Mutarotation is the change in the '' optical rotation'' because of the change in the equilibrium between two anomers, when the corresponding stereocenters interconvert. Cyclic
sugar
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or do ...
s show mutarotation as α and β
anomeric
In carbohydrate chemistry, a pair of anomers () is a pair of near-identical stereoisomers that differ at only the anomeric carbon, the carbon that bears the aldehyde or ketone functional group in the sugar's open-chain form. However, in order fo ...
forms interconvert.
The optical rotation of the solution depends on the optical rotation of each anomer and their ratio in the solution.
Mutarotation was discovered by French chemist
Augustin-Pierre Dubrunfaut
Augustin-Pierre Dubrunfaut (; 1 September 1797, Lille – 7 October 1881) was a French chemist.
Mutarotation was discovered by Dubrunfaut in 1844, when he noticed that the specific rotation of aqueous sugar solution changes with time. In the same ...
in 1844, when he noticed that the specific rotation of aqueous sugar solution changes with time.
Measurement
The α and β anomers are diastereomers of each other and usually have different specific rotations. A solution or liquid sample of a pure α anomer will rotate plane polarised light by a different amount and/or in the opposite direction than the pure β anomer of that compound. The optical rotation of the solution depends on the optical rotation of each anomer and their ratio in the solution.
For example, if a solution of β-D-
glucopyranose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, usi ...
is dissolved in water, its specific optical rotation will be +18.7°. Over time, some of the β-D-glucopyranose will undergo mutarotation to become α-D-glucopyranose, which has an optical rotation of +112.2°. The rotation of the solution will increase from +18.7° to an equilibrium value of +52.7° as some of the β form is converted to the α form. The equilibrium mixture is about 64% of β-D-glucopyranose and about 36% of α-D-glucopyranose, though there are also traces of the other forms including furanoses and open chained form.
:
The observed rotation of the sample is the weighted sum of the optical rotation of each anomer weighted by the amount of that anomer present. Therefore, one can use a
polarimeter
A polarimeter is a scientific instrument used to measure the angle of rotation caused by passing polarized light through an optically active substance.enantiomeric excess, as long as one knows the rotation of each pure anomer. One can monitor the mutarotation process over time or determine the equilibrium mixture by observing the optical rotation and how it changes.
Carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may o ...