Mutarotation

1-Mutarotation Is The Change In Optical Rotation Observed When Pure α- or β- Anomers Are Dissolved In Water (or other solvents)

In our recent post on ring-chain tautomerism, we said that there are two isomers of D-glucose in its 6-membered ring (“pyranose”) form.

These two diastereomers – which, to make matters more confusing, are called “anomers” in the context of sugar chemistry –  differ in the orientation of the hydroxyl group on C-1. (Note that C-1 is a hemiacetal. )

• ➡️In the “alpha” (α) anomer, the OH group on C-1 is on the opposite side of the ring as the chain on C-5.
• ➡️In the “beta” (β) anomer, the OH group on C-1 is on the same side of the ring as the C-5 substituent.

• Each of these two forms can be synthesized and isolated as pure compounds.

  
  • ●The alpha (α)  anomer of D-glucose has a specific rotation of +112 degrees in water.
  • ●The beta (β)  anomer of D-glucose has a specific rotation of +19 degrees. (18.7 actually, but rounding up to 19).

  Here’s the interesting thing. When either anomer is dissolved in water, the value of the specific rotation changes over time, eventually reaching the same value of +52.5°. 

• • ●The specific rotation of α-D-glucopyranose decreases from +112° to +52.5°.
  • ●The specific rotation of β-D-glucopyranose increases from +19° to +52.5°.

  This behaviour is called mutarotation (literally, “change in rotation”).

• Hold on.  Isn’t specific rotation of a molecule supposed to remain the same?

  Yes – if it is indeed the same molecule! 

  And therein lies the answer to the puzzle. For when the solutions whose specific rotations have changed to +52.5° are analyzed, they are found to no longer consist of 100% alpha (α) or 100% beta (β) anomers, but instead a ratio of alpha (α) (36%) and beta (β) (64% ) isomers.