The monosaccharides having asymmetric carbon atoms exhibit isomerism.
Asymmetric carbon atom- It is the carbon atom that is attached to four different groups (Figure-1)
Figure-1- An asymmetric carbon atom with 4 different attachments
All monosaccharides except- Dihydroxy acetone, have asymmetric carbon atoms (Figure-2)
Figure-2- Glyceraldehyde has an asymmetric carbon atom whereas dihydroxyacetone lacks, thus it does not have isomers.
Based on the presence of asymmetric carbon atoms the following types of isomerism of monosaccharides are observed in the human system-
1) D and L isomerism- The orientation of the —H and —OH groups around the carbon atom adjacent to the terminal primary alcohol carbon (carbon 5 in glucose) determines whether the sugar belongs to the D or L series. When the —OH group on this carbon is on the right (as seen in figure-3), the sugar is the D isomer; when it is on the left, it is the L isomer (figure-3)
Figure-3- D and L isomers of Glucose
Glyceraldehyde has a single asymmetric carbon and, thus, there are two stereoisomers of this sugar.
D-Glyceraldehyde and L-glyceraldehyde . The D and L isomers of monosaccharides are called enantiomers, as they are mirror images of each other (Figure-4)
Figure-4- D and L Isomers of Glyceraldehyde
- Most of the monosaccharides occurring in mammals are D sugars, and the enzymes responsible for their metabolism are specific for this configuration.
- D-Ribose, the carbohydrate component of RNA, is a five-carbon aldose.
- D-Glucose, D-mannose, and D -galactose are abundant six-carbon aldoses.
- Some sugars naturally occur in the L form e.g. L-Arabinose and L-Fucose are found in glycoproteins.
2) Optical Isomerism- The presence of asymmetric carbon atoms also confers optical activity on the compound. When a beam of plane-polarized light is passed through a solution of an optical isomer, it rotates either to the right, dextrorotatory (+), or to the left, levorotatory (–). The direction of rotation of polarized light is independent of the stereochemistry of the sugar, so it may be designated D (–), D (+), L (–), or L (+) – figure-5
For example, the naturally occurring form of fructose is the D (–) isomer. In solution, glucose is dextrorotatory, and glucose solutions are sometimes known as dextrose.
Measurement of optical activity in chiral or asymmetric molecules using plane polarized light is called Polarimetry. The measurement of optical activity is done by an instrument called Polarimeter.
Figure-5- Rotation of polarized light by an optically active solution
The compounds with the same molecular formula, but differing in spatial configuration of the attached groups around a single carbon atomonly are called epimers.
In hexoses, isomers differing as a result of variations in configuration of the —OH and —H on carbon atoms 2, 3, and 4 are known as epimers. Biologically, the most important epimers of glucose are mannose and galactose, formed by epimerization at carbons 2(figure-6) and 4(figure-7), respectively.
Figure-6- Glucose and Mannose are C2 epimers.
Figure-7- Glucose and galactose are C4 epimers
Mannose and Galactose are not epimers of each other as they differ in configuration around 2 carbon atoms (figure-8).
Figure-8- Relationship of glucose, galactose and mannose
4) Aldose-ketose isomerism
Compounds with the same molecular formula but differing in nature of functional group (aldehyde or keto) are aldose ketose isomers.
Examples- Fructose and Glucose are aldose ketose isomers.
Figure-9- Aldose ketose isomers (D-Glucose and D-Fructose)
Fructose has the same molecular formula as glucose but it differs in its structural formula, since there is a potential keto group in position 2, the anomeric carbon of fructose (Figure-9), whereas there is a potential aldehyde group in position 1, the anomeric carbon of glucose.
Glyceraldehyde and Dihydroxyacetone, Ribose and Ribulose are other examples of aldose -ketose isomers.
In biological system the monosaccharides tend to exist in a ring form. The ring structure of an aldose is a hemiacetal, since it is formed by combination of an aldehyde (C1) and an alcohol group (Mostly C5). Similarly, the ring structure of a ketose is a hemiketal. The ring can open and reclose allowing the rotation to occur around the carbon bearing the reactive carbonyl group yielding two possible configurations- α and β of the hemiacetal and hemiketal. The carbon about which this rotation occurs is called Anomeric carbon and the two stereoisomers are called Anomers. In alpha anomer the orientation of the OH group is towards the right side whereas in the beta anomer, it is towards the left side. (Figure-10).
Figure-10- Alpha and beta anomers of glucose
When drawn in the Haworth projection, the α configuration places the hydroxyl downward. While the β is the reverse (figure-11).
Figure-11- Alpha and beta anomers of glucose drawn in Haworth projection
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