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Cyclic forms of monosaccharides

In solution, the small monosaccharides are found in a linear form, while the aldoses and ketoses of five or more carbon atoms are not linear molecules but are found forming cycles or rings.

The linear structure is called the Fischer projection, and the cycled structure, the Haworth projection. In Haworth's representation, the carbon chain is cycled on a plane. The radicals in the chain lie above or below that plane.

Cyclization occurs by the formation of an intramolecular bond in which the carbonyl group reacts with the hydroxyl group (OH) of the asymmetric carbon  furthest from the functional group.

  • If the carbonyl group was part of an aldehyde (aldose) the bond is of the hemiacetal type.
  • If the carbonyl group was part of a ketone ( ketose ) the bond will be of the hemiketal type.

When it is cyclized, the carbon that carried the functional group goes from not being asymmetric to being so, so two new optical isomers will appear. These new optical isomers are called anomers and are the α and β forms and the carbon that carried the functional group, 1 in aldoses and 2 in ketoses, is now called anomeric.

The five-carbon aldoses and the six-carbon ketoses form five-sided rings called furanose, and the six- sided aldoses pyranose. Thus we will have α-D-fructofuranose or β-L-galactopyranose.

Video: Cyclization of monosaccharides.

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