mutarotace

Mutarotace (mutarotation) is a phenomenon observed in chiral compounds, most notably in sugars, where the specific optical rotation changes over time as the compound reaches equilibrium between its anomeric forms. The process occurs because the compound exists as two or more structural isomers that rotate plane‑polarised light differently. As the equilibrium between these isomers shifts, the measured optical rotation changes until a constant value is achieved.

In sugars, mutarotation arises from the interconversion of α‑ and β‑anomers about the anomeric carbon. The reaction is catalysed by acid or base and involves the temporary opening of the ring to a linear aldehyde or ketone form. During this transient state, the configuration at the anomeric centre can be lost, allowing the molecule to form the other anomer when the ring closes. Common examples are D‑glucose, D‑fructose, and other hexoses; for D‑glucose, the specific rotation slowly changes from about +112° (α‑anomer) towards +52° (β‑anomer) until the equilibrium mixture of approximately 36 % α and 64 % β is reached.

The change in optical rotation is monitored using polarimetry. The rate of mutarotation depends on concentration, temperature and the presence of catalysts, and it is often described by a first‑order kinetic equation. Because the equilibrium ratio of anomers is a function of temperature, mutarotation can be used to determine thermodynamic parameters such as enthalpy and entropy changes of the anomerisation process.

Mutarotation is important in carbohydrate chemistry, biochemistry and food science. It affects the physical properties of sugar solutions, influences the mechanisms of enzyme catalysis that involve sugars, and provides a simple experimental method for studying stereochemical interconversion. Although mutarotation itself is not a reaction that alters the overall chemical formula of the molecule, the underlying isomerisation is fundamental to the behaviour of many biologically relevant carbohydrates.