Mutarotation is the term given to the change in the specific rotation of a cyclic monosaccharide as it reaches an equilibrium between its α and β anomeric forms. It was discovered by Dubrunfaut in 1846, when he noticed that the specific rotation of aqueous sugar solution changes with time. [1] 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 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. Thus the rotation of the solution will increase from +18.7 to an equilibrium value of +52.5 as some of the β form is converted to the α form. The equilibrium mixture is actually about 64% of β-D-glucopyranose and about 36% of α-D-glucopyranose, though there are also with traces of the other forms including furanoses and open chained form.[2]
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Mechanism
Though the cyclic forms are usually heavily favoured, liquid monosaccharides (or monosaccharaides in aqueous solution) are always in equilibrium with their straight-chain forms. This equilibrium is established as the hemiacetal bond between C1 (the only carbon bound to two oxygens) and C5 is cleaved (forming the straight-chain compound) and reformed (forming the cyclic compound). When the hemiacetal bond is reformed, the OH group on C5 may attack either of the two stereochemically distinct sides of the aldehyde group that contains C1. Which side it actually does attack on decides whether the α or β anomer is formed.
If the reaction took place in amphoteric solution such as 2-pyridone, the rate of mutarotation would be much faster [3]
Observed effects
These 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 observed optical 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 to measure the rotation of a sample and then calculate the ratio of the two anomers present from the 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.
See also
References
- ^ Derek Horton (2008). "The Development of Carbohydrate Chemistry and Biology". Carbohydrate Chemistry, Biology and Medical Applications: 1-28. doi:.
- ^ Francis Carey (2000). Organic Chemistry (4th, McGraw-Hill Higher Education Press ed.).
- ^ Floyd H. Dean, Pyranose mutarotation. Journal of Colloid and Interface Science,1967,24(2) Pages 280-281 doi:10.1016/0021-9797(67)90235-4
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