Question

What are the structural differences between α and β glucose? Each of these two cyclic forms of glucose is the building block to form two different polymers. Explain

Short answer

The difference of α and β glucose is in the angle of attachment of the hydroxyl group. Cellulose - β glucose forms a linear chain; starch - α glucose forms a helical structure.

Step-by-step solution

structural differences between α and β glucose

When the pyranose or furanose ring is closed, the planar carbonyl group is converted to an asymmetric carbon in the hemiacetal. Depending on which side of the (protonated) carbonyl group is attacked, the hemiacetal group can be directed either up or down. These two orientations of the hemiacetal group give diastereomeric products called anomers.

A way to remember anomers is to note that the anomeric hydroxyl group is trans to the terminal${\text{- CH}}_{\text{2}}\text{OH}$ group in the anomer, but is cis in the β anomer. This rule applies to all D and L series sugars as well as furanoses.

To Explain the two cyclic forms of glucose

Cellulose is composed of D-glucose units linked by $\beta -1,4$glycosidic bonds. This bonding arrangement is rather rigid and very stable, giving cellulose desirable properties for structural material. Because the monomer unit of the cellulose is $\beta$ glucose, the cellulose is mostly a linear chain.

On the other hand, starch is also a polymer of glucose but both amylose and amylopectin have$\alpha -1,4$ glycosidic bonds. As a result of the bond angles in the alpha acetal linkage, starch forms a helical structure instead of a linear chain.

Hence The difference of α and β glucose is in the angle of attachment of the hydroxyl group. Cellulose - β glucose - a linear chain; starch - α glucose - a helical structure.