How can you use polarization to check whether or not a sugar is reducing?

Answer

Dear Lisa,

A reducing sugar shows mutarotation precisely because it is in equilibrium with the open chain form! After all, in the open-chain form there is an aldehyde group in the structure of the sugar. This aldehyde group can itself be oxidized to a carboxylic acid and will therefore be able to reduce another compound in a redox reaction.

Now to explain this further and try to make as clear a distinction as possible, let’s separate the different elements.

To be reducing refers to “being able to participate in a redox reaction or equilibrium” thereby acting “as a reductant” and this in turn implies that the compound itself is being oxidized. As mentioned, this has to do with the presence of an aldehyde function that, when oxidized, is converted into a carboxylic acid function:

-HC=O + H₂O -> -CO₂H + 2e^– +2H⁺

What about the open chain form? Simple sugars are actually polyols with an aldehyde or a ketone function. Let us focus on those sugars that contain an aldehyde function. For example, they have several alcohol functions such as: CH₂OH-CHOH-CHOH-CHOH-CHOH-HC=O. Now a molecule that has both alcohol functions and in this case an aldehyde function can react internally whereby one alcohol function reacts with the aldehyde function to form a so-called hemiacetal functional group. The latter group is a functional group comprising both an alcohol and an ether function. The reaction is shown in appendix under 1. Now such a reaction in a sugar results in a ring closure. secondly, for many sugars this ring structure is more stable than the open chain form and is therefore more common. That is why sugars in a book are more likely to be depicted in their ring shape and to a lesser extent as a linear structure. On the other hand, in water this is a pure equilibrium reaction and the ring structures and open chain structures will coexist. consequently in solution an aldehyde function is available and when converted to a carboxylic acid function the equilibrium will shift from ring structure to open chain structure. See appendix under 2.

What is Mutation Rotation? This is a phenomenon/reaction that is only observed in a device called a polarimeter (http://nl.wikipedia.org/wiki/Polarimeter). NOW with this a change of the plane of polarization of polarized light can be measured.

What is that polarized light? In principle, light vibrates in all possible planes, by using a polarization filter (http://nl.wikipedia.org/wiki/Polarisatiefilter) you can obtain polarized light that only vibrates in one plane. If you then place another polarizer filter 10 cm away, the light that passed through the first will only pass through the second if they are both positioned exactly the same way. Such a device then makes it possible to check whether a solution placed between the two polarization filters and containing a specific substance can change this plane of polarization of the light.

Are there such substances? Yes, substances that are asymmetrical in their construction will appear in two forms “the image” and “the mirror image” and these are two different objects in the same way that “a left-turning spiral staircase” is different from “a right-turning spiral staircase”. They too relate as “image” and “mirror image” and are indeed different.

Now these kinds of asymmetric substances interact with light in such a way that the plane of polarization of polarized light is twisted and the two antipodes do so in opposite directions. If the “image” rotates the plane of polarization to the right, “the mirror image molecule” will rotate the light to the left and by the same amount. Now that angle of rotation depends on the structure of the asymmetric molecule as a whole. So the degree that the plane of polarization rotates is typical for a particular structure.

Now suppose that one asymmetric molecule undergoes a reaction in which it is converted into another asymmetric molecule and they both have different angles of rotation and in the ideal case even in different directions, then one could follow such a reaction in a polarimeter just by walking in the loop of the time regularly measure the polarization. It will then be seen that at the beginning of the reaction the angle of rotation of the initial product is measured and at the end the angle of rotation of the final product is observed.

Now what is mutarotation? Now this is a reaction where sugars, which are isomers of each other, convert into each other in solution via the open chain form. Since sugars are asymmetric molecules in all their isomers, they each have a typical angle of rotation and this conversion from a pure one sugar to a mixture of isomeric sugars can be followed in a polarimeter.

I hope Liza this brings some clarification. These are different concepts/phenomena that occur for sugars, but that have to do with different aspects of their behavior.

Regards