Biomolecules
2.0 Monosaccharides
2.0 Monosaccharides
They are further classified on the basis of number of carbon atoms and the functional group present:
1. Aldohexose:
- Monosaccharide contains an aldehyde group are called aldose or aldohexose.
- Molecular formula: $C_6H_{12}O_6$.
- Structure:
- Due to four asymmetric carbon atoms, there are sixteen optical isomers or eight pairs of enantiomers (8D and 8L).
Glucose:
- It is an aldohexose and also known as dextrose because it occurs in nature principally as the optically active dextrorotatory isomers.
- It is the monomer of many larger carbohydrates, namely starch, cellulose.
- It is a strong reducing agent (reducing both fehling’s solution and ammonium silver nitrate) as the anomeric carbon contains free hydroxyl group.
- Molecular formula: $C_6H_{12}O_6$.
The six membered cyclic structure of glucose is called pyranose structure ($\alpha $ and $\beta$), in analogy with pyran. Pyran is a cyclic organic compound with one oxygen atom and five carbon atoms in the ring.
Preparation:
- From sucrose: $${C_{12}}{H_{22}}{O_{11}} + {H_2}O\mathop \to \limits^{{H^ + }} {C_6}{H_{12}}{O_6}\left( {{\text{Glucose}}} \right) + {C_6}{H_{12}}{O_6}\left( {{\text{Fructose}}} \right)$$
- From starch: $${({C_6}{H_{10}}{O_5})_n} + n{H_2}O\mathop \to \limits^{{H^ + }} n{C_6}{H_{12}}{O_6}$$
Important reactions of glucose:
Cyclic structure of glucose:
Some important facts can be explained by the cyclic structure:
- Despite having the aldehyde group, glucose does not give $2,4$-DNP test, Schiff’s test and it does not form the hydrogensulphite addition product with $NaHSO_3$ because the reaction is reversible and equilibrium concentration is not disturbed.
- Glucose is found to exists in two different crystalline forms which are named as $\alpha$ and $\beta$. The $\alpha$-form of glucose (melting point $419\ K$) is obtained by crystallization from concentrated solution of glucose at $303\ K$ while the $\beta$-form (melting point $423\ K$) is obtained by crystallization from hot and saturated aqueous solution at $371\ K$.
Two cyclic forms exists in equilibrium with open chain structure:
The two cyclic hemiacetal forms of glucose differ only in the configuration of the hydroxyl group at $C_1$, called anomeric carbon (the aldehyde carbon before cyclisation). Such isomers, i.e., $\alpha$ form and $\beta$ form, are called anomers.
- The pentaacetate of glucose does not react with hydroxylamine indicating the absence of free $–CHO$ group because $–OH$ group at anomeric carbon is bounded by ester linkage. Hence $\alpha$ and $\beta$ form cannot convert into open chain structure which contains the aldehyde group.
2. Ketohexose:
- Monosaccharide contains a keto group are called ketose or ketohexose.
- The only important ketohexose is Fructose. It is obtained along with glucose by the hydrolysis of disaccharide, sucrose.
Fructose:
- Molecular formulae: $C_6H_{12}O_6$.
- It is a laevorotatory compound.
- Open chain structure of $D$-(-)-Fructose is
- Cyclic structures of two anomers of fructose are represented by Haworth structures:
