Introduction

Occurrence and Structure of Monosaccharides and structures of disaccharides and polysaccharides
    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 LThe 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









Last modified: Tuesday, 27 March 2012, 5:46 PM