Elementary plant Biochemistry and Biotechnology (2+1)

Cyclic structure of Monosaccharides

The monosaccharides exist either in cyclic or acyclic form. There are many evidences to show that the pentose and hexose monosaccharides are present in cyclic form. The evidences are

1. Glucose and other aldoses fail to give the Schiff 's test for aldehydes.
2. Solid glucose is quite inert to oxygen whereas aldehydes are easily auto-oxidizable.
3. Glucose and other aldoses do not form bisulfite or aldehyde ammonia compound.
4. Glucose pentaacetate does not react with hydroxylamine.
5. Presence of two forms of glucose with different physical and chemical properties.
6. X-ray analysis definitely proves the existence of the ring structure and also the size of the ring.
7. Mutarotation.

When an aldehyde or a ketone group is present in a molecule that also possesses hydroxyl groups, an intramolecular arrangement may occur to form a hemiacetal or a hemiketal, respectively.This intramolecular hemiacetal or hemiketal is the basis for the cyclic structure of the sugars. Hence, Haworth (an English chemist) proposed a cyclic hemiacetal structure that accounts completely for its chemical properties

Two types of ring structures are possible, the five-membered furanose and the six-membered pyranose ring if the carbonyl group interact with hydroxyl group.These names are derived from the parent compounds 'furan' and 'pyran'.

The most common ring structure for aldohexoses is the pyranose ring structure that involves the first carbonyl carbon and the hydroxyl group attached to the fifth carbon.The furanose ring structure is formed by interaction of carbonyl carbon with the hydroxyl group attached to the fourth carbon.

This furanose form is less stable than the pyranose strucure and is not very common among aldohexoses.Very seldom is a seven-membered ring formed.Fructose exists in solution and in compounds as a furanose; however, in the crystalline state only the pyranose ring is believed to exist.

Ribose occurs as the furanose structure in many important biological compounds.A new asymmetric carbon is introduced in the molecule due to this rearrangement. As a result of this new asymmetric centre, two isomers are formed. Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom are called anomers and the carbon is referred as anomeric carbon.

When the newly formed hydroxyl group in C1and the ring are on the same orientation, it is a α - anomer.When the newly formed hydroxyl group in C1 and the ring are on opposite orientation, it is ß - anomer.While writing the cyclic form (Haworth) of monosaccharides it is sometimes difficult to judge whether an OH group should be above or below the plane of the ring.

A few rules can be followed for writing Haworth's structure for carbohydrates.Write the oxygen at the upper right hand corner of the ring structure (pyranose) and the carbons clockwise around the ring. At the fifth carbon it is necessary to rotate the bond to 90o to make the ring closure. For the D-family of sugars, it is customary to write the terminal CH2OH above the plane of the ring.

If the hydroxyl group or hydrogen atom occurs on the right-hand side of the carbon chain in the linear structure it is placed below the plane of the ring in the cyclic structure.Conversely, if the hydroxyl group or hydrogen atom is on the left-hand side of the carbon chain, it is placed above the plane of the ring in the structure formula.