation of the resulting mixture is opposite (or inverted) compared to the original sucrose.
Lactose: Lactose is a disaccharide sugar found in milk and dairy products. It is composed of two monosaccharides, glucose and galactose, linked together. Lactose plays an important role in the classification of carbohydrates, the reactions of monosaccharides, and the formation of disaccharides.
Levorotatory: Levorotatory refers to the ability of a chiral molecule to rotate the plane of polarized light in a counterclockwise direction when viewed from the direction of the light source. This property is closely related to the concepts of optical activity, enantiomers, and the tetrahedral carbon structure.
Maltose: Maltose, also known as malt sugar, is a disaccharide composed of two glucose molecules linked together. It is an important intermediate in the breakdown of starch and plays a crucial role in the production of fermented beverages like beer and wine.
Monosaccharides: Monosaccharides are the most basic units of carbohydrates, serving as the building blocks for more complex carbohydrate structures. They are simple sugars that cannot be broken down into smaller sugar molecules through hydrolysis.
Mutarotation: Mutarotation is the spontaneous interconversion between the '$\alpha$-' and '$\beta$-'anomeric forms of a monosaccharide in aqueous solution. This process occurs as the monosaccharide forms a cyclic structure and the orientation of the hydroxyl group on the anomeric carbon changes.
Pyranose: Pyranose is a cyclic structure formed by monosaccharides, where the sugar ring contains five carbon atoms and one oxygen atom. This ring structure is a key feature of carbohydrates and plays a crucial role in understanding their stereochemistry, cyclic structures, and the classification of essential monosaccharides and disaccharides.
Reducing Sugar: Reducing sugars are monosaccharides or disaccharides that can donate electrons and act as reducing agents, meaning they can reduce other compounds by donating hydrogen atoms or electrons. This property is particularly important in the context of cyclic structures of monosaccharides and the formation of disaccharides.
Reducing sugars: Reducing sugars are carbohydrates that can act as reducing agents due to their free aldehyde group or a ketone group that can isomerize to an aldehyde, enabling them to donate electrons to other molecules. These sugars undergo specific reactions that can help identify them, such as the Benedict's test.
Sucrose: Sucrose is a disaccharide composed of one molecule of glucose and one molecule of fructose. It is the primary sweet-tasting carbohydrate found in many plants and is commonly known as table sugar. Sucrose is an important carbohydrate in the context of carbohydrate classification and disaccharides.
α-D-glucose: α-D-glucose is a monosaccharide, the most abundant naturally occurring sugar, and a key component of disaccharides like sucrose. It is the primary source of energy for many organisms and plays a crucial role in various metabolic processes.
α(1→2)β Glycosidic Bond: The α(1→2)β glycosidic bond is a type of covalent linkage that connects two monosaccharide units in disaccharides. It is characterized by the orientation of the anomeric carbon and the position of the glycosidic oxygen atom between the two sugar residues.
α(1→4) Glycosidic Bond: The α(1→4) glycosidic bond is a type of covalent bond that connects two monosaccharide units, forming a disaccharide. This specific bond is found in important disaccharides like sucrose and starch, and it plays a crucial role in the structure and function of these carbohydrates.
β-D-fructose: β-D-fructose is a monosaccharide, a type of simple sugar, that is commonly found in fruits, honey, and some vegetables. It is one of the three dietary monosaccharides, along with glucose and galactose, that are the building blocks of more complex carbohydrates. β-D-fructose is particularly relevant in the context of disaccharides, as it is a component of the disaccharide sucrose.
β-D-galactose: β-D-galactose is a monosaccharide, a type of simple sugar, that is an epimer of glucose. It is a key component of the disaccharide lactose, which is found in mammalian milk. β-D-galactose is an important carbohydrate in various biological processes and metabolic pathways.
β-D-glucose: β-D-glucose is a monosaccharide, the most abundant form of glucose found in nature. It is the primary source of energy for many organisms and plays a crucial role in the formation of disaccharides, which are the focus of the 25.8 Disaccharides topic.
β(1→4) Glycosidic Bond: A β(1→4) glycosidic bond is a type of covalent linkage that connects two monosaccharide units, forming a disaccharide. This specific bond is found in important carbohydrate structures, such as cellulose and chitin, and plays a crucial role in their chemical and structural properties.