Configuration of Monosaccharides

Monosaccharides are further classified on the basis of three criteria :

1. On the basis of presence of an aldehydic group or a ketonic group monosaccharides are divided into Aldoses and ketoses repectively. 2. On the basis of the number of carbon atoms in their molecules aldoses and ketoses are further divided in to following sub-classes :

Number of carbon atoms in Aldoses Ketoses monosaccharides

3 Aldotriose ( eg.glyceraldehyde, - C3H6O3 )

4 Aldotetrose (eg.erythrose and Ketotetrose(eg.erythrulose,C4H8O2) threose ,C4H8O2) 5 Aldopentose (eg.arbinose Ketoentose ( xylulose, ribulose ,xylose,ribose, C5H10O5) etc. C5H10O5) 6 Aldohexoses( glucose Ketohexoses ( fructose, sorbose ,mannose, galactose,C6H12O6) etc. C6H12O6)

3. D and L designation of monosaccharides: ie D and L terminology

D and L nomenclature of carbohydrates is a relative method of configuring optically active compounds.It is the stereochemical configuration of the chiral carbon atom farthest from the carbonyl group.Here relative term is used as this method of configuration is based on glyceraldehyde molecule, the simplest and first member of monosaccharides containing a stereocentre.

Glyceraldehyde HOCH2-CHOH-CHO , exist in two enantiomeric forms ie. D-form and L-form.

The above figures show the spatial relationship of the groups attached to asymmetric carbon atom in each case. The two forms are mirror images that cannot be superimposed, that is, they are enantiomers. These two compounds serve as configurational standards for all monosaccharides.

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Ball and Stick model of Glyceraldehyde

To make it simple the three dimensional structure is written as two dimensional projection to represent an optically active compound such as D- and L-glyceraldehyde. A simple way to represent it is in the form of Fischer Projection. In this type of representation, glyceraldehyde has the aldehyde group at the top . The horizontal bonds in the Fischer projection are defined as coming out towards the viewer. The vertical bonds go away from the viewer.

OR

Relative Configuration: In D-glyceraldehyde, the prefix letter D means that the hydroxyl group on the carbon number 2 is projected to the right in the Fisher projection and in L-glyceraldehyde the prefix letter L means that the hydroxyl group is projected to the left. D and L-glyceraldehyde may be represented as shown above.

Glyceraldehydes, both D- and L- are considered the reference point for designating and drawing all other monosaccharides , considered to be derived from them.In carbohydrate chemistry, in Fisher projection formulas the aldehyde and ketone groups always written at the top of the structure. According to the definition if the hydroxyl group on the asymmetric carbon atom, farthest from aldehyde or ketone group projects to the right the compound belongs to the D-family and if on the left side it is a member of the L-family.

Optical activity in monosaccharides: The maximum number of optical isomers of a sugar is related to the number of asymmetric carbon atoms in the molecule and is calculated by the formula given below

Maximum Number of Optical Isomers = 2n, where n is the number of asymmetric carbon atoms.

In glyceraldehydes there is only one asymmetric carbon atom so the number of optical isomers is 21.

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Erythro and Threo Diastereomers: Erythro and Threo system of nomenclature is used only in aldotetroses having two chirality or asymmetric centre giving four stereoisomers ( 22=4).Among these four stereoisomers two are D-sugars and two are L-sugars. When Fisher projections are drawn for stereoisomers having two adjacent chirality centres, the pair of enantiomers with similar groups on the same side of the carbon chain is the erythro enantiomers and that on opposite sides are called threo enantiomers. The names of the erythro and threo pairs of enantiomers, infact, originated from the name of the aldotetroses, erythrose and threose.

Erythrose and threose are diastereomers.

Epimers: Epimers are a specific type of stereoisomer that have multiple stereocenters, but only differ from one another by the configuration at one of the stereogenic centers. A stereocenter or stereogenic center is a carbon atom that has four different atoms or groups of atoms bonded to it. Thus epimers are always diastereomers.

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REFERENCES

1. Organic Chemistry Vol III by Dr. Jagdamba Singh and Dr. L.D.S. Yadav 2. Organic Chemistry-III by Alok Bariyar and Sudha Goyal 3. Organic Chemistry Vol II by I.L. Finar 4. A textbook of Organic Chemistry by Arun Bahl and B.S. Bahl

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