Glucose, also known as dextrose is the most common simple sugar (monosaccharide). It has two isomeric structures, i.e. dextrose-(D) glucose and Lactose-(L) glucose. The dextrose-(D) glucose contains two further isomers, i.e. alpha (α) and beta (β) glucose. The key difference between alpha (α) and beta (β) glucose is the orientation of hydroxyl (-OH) group attached to the first carbon atom.
The two dimensional graphical representation of these isomeric glucose structures shows that the α-glucose has (1-hydroxyl) and (4-hydroxyl) orientations on the same side. On the other hand, β-glucose has the orientations of (1-hydroxyl) and (4-hydroxyl) on opposite sides. By relative stability, alpha and beta glucoses exist naturally as 36 : 64 respectively.
The following figure clearly depicts the geometrical orientation of hydroxyl groups on alpha (α) glucose and beta (β) glucose structures.
Besides this main difference i.e. different orientation of hydroxyl group at first carbon atom, the rest of entire structures of alpha (α) glucose and beta (β) glucose are the same.
Difference between alpha(α) and beta(β) glucose
|Alpha Glucose||Beta Glucose|
|Alpha(α) glucose has hydroxyl groups of 1 and 4 positions on same sides||Beta(β) glucose has hydroxyl groups of 1 and 4 positions on opposite sides|
|Alpha(α) glucose is less stable due to the steric hinderance of OH groups, being on same sides||Beta(β) glucose is more stable due to opposite sides of OH groups|
|Alpha glucose is higher in energy than beta glucose||Beta glucose is lesser energetic|
|It has low melting point i.e. 146 °C||It has higher melting point i.e. 150 °C|
|It has a specific rotation of 112.2 degrees||It has a specific rotation of 18.7 degrees|
|Alpha glucose structure is easily broken down by enzymes due to its high reactivity||Beta glucose structure is resistant against enzyme action due to its low reactivity|
|The glycosidic bond between two alpha glucose structures results in the formation of maltose||The glycosidic bond between two beta glucose structures results in the formation of cellobiose|
|It can only be crystallized in the form of α-glucopyranose||It can be crystallized in the form of β-glucopyranose or β-glucopyranose hydrate|
|Polymerization of alpha(α) glucose yields starch||Polymerization of beta(β) glucose yields cellulose|
|Food sources of alpha(α) glucose include whole grains, potatoes, beans and corn, and sucrose, etc||Food sources of beta(β) glucose include fibrous foods like legumes, nuts, yeast, algae, and mushrooms, etc|
Similarities among alpha (α) and beta (β) glucose
- Both alpha(α) and beta(β) glucose structures are simple sugar monomers.
- They can be crystallized from their aqueous solutions, although, alpha(α) glucose cannot be crystallized as α-glucopyranose hydrate.
- They both have the same number of chiral carbon centers i.e. 4.
- Both alpha(α) and beta(β) glucose are optically active organic compounds.
Alpha(α)-D glucose is an isomer of dextrose(D) glucose with the identification of having hydroxyl (-OH) groups on 1 and 4 positions on the same sides of the plane. It can also be referred to as an orientation in which the hydroxyl group of 1 position is on the opposite side to that of (-CH2OH).
This isomer of glucose is less stable than the beta(β) glucose because of the steric hindrance of hydroxyl groups, being on the same sides. This makes the natural abundance of alpha glucose, 36%. The position of the hydroxyl group has a great influence on the properties of compounds for which, alpha glucose is generally more reactive than beta glucose. It has a melting point of 146 °C and a specific rotation of 112.2°. Moreover, it can be crystallized from its aqueous solution in the form of α-glucopyranose.
The chair conformation of alpha dextrose glucose [α-D glucose] shows the same sides of hydroxyl groups of 1 and 4 positions, i.e. lower to the plane in the above diagram.
Examples of Alpha(α) Glucose yields are:
- Maltose is made by the joining of two α-glucose molecules.
- Sucrose is made from one α-glucose and one fructose molecule.
- Lactose is made from one α-glucose and one galactose molecule.
Beta(β) glucose is also an isomer of dextrose(D) glucose. It has the identification of having hydroxyl (-OH) groups of 1 and 4 positions on opposite sides. The position of the hydroxyl group of 1 position is on the same side as (-CH2OH).
The beta glucose isomer is more stable than alpha glucose due to the reduced steric hindrance as the bulky groups are away from each other. This makes beta its natural abundance 64%. Beta(β) glucose has a melting point of 150 °C and a specific rotation of 18.7°. Beta glucose can be crystallized from the aqueous solutions as β-glucopyranose and β-glucopyranose hydrate.
The chair conformation of beta dextrose glucose [β-D glucose] shows the opposite sides of 1 and 4 positioned hydroxyl groups.
Beta(β) Glucose is the monomer unit in cellulose fibers. These fibers are linear due to the beta acetal linkages in beta glucose. Acetal is another name for these glucose monomers under the condition that they are duly protonated. The D-glucose monomers can cyclize to form hemiacetal structures. Under acidic conditions, these hemiacetals can react with further alcoholic groups to form acetals.
To read more, visit Acetal and Hemiacetal.
Key Differences between α and β Glucose
Alpha and Beta glucose are both aldose structures, and neither of them is from the ketose family. Aldose and Ketose terms are really important in knowing about sugar molecules especially, glucose and fructose.
Chiral centers in Alpha and Beta Glucose
Alpha and beta glucose, both contain similar chiral centers, although cyclic and open chain forms have different numbers of chiral centers. Cyclic glucose (α and β) structures have 5 chiral centers while open chains have 4.
Read more at, Chiral vs Achiral vs Meso compounds
What is beta glucose?
Beta glucose is an isomer of dextrose(D) glucose having a hydroxyl group on 1 position, on the opposite side, as the hydroxyl group of position 4. An example of a polymer having beta glucose monomers is cellulose. It has a high melting point and more stability.
What is alpha glucose?
It is an isomer of D-glucose having a hydroxyl group located downward. It has a low melting point and high stability.
Alpha glucose is an isomer of dextrose(D) glucose having a hydroxyl group on 1 position, on the same side, as the hydroxyl group of position 4. An example of a polymer having alpha glucose monomers is maltose. It has a lower melting point and less stability.
What are the functions of alpha and beta glucose?
Alpha and beta glucose are very important for living organisms. Alpha glucose is the building block of starch while beta glucose is the building block of cellulose. Amylopectin, lactose, maltose, and galactose, etc also contain alpha and beta glucose as their building blocks.
How to remember alpha vs beta glucose efficiently?
The difference between alpha and beta structures of glucose is well remembered by the difference of orientation of the hydroxyl group. Alpha is a same-sided 1-4 hydroxyl groups, while beta refers to opposite-sided 1-4 hydroxyl groups.
Are alpha-D-glucose and beta-D-glucose enantiomers?
Enantiomers are the non-superimposable mirror images of the isomeric structures of same molecule. As alpha and beta glucose do not meet the criterion, they are not enantiomers.
What is the difference between cellulose and cellulase?
Cellulose is an example of a polymer formed by beta-glucose monomers, which is a carbohydrate. Cellulase is an enzyme used to break down cellulose.