The conversion of an optically pure isomer (enantiomer) into a mixture of equal amounts of both dextro and levo forms is called as:
1. Stereo Isomers or Geometric Isomers
a. The presence of asymmetric carbon atoms allows the formation of isomers.
b. The compounds which have the same structural formula but differ only in spatial configuration
c. are called Stereo Isomers.
d. Glucose with → 4 asymmetric carbon atoms has 2n (16) isomers, n is the no of asymmetric carbon atoms
2. D and L isomer:- When OH-gp around the carbon atom adjacent to the terminal primary alcohol carbon (eg. carbon atom 5 in glucose) is on the right, sugar is a member of the D-series; when it is on the left, it is a member of L-series. The majority of the mono-saccharides occurring in mammalian metabolism are of the D-configuration.
3. Optic isomer -
a. When a beam of polarized light is passed through a solution exhibiting optical activity, it will be rotated to the right or to the left in accordance with the type of compound i.e. the optical isomer, which is present.
b. If the rotation of polarized light to the right, is said be dextrorotation and a plus (+) sign is used to designate the fact; and it the rotation of the beam to the left, is said to be Levorotary action, and is designated by a minus (-) sign; optical activity also due to presence of asymmetrical carbon atoms
c. When equal amount of dextrorotatory and levorotatory isomers are present, the resulting mixture has no optical activity since the activities of each isomer cancel one another. Such mixture is said to be RACEMIC or a DL mixture
d. The separation of optically active isomers from a racemic mixture is called RESOLUTION-
e. Note that —. STEREOISOMERISM and OPTICAL ISOMERISM are independent properties.
4. Epimers - Isomers formed as a result of interchange of the — OH and —H on carbon atoms 2,3, and 4 of glucose are known as epimers. Biologically, the most important epimers of glucose are mannose and galactose formed by epimerization at carbon 2 and 4 respectively. ( enzyme — Epimerase).
5. α andβ-isomers →due to Mutarotation
a. Crystalline glucose is α-D-glucopyranose.
b. The cyclic structure is retained in solution but isomerism takes place about position 1 (Carbon 1), the carbonyl or anomeric carbon atom, to give a mixture of α-glucopyranose (38%) and β- glucopyranose (62%). This equilibrium is accompanied by optical rotation (MUTAROTATION) as the hemiacetal ring opens and reforms with change of position of the H and- OH groups on carbon 1.
For eg. →as soon as D-glucose is dissolved in water its specific rotation is +1110. Gradually, and quicker if NH3 is present, this value falls, finally remains constant at ±52.5° This phenomenon is said to be mutarotation.
Note: Sucrose (Cane sugar):-
a. It is hydrolyzed to glucose and fructose by the enzyme invertase (Sucrase) in the GIT.
b. Sucrose has no free aldehyde or keto groups because the linkage is between the Aldehyde gp of glucose and keto gp of fructose. Hence, it is a non-reducing sugar. It does not exhibit mutarotation and cannot exist in αor β forms.
c. The specific rotation of sucrose solution is +66.5° During hydrolysis this rotation change to — 19.50; since the Levo-rotation of fructose is greater than dextro-rotation of glucose. This product of the hydrolysis is used to be referred to as “ INVERT SUGAR”