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Representation of ring structure of monosaccharides

The ring structures of monosaccharides can be represented as follows :

1) Fischer projection

The acyclic structure of a sugar is commonly shown by using a Fischer projection. A Fischer projection is sometimes used to illustrate the cyclic hemiacetal form of a sugar. The presence of an aldehyde group and the hydroxyl groups in an aldose make it possible for these compounds to undergo intramolecular reactions to form cyclic hemiacetal. These cyclic hemiacetals, are often more stable than is the open-chain form of the sugar. The ketoses (keto group containing sugars) form hemiketal rings

 Hemiacetal

Figure-1- In Fischer projection, α anomer has the orientation of OH towards the right side, whereas, in beta anomer, it is towards the left side.

2) Haworth projection

Rules for drawing Haworth projections

Either a six or 5-membered ring is drawn including oxygen as one atom.
Most aldohexoses are six-membered resembling Pyran- an organic compound having a 6 membered ring (Figure-2).
Aldotetroses, aldopentoses, ketohexoses are 5-membered resembling Furan- A five membered organic compound (Figure-2)

 Furan and Pyran

Figure-2- Structure of Furan and Pyran.

Numbering the rings

The numbering is done clockwise starting next to the oxygen (Figure-3)

 Numbering the rings

Figure-3- Numbering the rings

Pyranose and Furanose forms of Glucose (Figure-4)

 Pyranose and furanose forms

Figure-4- Glucopyranose and Glucofuranose forms

In Haworth configuration all groups to the right of carbon backbone in Fischer projection are oriented down (down -right) while all groups to the left of carbon backbone are oriented up, except those around C5,the reverse orientation occurs (Figure-5).

 Alpha and beta anomers (Figure-5)

 Anomers

Figure-5- When drawn in the Haworth projection, the α configuration places the hydroxyl downward, while the β is the reverse.

Mutarotation

Carbohydrates can change spontaneously between α and β configurations through intermediate open chain formation, this leads to a process known as Mutarotation. There is gradual change in optical rotation of the solution. The initial optical rotation is changed to a constant optical rotation characteristic of that sugar.

This can be explained in reference to two experiments

1) When D Glucose is crystallized at room temperature and a fresh solution is prepared, its specific rotation of polarized light is +112ο, but after 12-18 hours it changes to +52.5 ο

2) If the initial crystallization takes place at 98 ο and then solubilized, the specific rotation is found to be +19 ο, which also changes to +52.5 ο within a few hours.

This change in rotation with time is called Mutarotation.

Explanation

At room temperature the alpha form predominates and the specific rotation is+112ο, there is transient ring opening and change in configuration. In the second condition when the crystallization takes place at 98 ο, the Beta form predominates and the specific rotation is+19 ο. Both undergo Mutarotation and at equilibrium one-third molecules are α type and two third are β variety to get the specific rotation of+52.5 ο (Figure-6) . 

 Mutarotation

 Figure-6- Mutarotation of Glucose

 The constant specific rotation is due to the resultant net optical activity of both alpha and beta anomers.

 

 

 

 

 

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