Question

Does lactose mutarotate? Is it a reducing sugar? Explain. Draw the two anomeric forms of lactose.

Short answer

Lactose can mutarotate since it has free anomeric end which is shown by the figure below.

Free anomeric end which is able to mutarotate.

The two anomeric forms of lactose (α and β) are shown below.

α anomer

β anomer

Step-by-step solution

A concept:

Lactose can be obtained commercially from cow’s milk after the removal of emulsified fat and casein. Hydrolysis of lactose with dilute acids forms an equimolar mixture of D-glucose and D-galactose which means that the lactose molecule is made up of one unit each of D-glucose and D-galactose.

Lactose mutarotate or not:

Lactose is a reducing sugar since it has only one of its two anomeric carbon atoms involved in the glycosidic bond. This means that lactose can be converted to an open chain form with an aldehyde (-CHO) group.

Lactose can mutarotate since it has free anomeric end which is shown by the figure below.

Free anomeric end which is able to mutarotate.

The two anomeric forms of lactose (α and β) are shown below.

α anomer

β anomer