Monosaccharides (Structural Aspects)- part 1

Structural Aspects of Carbohydrates

Monosaccharides are carbohydrates which consist of a single polyhydroxy aldehyde or ketone unit and are also called as simple sugars. These are classes of sugars that can’t be hydrolyzed to simpler sugars. However, they can polymerize to form oligosaccharides and polysaccharides. They have various structural aspects which include:


Stereoisomers are compounds with same structural formulae but differ in their spatial configuration. It is an important character of monosaccharides. A carbon is said to be asymmetric when it is attached to four different atoms or groups. The number  of  asymmetric  carbon  atoms  (n) determines  the  possible  isomers  of  a  given compound which  is  equal  to  2n.

E.g. Glucose contains 4 asymmetric carbons, and thus has 16 isomers.


Optical activity

This is a characteristic feature of compounds with asymmetric carbon atom. When a beam of polarized light is passed through a solution of an optical isomer, it will be rotated either to right or left. The terms dextrorotatory (+) and levorotatory (-) are used to compounds that respectively rotate plane of polarized light to right or to left.  An optical isomer may be designated as D(+), D(-), L(+) and L(-) based on its structural relation with glyceraldehyde.



Mutarotation is defined as change in specific optical rotation representing interconversion of anomeric forms. E.g. α and β anomers of D-glucose interconvert in aqueous solution by a process called mutarotation, in which one ring form (say, α anomer) opens briefly into linear form, then closes again to produce β anomer to form equilibrium mixture. This mixture consists of about one-third α -D-glucose, two-thirds β -D-glucose, and very small amounts of the linear and five-membered ring (glucofuranose) forms.




These anomers of glucose have different optical rotations. The specific optical rotation of a freshly prepared glucose (α anomer) solution in water is +112.2° which gradually changes and attains equilibrium with a constant value of +52.7°. ln presence of alkali, decrease in optical rotation is rapid. The optical rotation of β -glucose is +18.7°.



The process of shifting a hydrogen atom from one carbon atom to another to produce enediols is known as tautomerization. Sugars possessing anomeric carbon atom undergo tautomerization in alkaline solutions.




When glucose is kept in alkaline solution for several hours, it undergoes isomerization to form D-fructose and D-mannose. This reaction is known as Lobry de Bruyn-von Ekenstein transformation which results in formation of a common intermediate namely enediol. The enediols are highly reactive, hence sugars in alkaline solution are powerful reducing agents.


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