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Components of Cells
The Macromolecules
Glucose: anomers

Rotating polarized light

Pure glucose, in the form of solid crystals, was isolated in 1895 - but there was a problem; there appeared to be two different forms of the molecule which crystallized from two different versions of the molecule. Both versions had all the same chemical properties, but when they were dissolved in water, one version twisted polarized light by 112 degrees, whereas the second version only twisted polarized light by 19 degrees.

If solutions of both forms were allowed to sit at room temperature for a few minutes and then re-tested, both had changed, and now both twisted polarized light by 52.7 degrees. This change in the ability of glucose solutions to rotate the orientation of polarized light is called mutarotation, and strongly indicated that the simple, linear formula for glucose needed to be modified.

There was, it appeared, to be at least two different structural forms of the glucose molecule that could not be explained by the simpler version of the molecular model.

What happened to the aldehyde?

It is possible to react all the alcohol (-OH) groups on the glucose molecule with reagents such as methyiodide (CH3I) and silver hydroxide (AgOH) to produce a highly modified form of glucose in which five of the carbon atoms were now linked to methylether groups (-O-CH3), instead of the simpler, alcohol groups (-OH).

When this reaction was carried out on both forms of the glucose molecule (see above), neither version of the molecule showed any of the expected properties of an aldehyde (-CHO), a functional group that should still have been at one end of the molecule if the simple formula was correct.

Where was the aldehyde functional group? What had happened to it?

If one of the methylether groups (-O-CH3) was gently removed from the pentamethylglucose molecule ("5-methyl" glucose), to form hexamethylglucose ("4-methyl" glucose), the properties of the aldehyde functional group returned. Could this mean that the aldehyde group in the basic glucose molecule had been somehow modified into an alcohol group? Very strange!

How could all this be explained? How could the 6-carbon-linear chain-6-hydroxyl version of the glucose molecule (called a pentahydroxylhexanal version!) be changed (without changing its chemical properties)?

Circular hemiacetals

A simple solution to these puzzling properties of the glucose molecule came with the realization that the linear model had an aldehyde group (-CHO) with its carbonyl (-CO) at one end, and further down the molecule there were alcohol (-OH) groups.

It was well known that carbonyl groups (-CO) could react with alcohol groups (-OH) to produce a new arrangement called a hemiacetal (OH-C-OR). Molecules such as 5-hydoxypentanal could do this, so why not glucose?

If the linear molecule is tipped on its side, and one of the bonds between carbon atom number 5 (C5) and carbon atom number 4 (C4) was rotated, suddenly the alcohol group on the C5 carbon atom comes very close to the aldehyde group on carbon number one (C1).

The two reactive groups interact with one another forming a six membered hemiacetal ring with all the new properties needed to explain the rotation of polarized light and the mysterious disappearance of the aldehyde group.

The glucose molecule, it appeared, should be drawn in the form of a ring, and the arrangement of the atoms attached to the first carbon atom (C1) could take two different configurations! This could explain the two different rotations of the polarized light, and why the two forms of glucose could gradually blend into a compromise mixture of the two that had a different rotation of the polarized light.

The two different versions of the glucose molecule, in which all the atoms are the same, but arranged differently in three dimensional space, are called anomers.


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© 2004, Professor John Blamire