I. Structure of Monosaccharide
(A) the three-dimensional structure and configuration of monosaccharides
1. Stereoisomers of monosaccharides
Monosaccharide molecules are asymmetric molecules with optical rotation. Taking glyceraldehyde as an example, the 2-position carbon in the molecule is an asymmetric carbon atom, which is connected with four different atoms and groups H, CH2OH, OH and CHO respectively. There are two arrangements of this structure, one is D- glyceraldehyde and the other is L- glyceraldehyde. When writing D-structure, put hydroxyl group on the right; On the left is the l-type hydroxyl group. The optical rotation of D- glyceraldehyde is right-handed, and that of L- glyceraldehyde is left-handed. D- glyceraldehyde and L- glyceraldehyde are stereoisomers with different configurations. Therefore, both D-form and L-form glyceraldehyde are enantiomers, and the structure with enantiomers is also called "chiral" structure.
Because the direction and degree of optical rotation are determined by the direction of hydroxyl groups on all asymmetric atoms in the molecule, and the configuration is only related to the direction of hydroxyl groups of asymmetric carbon atoms farthest from carbonyl groups in the molecule, the configurations D and L of monosaccharides do not necessarily correspond to dextrorotation and levorotation. The dextrorotatory rotation of monosaccharide is D or (+), and the levorotatory rotation is L or (-).
Monosaccharides from trisaccharides (glyceraldehyde) all have asymmetric carbon atoms. Compounds containing n asymmetric carbon atoms should have 2n stereoisomers.
2. Configuration of monosaccharide
D- type and L- type sugars are relative configurations determined by comparing glyceraldehyde as a standard. The configuration of sugar depends on the direction of hydroxyl groups on asymmetric carbon atoms farthest from carbonyl groups. If it is the same as D- glyceraldehyde, it is type d. If it is the same as L- glyceraldehyde, it is L-shaped. Aldose can be derived from glyceraldehyde by gradually increasing the carbon chain. The configuration of ketose was also determined by the same method. The carbon atoms summarized by the following sugars have the same configuration, and they are all D-type sugars.
(2) Structure and conformation of monosaccharide
There are many kinds of monosaccharides, among which glucose (free and bound) is the most abundant and widely distributed in nature.
Although the structure and properties of monosaccharides are different, there are many similarities. The structure and properties of glucose are representative. Taking glucose as an example, this paper expounds the molecular structure of monosaccharide.
Glucose is the most important hexose, so it is called glucose because it originally existed in grapes. Its molecular formula is C6H 12O6. Naturally occurring is D- glucose.
1. Chain structural formula
Experiments show that the chain structure of D- glucose is:
The above structural formula can be simplified, "├" represents the position of hydroxyl group in carbon chain and asymmetric carbon atom, and "△" represents aldehyde group.
"-—CHO", "-"stands for hydroxyl "-OH" and "○" stands for the first alcohol group, then the structural formula of glucose is simplified as (a), and the structural formulas of D mannose and D galactose, which belong to aldose with glucose, are simplified as (b) and (c) respectively.
(a)D- glucose (b)D- mannose (c)D- galactose
2. Ring structure
Physical and chemical methods prove that monosaccharides exist not only in linear structure, but also in cyclic structure. Because monosaccharide molecules have both carbonyl groups and hydroxyl groups, a ring can be formed in the molecule due to the formation of hemiacetal (or hemiketal). That is, the oxygen in one hydroxyl group on the carbon chain is connected with the carbon atom of carbonyl group to form a ring, and the hydrogen atom in the hydroxyl group is added to the oxygen of carbonyl group. Experiments show that, in general, hexose is a semi-acetal formed by hydroxyl and carbonyl on the fifth carbon atom, forming a six-membered ring. For example, D- glucose can form the following two cyclic hemiacetals:
Hemiacetal α-D- glucuronic aldehyde D-glucose hemiacetal β-D- glucose
37% 0. 1% 63%
D- glucose is converted from aldehyde to hemiacetal, and C 1 is converted into chiral carbon atoms, forming a pair of optical isomers. Generally speaking, the hydroxyl group on the newly formed chiral carbon atom (called hemiacetal hydroxyl group) and the hydroxyl group on the carbon atom that determines the monosaccharide configuration (C5 in hexose) are on the same side of the carbon chain, which is called α-glucose and written as α-D- glucose; What is not on the same side is called β-glucose, which is written as β-d- glucose. However, these two isomers are not enantiomers, but the hydroxyl groups on 1 carbon are in different directions, so they are called anomers. Hemiacetal hydroxyl group is more active than other hydroxyl groups, and many important properties of sugar are related to it.
