The structure and characteristics of monosaccharides:
1. The open-chain structure and configuration of monosaccharides
The open-chain structure of monosaccharides can be determined by Fisher ( Fischer projection, that is, the carbon chain is placed on a vertical line, with the No. 1 carbon atom in the main chain on top, the intersection of the "cross" line represents the carbon atom in the chain, and each carbon atom in the chain is connected to a Hydrogen atoms and a hydroxyl group (or two hydrogen atoms connected) are located on the left and right sides of the carbon chain respectively.
Characteristics of monosaccharides
Monosaccharides mainly exist in the form of cyclic structures in solution, crystallized state and in vivo. The cyclic structure of monosaccharides is formed by the hemiacetal (or hemiketal) reaction between the carbonyl group and the hydroxyl group.
The most common ones are five-membered rings and six-membered rings. The hydroxyl group of the hemiacetal (or hemiketal) formed is on the same side of the carbon chain as the hydroxyl group on the carbon atom that determines the configuration of the monosaccharide. - type, and the one on the opposite side is β-type. The a-type and β-type cyclic structures of monosaccharides can be converted into each other through chain structures.
Disaccharide structure and characteristics:
Structure:
1. Reducing disaccharide
Reducing disaccharide is composed of one molecule It is formed by dehydration condensation between the hemiacetal hydroxyl group of a monosaccharide and the alcoholic hydroxyl group of another monosaccharide molecule. There is also a hemiacetal hydroxyl group in the molecule, which can be opened into a chain. Important reducing disaccharides are maltose, cellobiose and lactose.
2. Non-reducing disaccharides
Non-reducing disaccharides are formed by dehydration between the hemiacetal (or hemiketal) hydroxyl groups of two monosaccharide molecules. There is no hemiacetal hydroxyl group and cannot be opened into a chain.
Features:
Maltose and cellobiose are both reducing disaccharides composed of two molecules of glucose connected to each other by the first and fourth carbon atoms through oxygen atoms. The difference The only difference lies in the configuration of the hemiacetal hydroxyl group in the glucose unit.
In maltose, the hemiacetal hydroxyl group of the glucose unit that forms the glycoside is a-formula, so the bond formed with C4 of another glucose molecule is called a-1,4-glycoside bond, while cellobiose The two glucose units are connected by a β-1,4-glycoside bond. Both maltose and cellobiose have a and two beta isomers.
Extended information
Related knowledge
Saccharides are also called carbohydrates and are widely distributed in nature, such as glucose, starch, cellulose, glycogen, etc. All are sugars. Carbohydrates are important components of animals and plants and are the main source of energy required by all organisms to maintain life activities. From the chemical structure point of view, they are polyhydroxy aldehydes, ketones or condensates of polyhydroxy aldehydes and ketones.
According to whether it can be hydrolyzed and the products of hydrolysis, sugars can be divided into monosaccharides, oligosaccharides and polysaccharides.
Monosaccharides are sugars that cannot be hydrolyzed into smaller molecules, such as glucose, ribose, fructose, etc.
Oligosaccharides are sugars that can be hydrolyzed into 2 to 10 monosaccharide molecules, the most important of which are disaccharides that can be hydrolyzed into two molecules of monosaccharides, such as sucrose, maltose, lactose, etc.
Polysaccharides refer to sugars that can be hydrolyzed into many monosaccharide molecules, such as starch, cellulose, glycogen, etc.
China Reference Materials Network-Structures and properties of monosaccharides and disaccharides