- Isomerism in Carbohydrates– Different compounds or structures having same molecular formula is called isomers and the phenomenon existing is called isomerism.
- Carbon to which four different substituent groups are attached called as Chiral or asymmetric carbon atom.
- Lebervon’t Hoff Rule– Relation between asymmetric carbon atom and number of stereoisomers possible. (No. Of isomers= 2n)
- Types of Isomers In Carbohydrates- 1) Stereoisomerism 2) Optical isomerism
1) Stereoisomers- compounds which have same structural formula but differ in spatial configuration of H and OH group around asymmetric carbon atoms.
- Enantiomers (D and L isomerism) – *D sugars are naturally occurring. Examples are
- D Glucose and L Glucose
- D Fructose and L Fructose
- D Mannose and L Mannose
- D Glyceraldehyde and L Glyceraldehyde.
- Optical Activity- The presence of asymmetric carbon atom causes optical activity.
- When a beam of plane-polarized light is passed through a solution of carbohydrates, it will rotate the light either to right or to left.
- Depending on rotation it is called as
a. Levorotatory (l or -) anticlockwise b. Dexarotatory (d or +) clockwise
(Rotate plane polarised light to left) (Rotate plane polarised light to right)
D Fructose D glucose ( Dextrose)
- Racemic Mixture – Equimolar mixture of optical isomers which has no net reaction of plane polarized light.
- Epimerism (Diastereoisomerism) – When sugars are different from one another, only in configuration with regard to a single carbon atom, other than the Glucose reference carbon atom, they are called epimers.
Epimers of glucose–
- 2nd Epimer of Glucose – Mannose
- 3rd Epimer of Glucose – Allose
- 4th Epimer of Glucose – Galactose.
- Anomerism– Formation of ring structure in monosaccharides results in creation of an additional asymmetric carbon called Anomeric Carbon atom.
- Difference in orientation of H and OH group around the anomeric carbon atom results in Anomerism and the resulting isomers are called α and β anomers.
- Examples of Anomerism:
- αDGlucose and β DGlucose
- α D Fructose and β D Fructose.
- Mutarotation– is the change in the specific optical rotation of plane polarised light with time.
- The optical rotation of α glucose is +112.20
- The optical rotation of β glucose is 18.70 ( 190)
- The optical rotation of Fructose is -920
- An equilibrium with a constant value is +52.70
- Benedict’s Reaction- Standard lab test to diagnose Diabetes mellitus.
- Shapes of Osazones-
- Needle-shaped/Broomstick/sheaves of corn – Glucose, Fructose, Mannose
- Pincushion with pins/Hedgehog/ Flower of Touch –me-not – Lactose
- Sunflower Petal-shaped – Maltose
- Sucrose is also called as invert sugar.
- Monosaccharide with no Asymmetric Carbon atom- Dihydroxyacetone.
- Aldose sugar is converted to ketose and vice versa by enzyme isomerase.
- Glucose and mannose are C2 epimers and glucose & galactose are C4 epimers.
- Maltose is a reducing disacharide.
- Lactose is a reducing dissacharide.
- Sucrose is a non-reducing sugar.
- Benedict’s quantitative reagent detects any reducing sugar.
- Fehling solution contains CuSO4 and Ruchelle salt .
- Glucose oxidase method is used for glucose.
- Perioxidase and oxidase enzymes are used for estimation of glucose.