mutarotare

Mutarotation is a process observed in sugars where the optical rotation of a solution changes over time until it reaches a constant equilibrium value. This phenomenon is specifically related to the anomeric carbons of cyclic sugars, which exist in equilibrium between two different forms called anomers. These anomers, denoted as alpha (α) and beta (β), differ in the spatial orientation of the hydroxyl group attached to the anomeric carbon.

When a solid sugar, which is typically in a pure anomeric form (either α or β), is dissolved in a solvent, it can interconvert between its anomeric forms. This interconversion occurs through an open-chain aldehyde or ketone intermediate. The equilibrium mixture usually contains both the α and β anomers, along with a small amount of the open-chain form.

Because each anomer has a slightly different specific optical rotation, the overall optical rotation of the solution changes as the proportions of the anomers shift towards equilibrium. The rate of mutarotation depends on factors such as temperature, solvent, and pH. Once equilibrium is reached, the optical rotation of the solution remains constant. This process is a characteristic property of reducing sugars and is a key aspect of their chemical behavior.