3) Isomerismofmonosaccharide (stereoisomerism, cyclo-, oxo-tautomerism, conformation).

Dissolving monosaccharides in aqueous base causes them to undergo enolizations and a series of keto-enoltautomerizations that lead to isomerizations.

Carbohydrate conformation refers to the overall three-dimensional structure adopted by a carbohydrate (saccharide) molecule as a result of the through-bond and through-space physical forces it experiences arising from its molecular structure.

Different kinds of isomerism

Stereoisomers: These are compounds identical in composition and different in spatial configurations. By convention, one of these two forms is designated the D isomer, the other the L isomer. (The absolute configurations of sugars are known from x-ray crystallography.) The D and L designation is based on the configuration of single asymmetric carbon in Glyceraldehyde. When the hydroxyl group on the reference carbon is on the right in the projection formula, the sugar is the D isomer; when on the left, it is the L isomer.Those in which the configuration at this reference carbon is the same as that of D-glyceraldehyde are designated D isomers, and those with the same configuration as Lglyceraldehyde are L isomers.

In general, a molecule with n chiral centers can have 2ⁿ stereoisomers. Glyceraldehyde has 2¹⁼ 2; the aldohexoses, with four chiral centers, have 2⁴ = 16 stereoisomers.

The stereoisomers of monosaccharides other than glyceraldehydes is based on asymmetric or chiral carbon atom just adjacent to terminal primary alcohol group e.g. C5 in glucose and C4 in ribose. This carbon is the last but one carbon atom or

chiral center most distant from the carbonyl carbon as shown in the figure below

Of the 16 possible aldohexoses, eight are D forms and eight are L. Most of the hexoses of living organisms are D isomers.

Functional isomers

These are monosaccharides which have the same molecular formula but have different functional groups. For example: Glucose and Fructose both have the same molecular formula ( C₆H₁₂O₆ ) but have different structural formulae. Glucose has an aldehyde group and fructose has a ketose group.

DIASTEREOMERS :They are two non-identical monosaccharides which are of the same type (either both aldoses or both ketoses), have the same stereochemistry at their highest-numbered stereocenter ( this ensures that they are not mirror images of each other, and have the same number of carbons (e.g. if both are hexoses). They differ in the spatial orientation of hydroxyl groups. If they differ only in one position then they are called epimers. They are a kind of positional isomers. Example: glucose, galactose and mannose. Anomersdiscussedbelowarealso a kindofepimers.

Optical isomers:

These are isomers which are optically active and have non super imposable mirror images. When polarized light is passed through these isomers they either rotate the plane of polarized light to the right and are called dextrorotatory or to the left and are called levorotatory. The isomer which rotates the plane of light to the right is designated by small d and the one to the left is designated by small l. hence D- glucose can be dextrorotatory or levorotatory. When equal amounts of detro and levorotatory isomers are present then the resultant mixture has no optical activity, since the activity of each isomer cancel each other out. Such mixtures are called racemic mixture.

Tautomers are isomers (constitutional isomers) of organic compounds that readily interconvert by a chemical reaction called tautomerization. This reaction commonly results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond. The concept of tautomerizations is called tautomerism.