Anomers

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Anomers are a kind of diastereoisomers[1]. As the name suggests, they are isomers of each other.

There are 2 forms of anomers, namely alpha and beta. They are identified by the direction that the -OH group is pointing to on the first carbon (C1) on the cyclic sugar. Thus, the carbon is called an anomeric carbon[2]. An alpha-glucose has its -OH perpendicular to the ring. A beta-glucose has its -OH parallel to the ring. It is possible to distinguish the two by the directions that the molecule rotates under plane-polarise light[3].

Alpha and beta anomers are formed from an open-chain sugar by mutarotation[4].In the process of mutarotation, a solution of both anomers are heated at a high temperature, giving rise to individual monomers. The solution is then added into water. Through contact with water, the sugar monomers appear to be opening up their ring structure to form straight chain sugars. In addition, these straight chain sugars are converted back into its ring structure. The reaction is said to be at a dynamic equilibrium, where the rate of forward reaction is equal to the rate of backward reaction[5]. The percentages of each type of anomers present are as follow: 25% are alpha anomers and 75% are beta anomers[6]. Anomers in the form of Fischer projection (open-chain sugar) makes up less than 1% of the total solution[7].

Under a dynamic equilibrium, 6-carbon pyranose rings are usually formed as compared to 5-carbon furanose rings[8].

Condensation reaction catalyses the formation of disaccharides between 2 monosaccharides with the elimination of a water molecule.

Determining which anomer is alpha and which is beta is easy when looking at the Haworth projection; alpha anomers have different stereochemistry at carbon 1 and carbon 5, whereas beta anomers have the same stereochemistry at carbon 1 and 5.

References

  1. Berg J., Tymoczko J. and Stryer L. (2012) Biochemistry, 7th Edition, New York: Kate Ahr Parker. pg331
  2. Berg J., Tymoczko J. and Stryer L. (2012) Biochemistry, 7th Edition, New York: Kate Ahr Parker. pg333
  3. Cornell College (2012) Reactions of monosaccharides. Available at: http://www.cornellcollege.edu/chemistry/courses/cardon/organic/326chorxns.html (Last accessed 30.11.1012)
  4. Professional Education,Testing and Certification Organization International (2012) Organic Chemistry 16.4 Cyclic Structures of Monosaccharides. Available at: http://www.peoi.org/Courses/Coursesen/chemorg/ch/ch16d.html (Last accessed 29.11.2012)
  5. Professional Education,Testing and Certification Organization International (2012) Organic Chemistry 16.4 Cyclic Structures of Monosaccharides. Available at: http://www.peoi.org/Courses/Coursesen/chemorg/ch/ch16d.html (Last accessed 29.11.2012)
  6. Berg J., Tymoczko J. and Stryer L. (2012) Biochemistry, 7th Edition, New York: Kate Ahr Parker. pg333
  7. Russel KC (1999) Haworth Projections. Available at: http://www.nku.edu/~russellk/tutorial/sugar/haworth.html (Last accessed 30.11.2012)
  8. Solano Community College (2007) Drawing Haworth Projections. Available at: http://mathsci.solano.edu/users/cspillne/Chem%2011%20Handouts%20and%20Worksheets/Unit%203/Drawing%20Haworth%20Projections.doc (Last accessed 29.11.2012)

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