Editing Mutarotation (section)

== Measurement ==
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The α and β anomers are [[diastereomer]]s of each other and usually have different [[specific rotation]]s. A solution or liquid sample of a pure α anomer will rotate [[Polarization (waves)|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.<ref>{{Cite book |last=Schmiermund |first=Torsten |url=https://books.google.com/books?id=PjYyEAAAQBAJ&newbks=1&newbks_redir=0&lpg=PP1&dq=stereoisomers%20and%20optical%20rotation&pg=PP1#v=onepage&q=stereoisomers%20and%20optical%20rotation&f=false |title=Introduction to Stereochemistry: For Students and Trainees |date=2021-06-07 |publisher=Springer Nature |isbn=978-3-658-32035-5 |language=en}}</ref>

For example, if a solution of β-<small>D</small>-[[glucopyranose]] is dissolved in water, its specific optical rotation will be +18.7°. Over time, some of the β-<small>D</small>-glucopyranose will undergo mutarotation to become α-<small>D</small>-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-<small>D</small>-glucopyranose and about 36% of α-<small>D</small>-glucopyranose, though there are also traces of the other forms including furanoses and open chained form.

:[[Image:Α- and β-D-glucopyranose.svg|500px|left|class=skin-invert-image]]{{clear-left}}

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]] 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.