2.8 Isomers and Stereoisomers of Glucose | Molecular Biology | Biology 11

A detailed breakdown of compounds with the same molecular formula but different structural arrangements, focusing on the various types of stereoisomers found in glucose.

Isomerism

A phenomenon where compounds share the same molecular formula but differ in their structural arrangement.

Example: Glucose and Fructose both have the formula $C_{6} H_{12} O_{6}$ , but glucose is an aldo sugar (aldehyde group) while fructose is a keto sugar (ketone group). They are considered functional group isomers.

Stereoisomers are molecules with the same molecular formula and sequence of bonded atoms, but they differ in the three-dimensional orientation of their atoms in space. The number of possible stereoisomers can be calculated using the formula $2^{n}$ , where n is the number of asymmetric carbon atoms.

For Glucose: Glucose has 4 asymmetric carbon atoms (C-2, C-3, C-4, C-5), so it has $2^{4} = 16$ possible stereoisomers.

Key Carbon Atoms in Glucose Structure

Asymmetric Carbon: A carbon atom that is bonded to four different atoms or groups. In glucose, C-2, C-3, C-4, and C-5 are asymmetric.
Anomeric Carbon: The carbon atom in the ring structure that was formerly the carbonyl carbon (aldehyde or ketone group) in the open-chain form. In glucose, this is C-1.
Penultimate Carbon: The second-to-last carbon in the chain, which is also the last asymmetric carbon. In glucose, this is C-5.

Types of Stereoisomers

1. Anomers

Stereoisomers that differ in the position of the -H and -OH groups only at the anomeric carbon (C-1).
α-glucose: The -OH group on the anomeric carbon is pointing downward.
β-glucose: The -OH group on the anomeric carbon is pointing upward.

Anomers of glucose — Figure 2.15: Anomers (α and β stereoisomers) of glucose

2. Enantiomers

Stereoisomers that are non-superimposable mirror images of each other. The D and L forms of a sugar are enantiomers.
D-glucose (Dextro): The "right-handed" form. The -OH group on the penultimate carbon (C-5) is on the right side.
L-glucose (Levo): The "left-handed" form. The -OH group on the penultimate carbon (C-5) is on the left side.

Enantiomers of glucose — Figure 2.16: Enantiomers (D-glucose and L-glucose)

3. Diastereoisomers

Stereoisomers that are not mirror images of each other. They differ in the arrangement of -H and -OH groups at more than one asymmetric carbon.
Example: D-Glucose and D-Altrose are diastereoisomers.

4. Epimers

A specific type of diastereoisomer. They differ in the arrangement of -H and -OH groups at only one asymmetric carbon (other than the anomeric carbon).
Example: D-Glucose and D-Mannose are epimers, differing only at C-2. D-Glucose and D-Galactose are also epimers, differing only at C-4.

Stereoisomers of D-Glucose — Figure 2.17: Eight stereoisomers of D-Glucose. The encircled -OH group at the penultimate carbon indicates they are all D-isomers.

Stereoisomer Type	Basis of Difference	Relationship	Example
Anomers	Position of -OH on anomeric carbon (C-1)	Isomers (α or β)	α-glucose vs. β-glucose
Enantiomers	Mirror images; position of -OH on penultimate carbon (C-5)	Non-superimposable mirror images (D or L)	D-glucose vs. L-glucose
Epimers	Position of -OH on a single asymmetric carbon	Not mirror images	D-glucose vs. D-Mannose (at C-2)
Diastereoisomers	Position of -OH on more than one asymmetric carbon	Not mirror images	D-glucose vs. D-Altrose

Biological Significance

Living organisms almost exclusively use D-sugars. Enzymes, like those in the human digestive system, are stereospecific and can only metabolize D-sugars. This is why artificial sweeteners can be made from L-sugars, as the body cannot metabolize them for energy.

Isomerism

A phenomenon where compounds share the same molecular formula but differ in their structural arrangement.

Example: Glucose and Fructose both have the formula

C_{6} H_{12} O_{6}

, but glucose is an aldo sugar (aldehyde group) while fructose is a keto sugar (ketone group). They are considered functional group isomers.

Stereoisomerism in Glucose

For Glucose: Glucose has 4 asymmetric carbon atoms (C-2, C-3, C-4, C-5), so it has

2^{4} = 16

possible stereoisomers.

Asymmetric Carbon: A carbon atom that is bonded to four different atoms or groups. In glucose, C-2, C-3, C-4, and C-5 are asymmetric.

Anomeric Carbon: The carbon atom in the ring structure that was formerly the carbonyl carbon (aldehyde or ketone group) in the open-chain form. In glucose, this is C-1.

Penultimate Carbon: The second-to-last carbon in the chain, which is also the last asymmetric carbon. In glucose, this is C-5.

Stereoisomers that differ in the position of the -H and -OH groups only at the anomeric carbon (C-1).

α-glucose: The -OH group on the anomeric carbon is pointing downward.

β-glucose: The -OH group on the anomeric carbon is pointing upward.

Figure 2.15: Anomers (α and β stereoisomers) of glucose

Stereoisomers that are non-superimposable mirror images of each other. The D and L forms of a sugar are enantiomers.

D-glucose (Dextro): The "right-handed" form. The -OH group on the penultimate carbon (C-5) is on the right side.

L-glucose (Levo): The "left-handed" form. The -OH group on the penultimate carbon (C-5) is on the left side.

Figure 2.16: Enantiomers (D-glucose and L-glucose)

Stereoisomers that are not mirror images of each other. They differ in the arrangement of -H and -OH groups at more than one asymmetric carbon.

Example: D-Glucose and D-Altrose are diastereoisomers.

A specific type of diastereoisomer. They differ in the arrangement of -H and -OH groups at only one asymmetric carbon (other than the anomeric carbon).

Example: D-Glucose and D-Mannose are epimers, differing only at C-2. D-Glucose and D-Galactose are also epimers, differing only at C-4.

Figure 2.17: Eight stereoisomers of D-Glucose. The encircled -OH group at the penultimate carbon indicates they are all D-isomers.

Stereoisomer Type	Basis of Difference	Relationship	Example
Anomers	Position of -OH on anomeric carbon (C-1)	Isomers (α or β)	α-glucose vs. β-glucose
Enantiomers	Mirror images; position of -OH on penultimate carbon (C-5)	Non-superimposable mirror images (D or L)	D-glucose vs. L-glucose
Epimers	Position of -OH on a single asymmetric carbon	Not mirror images	D-glucose vs. D-Mannose (at C-2)
Diastereoisomers	Position of -OH on more than one asymmetric carbon	Not mirror images	D-glucose vs. D-Altrose