Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.
Isomers & Light Rotation
Have you ever hung out with someone enough that you started picking up some of their habits and they started picking up some of yours? Eventually, you both mutate into a combination of each other. Over time you both change, which will also change the nature of the relationship.
Molecules also slowly undergo changes. One of these common changes is when a molecule changes from one isomer into another isomer.
For example, the sugar molecule D-glucose has two common isomers in the cyclic form: alpha and beta. In water the molecule can open up into the straight chain, and when it reforms the cyclic form it can form either the alpha or the beta form, thus allowing D-glucose to interchange between alpha and beta forms.
Now, recall that each molecule (in fact each isomer of a molecule) will rotate plane polarized light to a specific degree, and this is called the specific rotation. This means that alpha-D-Glucose will rotate plane polarized light at a different rotation than beta-D-Glucose will rotate it. The specific rotation for pure alpha-D-Glucose is 112 ° while the specific rotation for the pure beta-D-Glucose is 18.7 ° .
Okay, now that we've reviewed a bit of background knowledge, let's learn about mutarotation. Mutarotation refers to the change in specific rotation over time due to a change between isomers. 'Muta' means 'change', so it literally means a change in rotation.
The specific rotation of a molecule never changes, but the specific rotation of the entire solution can change. This is because the molecule can change between isomers in some cases (such as with glucose).
Let's look at what happens when we put pure alpha-D-Glucose into water and measure the specific rotation over time. At first, it starts out at 112° just as we would expect, but it slowly starts to change until it reaches 52.5° .
Now, let's look at what happens when we put pure beta-D-Glucose into water and measure the specific rotation over time. Once again, it starts out where we would expect it at 18.7°, but it slowly changes until it also reaches 52.5°.
This change in rotation within the entire sample is mutarotation. So, what exactly is happening here?
Well, we know that in water, glucose will form the open chain and then reform into the cyclic, either as alpha or as beta. The specific rotation will change based on the concentrations of alpha and beta forms in the sample.
In fact, if we have 50% alpha and 50% beta in a sample, the specific rotation will simply be right between the two (65.4°). Since the specific rotation at equilibrium is less than 65.4, we know that there is more beta-D-glucose at equilibrium than alpha-D-glucose. This makes sense, since the beta form (with the OH in the equatorial position) is the more stable form.
The most common mutarotations are with sugar molecules like fructose, maltose, ribose, and galactose. Only reducing sugars can undergo mutarotations, however, so sucrose (which you might recall is table sugar) cannot undergo mutarotations. Let's look at another example using fructose.
Beta-D-fructose has a specific rotation of -133.5° and alpha-D-fructose has a specific rotation of -86°. In water, it eventually reaches an equilibrium of -92°. So, we know that we end up with more alpha-D-fructose in the final equilibrium.
Every molecule rotates plane polarized light to a specific degree. This is called specific rotation. Even different isomers of the same molecule will have a different specific rotation. Since the concentration of isomers in solution is changing, the specific rotation of the solution is changing. This change in specific rotation is called mutarotation. Some molecules, such as the sugar molecules glucose and fructose, will change isomers in solution.
When isomers are mixed together, the specific rotation of the entire mixture changes based on the concentrations of each isomer in the solution. If the concentration is 50/50, the specific rotation will slowly reach an equilibrium point - in this case, right between the two. However, if there is more of, say, isomer 'a' than 'b', then the final specific rotation will be closer to a's original specific rotation.
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