📄 Occurrence & Structure

This lesson explains how monosaccharides are classified, why cyclic structures form, and how key derived sugars and oxidation products are identified in biochemistry.

OCCURRENCE AND STRUCTURE OF MONOSACCHARIDES

The simplest monosaccharide that possesses a hydroxyl group and a carbonyl group with an asymmetric carbon atom is the aldotriose -glyceraldehyde. (A carbon is said to be asymmetric if four different groups or atoms are attached to it. The carbon is also called as a chiral center).

• Glyceraldehyde is considered as a reference compound and it exists in two optically active forms, D and L

The two families of monosaccharides, D-and L occur based on the configuration of D and L glyceraldehydes. In general, the D-family of sugars occur in nature.

• For monosaccharides with two or more asymmetric carbons, the prefixes D or L refer to the configuration of the penultimate carbon (i.e, the asymmetric carbon farthest from the carbonyl carbon).
• If the hydroxyl group on the penultimate carbon is on the right-hand side of the carbon chain when the aldehyde or ketone group is written at the top of the formula it belongs to the D family and if on the left hand side it belongs to L family. The D or L has nothing to do with optical activity. D sugars may be dextro- or levorotatory.
• The important monosaccharides containing aldehyde group belonging to the D family are
• the aldotetrose - D-erythrose
• the aldopentoses - D-ribose, D-arabinose and D-xylose
• the aldohexoses - D-glucose, D-mannose and D-galactose
• The important monosaccharide belonging to the L-family is L-arabinose.
• The important ketoses are
  • Ketotriose - dihydroxy acetone (It is optically inactive since there is no asymmetric carbon);
• the ketotetrose - D-erythrulose;
• the ketopentoses - D-ribulose and D-xylulose
• the ketohexose - D-fructose

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.