Moreover, glucose also has conformation problems. According to X-ray diffraction, the five carbons and one oxygen in the glucopyranose ring are not on the same plane, and usually have the following conformation, in which the chair conformation is the most stable because it makes the molecular tensile strength the lowest and the electrostatic repulsion of each atom in the molecule the smallest.
Second, the properties of monosaccharides
The properties of monosaccharides are determined by their chemical composition and structure.
(1) Main physical properties
1. solubility
Monosaccharide is a colorless crystal. Because there are many hydroxyl groups in the molecule, it has great solubility in water and can often form supersaturated solution-syrup.
2. Aroma
All monosaccharides have sweetness, but the sweetness is different. Usually, the sweetness of sucrose is set to 100 for comparison.
Sugar, sucrose and fructose are converted into sugar * glucose, xylose, maltose, galactose and lactose.
Sweetness100173130 74 40 32 3216
* The mixture of glucose and fructose produced by sucrose hydrolysis is called invert sugar.
3. Optical rotation and variable rotation phenomena
All saccharides have chiral carbon atoms in their molecules, so they are optically active and belong to "optically active substances" (or optically active substances). The angle at which an optically active substance rotates the vibration plane of polarized light is called "optical rotation". The optical rotation of a substance varies with the concentration of the solution, the length of the liquid container, the temperature, the wavelength of the light wave and the nature of the solvent. However, under certain conditions, the optical rotation of different optically active substances is still a constant, which is usually expressed by specific rotation [α]. The specific rotation is defined as the optical rotation measured by placing a 1 ml solution containing 1 g solute in a liquid container with a length of 1 decimeter. The specific optical rotation of sugar is expressed by [α] D2 0. The calculation formula is as follows:
Where α: optical rotation measured by polarimeter.
C: the concentration of sugar (optical rotation) solution, expressed in grams of solute per milliliter of solution, and the solvent is water.
L: the length of the liquid container, in decimeter.
20: 20℃, which means that the specific curl is measured at 20℃.
D: that is, using sodium lamp as light source.
(II) Main chemical properties
Monosaccharide is a polyhydroxy aldehyde or a polyhydroxy ketone, so it has the properties of aldehyde group, ketone group and alcohol hydroxyl group, which can produce esterification and etherification of alcohol hydroxyl group and oxidation, reduction and addition of carbonyl group, as well as some special reactions caused by the interaction between hydroxyl group and carbonyl group. Monosaccharide exists in chain and ring equilibrium in aqueous solution. In some reactions, chain isomers participate in the reaction, while cyclic isomers are continuously transformed into chains, and finally all derivatives of chain isomers are generated. The main chemical properties of monosaccharides are as follows:
1. Properties of Aldehyde and Ketone Groups
(1) monosaccharide isomerization
(2) Oxidation (reduction) of monosaccharides
2. Properties produced by hydroxyl groups (alcoholic hydroxyl groups and hemiacetal hydroxyl groups)
(1) esterification
(2) Causality
(3) Glycosylation
Three. Important monosaccharides and their derivatives
Monosaccharide is the smallest unit of sugar. In the past half century, many monosaccharides have been discovered, including more than 600 aldoses and 180 ketoses and their derivatives. The number of monosaccharides in nature is less than the theoretical number of optical isomers, and there are many common ones, such as aldose, ketose, deoxysugar, branched-chain sugar, amino sugar and so on. The following are some important representatives (Table 3-3).
Because monosaccharide has many reactive groups, it can form a variety of monosaccharide derivatives, generally including the following categories:
1. glycosides
2. Monosaccharide phosphate ester
3. Amino sugar (amino sugar or sugar amine)
4. Sugar and acid
5. Sugar alcohols