Main Difference – D vs L Glucose
Isomerism is divided into two broad categories as structural isomerism and stereoisomerism. D and L isomers are stereoisomers that have the same chemical structure but are non-superimposable mirror images of each other. Glucose is a sugar molecule that is found as either D-Glucose or L-Glucose in nature. The main difference between D and L Glucose is that D-Glucose rotates plane polarized light clockwise whereas L-Glucose rotates plane polarized light anticlockwise.
1. What are the Methods of Representing D and L Isomers
– Fischer Projection, Haworth Projection, Chair Confirmation
2. What is D Glucose
– Definition, Structure, Properties
3. What is L Glucose
– Definition, Structure, Properties
4. What is the Difference Between D and L Glucose
– Comparison of Key Differences
Key Terms: Chain Confirmation, D Glucose, Fischer Projection, Glucose, Haworth Projection, Isomerism, L Glucose, Stereoisomerism, Sugar
What are the Methods of Representing D and L Isomers
It is important to learn methods of representing the D and L isomers first before discussing the difference between D and L glucose in depth. There are three methods for presenting the molecular structures of D and L isomers.
Fischer Projection
This is the most common and easiest way of drawing a chemical structure for an isomer. It is a 2D structure and is a linear structure.
Figure 1: Fischer Projection for D and L Glucose
Haworth Projection
This is a 3D representation of the molecule and it shows the cyclic structure of the molecule. The cycle is formed through the formation of a bridge through the oxygen atom of aldehyde group.
Figure 2: The Haworth Projection of D and L Glucose.
The cyclic structure has a bridge through Oxygen atom; thus, Glucose is called Glucopyranose in order to avoid any confusion.
Chair Confirmation
Figure 3: β-D-Glucose Chair Confirmation
The actual spatial structure of a molecule is given by the chair confirmation. But it is difficult to deal with this structure in comparisons and identifications. In order to avoid confusion, these D and L isomers are called α and β isomers. This method is the most accurate way of representation.
What is D-Glucose
D-Glucose is a sugar molecule that can rotate the plane polarized light in the clockwise direction. In the Fischer projection, D-Glucose shows four –OH groups on the sides of the main carbon chain. Three of the –OH groups are on the right side whereas other –OH group is on the left side (-OH group attached to the 3rd carbon atom).
Figure 4: Fischer projection of D-Glucose
In the above image, -OH group inside the green colored box is on the left side whereas other –OH groups are on the right side.
Figure 5: Haworth Projection of D-Glucose
In the Haworth projection of the D-Glucose, one of the –OH groups is directed upward whereas other –OH groups are downward.
D-Glucose is the form of Glucose that is found abundant in nature. It is the basic form of energy storage and living beings use D-Glucose to fulfil their energy needs.
What is L-Glucose
L-Glucose is the mirror image of D-Glucose. But this mirror image is non-superimposable with D-Glucose. Therefore, L-Glucose is considered as the enantiomer of D-Glucose. Since it is the mirror image, the –OH groups of the Fischer projection are located in completely opposite directions. This structure also has three –OH groups at the same side and other –OH group on the opposite side. But unlike in D-Glucose, three –OH groups are on the left side of the main carbon chain whereas other –OH group is located on the right side.
Figure 6: Fischer Projection of L-Glucose
In the Fischer projection of D-Glucose, the –OH groups are located in the opposite directions compared to the Fischer projection of D-Glucose.
Figure 7: Haworth projection of L-Glucose
When considering the Haworth projection, the three –OH groups are directed upward and the –OH group attached to the 3rd carbon is directed downward.
L-Glucose is not abundant in nature. But L-Glucose can be found in some fruits and vegetables. Since it is the non-superimposable mirror image of D-Glucose, the molecular formula and the molar mass are the same for D and L Glucose isomers. But the chemical properties and their occurrence would be different due to different spatial arrangements.
Difference Between D and L Glucose
Definition
D-Glucose: D-Glucose is a sugar molecule that is abundant in nature.
L-Glucose: L-Glucose is a sugar molecule that is less abundant in nature.
Rotation of Light
D-Glucose: D-Glucose can rotate plane polarized light in the clockwise direction.
L-Glucose: L-Glucose can rotate plane polarized light in anticlockwise direction.
Fischer Projection
D-Glucose: The Fischer projection of D-Glucose has a –OH group on the left side of the main carbon chain whereas other –OH groups are on the right side.
L-Glucose: The Fischer projection of L-Glucose has a –OH group on the right side of the main carbon chain whereas other –OH groups are on the left side.
Haworth Projection
D-Glucose: The Haworth projection of D-Glucose has a –OH group in the upward direction whereas other –OH groups are downward.
L-Glucose: The Haworth projection of L-Glucose has a –OH group in the downward direction whereas other –OH groups are upward.
Conclusion
Glucose can be found in nature as either D-Glucose or L-Glucose. The main difference between D and L Glucose is that D-Glucose rotates plane polarized light clockwise whereas L-Glucose rotates plane polarized light anticlockwise.
References:
1.“ D and L Configurations.”D and L Configurations. N.p., n.d. Web. Available here. 06 July 2017.
2.”Substance: Glucose.” Royal Society of Chemistry – Advancing excellence in the chemical sciences. N.p., n.d. Web. Available here. 06 July 2017.
Image Courtesy:
1. “DL-Glucose” By NEUROtiker – Own work, Public Domain) via Commons Wikimedia
2. “Haworth projection of α-D- and α-L-Glucopyranose” By Vaccinationist – Own work (Public Domain) via Commons Wikimedia
3. “Alpha-D-Glucose” By Yikrazuul – Own work (Public Domain) via Commons Wikimedia
4. “Beta-D-Glucose” By Yikrazuul – Own work (Public Domain) via Commons Wikimedia
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