mutarotation

Meaning: A cyclic sugar is a hemiacetal (or hemiketal) that can reopen to the free aldehyde or ketone and then close again. On reclosing, the hydroxyl can attack either face of the carbonyl carbon, so the anomeric carbon becomes either α or β. Acid or base speeds this up. A freshly dissolved pure anomer therefore converts to an equilibrium mixture, and the observed rotation changes with time. For D-glucose, α has [α] of about +112°, β about +19°, and the equilibrium mixture about +52.7°. The mixture is roughly 36% α and 64% β, with only a trace of open chain. β is favored because in β-D-glucopyranose every large substituent can sit equatorial.

Equation: [α] = αobs / (l × c), where [α] is specific rotation (degrees, usually reported with mL g⁻¹ dm⁻¹), αobs is observed rotation (degrees), l is path length (dm), and c is concentration (g/mL).

On the MCAT: A passage gives polarimeter readings over time for a freshly dissolved anomer. You predict that the rotation moves toward the same final value whichever anomer you start with. You may also be asked which sugars can do this, or why β predominates.

Common confusion: The two anomers are diastereomers that differ only at C1, not enantiomers, and no D to L change occurs. Only sugars with a free anomeric hemiacetal group mutarotate. Glycosides and sucrose are locked acetals, so they do not.