Biomolecules such as carbohydrates, lipids, proteins, and nucleic acid play important role in various metabolic activities of living bodies. Carbohydrates are a major source of energy in living bodies. They are polymeric forms of monosaccharide which are polyhydroxy aldehyde and ketone. The carbonyl group of the monosaccharide units involves in glycosidic linkage with each other to form polymeric forms.
On the basis of taste, they can be classified as sugar and non-sugar molecules. The sugar molecules are sweet in nature and usually soluble in water such as glucose, fructose. On the contrary, non-sugars are not sweet in taste and usually cannot dissolve in water such as starch, cellulose etc.
On the basis of monomer units, carbohydrates can be classified as a monosaccharide, oligosaccharide and polysaccharides. Monosaccharides are the simplest units of carbohydrates which cannot be further hydrolysis in simple units. The polymerisation of 2-10 units of monosaccharide units forms oligosaccharide such as lactose, maltose etc. The polysaccharides are polymeric forms of carbohydrates which are formed by polymerisation of more 100 units. Monosaccharide units such as glucose, fructose, and galactose are polymerised by glycosidic linkages to form polymeric form. The type of the glycosidic linkage affects the physical and chemical properties of molecules. Let’s discuss one of the oligosaccharide that is named as lactose.
One of the most important classes of biomolecules is carbohydrates. These biomolecules play a vital role in our daily life by providing us three necessities i.e., food (Starch), cloth (Cellulose) and shelter (Cellulose).
Carbohydrates are optically active compounds with hydroxy and carbonyl functional groups and general formula is Cx
. They are also known as saccharides, classified on the basis of the product of hydrolysis.
- Monosaccharide: The simplest carbohydrates which cannot further hydrolyze are called as Monosaccharides. The general formula for monosaccharide is (CH2O)n where n is 3 to 7. For example, Glucose, fructose, galactose.
- Oligosaccharide: The term “oligo” stands for “few”, hence oligosaccharides give a few number of monosaccharide on hydrolysis, generally two to ten monosaccharides. On the basis of number of monosaccharides ; oligosaccharides can be classified as;
- Disaccharide: They give two same or different monosaccharides on hydrolysis with general formula of C12H22O11. Like; sucrose, maltose, lactose etc.
- Trisaccharide: They give three same or different monosaccharides on hydrolysis with general formula of C18H32O16. Like;Raffinose.
- Tetrasaccharide: They give four same or different monosaccharides on hydrolysis and have general formula of C24H42O21. Like; stachyrose.
- Polysaccharide: Such type of saccharides forms a large number of monosaccharides on hydrolysis. The general formula of polysaccharides is (C6H10O5)n.
- Lactose is an example of disaccharide which upon hydrolysis gives two saccharides, glucose and galactose.
- It first discovered by Fabriccio Bartoletti in 1619 and later in 1780 identified as a sugar by Carl Wilhelm Scheele.
- This disaccharide present in cow milk and also known as milk sugar. The percentage of lactose sugar in cow’s milk is 4-6% while in human milk it is 5-8%.
- It formed during the manufacturing of cheese as a side product. Lactose is a white color solid carbohydrate with the formula of C12H22O11 and molecular mass is 342.30 g/mol. It is quite soluble in water around 22 gm in 100 ml of water.
- The melting point and boiling point of lactose is 475.8 K and 941.8 K respectively. Because of sweet nature of lactose it is included in the sugar class of carbohydrates.
- In lactose unit on hydrolysis yields an equi molar mixture of ß-D-glucose and ß-D-galactose. Remember that glucose and galactose both are aldose monosaccharide but differ in orientation of hydroxy group of C4 carbon atom.
- Generally hydrolysis of disaccharides takes place either in the presence of an acid or an enzyme under biochemical conditions. For example, the hydrolysis of lactose takes place in the presence of lactase enzyme.
- The level of lactase is high level, hence can easily digest milk, however the level of this enzyme decreases with age which further decreases the tolerance power for ingestion of lactose. This is also known as lactose intolerance.
Lactose a reducing sugar as it forms an osazone, undergo mutarotation and can also reduce Tollen’s and Fehling’s solution. The reducing nature of saccharides depends on the presence of carbonyl group in molecule. Generally in monosaccharides the carbonyl group is free to reduce Tollen’s reagent as well as Fehling’s solution. But when monosaccharide involves in the formation of disaccharides, the carbonyl group may or may not involve in hemiacetal linkage.
If the reducing centre that is carbonyl group in monosaccharide is not involving the hemiacetal linkage, the sugar will be reducing in nature. However the contribution of reducing centre in hemiacetal linkage makes the molecule non-reducing in nature. For example; in sucrose two monosaccharides units are linked through their reducing centre and make the molecule non-reducing.
While in lactose, reducing centre is not involved in hemiacetal linkage and it is a reducing sugar, hence show all reducing characteristics of carbonyl groups. Since in lactose galactose and glucose units are joined by an acetal oxygen bridge in the beta orientation, therefore it contains a ß-1?4 glycosidic linkage with the systematic name; ß-D-galactopyranosyl-(1?4)-D-glucose. As in each unit of lactose the C4
of glucose unit bonded with C1 of galactose unit by glycosidic linkage.
Hence, all the chemical and physical properties of carbohydrates cannot be explained by using their open chain structure, therefore the cyclic hemiacetal structure is given. These cyclic structures formed due to the formation of hemiacetal linkage between hydroxy group and carbonyl group of molecules.
These cyclic structures are differ from each other in the orientation of groups at C1
carbon atom which is also termed as anomeric carbon atom and these cyclic structures which are differ in their anomeric carbon atom are called as anomers
. Anomer in which the –OH group at anomeric carbon (C1
) is towards right side is called as a-form and another is called as $\beta $-form. The phenomenon of spontaneous change in specific rotation of these anomers is called as mutarotation
The molecular formula of lactose is C12
in which one β-D-glucopyranose ring bonded with one $\beta $-D-galactopyranose ring by glycosidic linkage through beta-linkage between C1
of galactose and C4
Lactose monohydrate is a naturally occurring disaccharide which mainly present in milk. There is water of crystallization in lactose crystal and because of that the molecular mass becomes 360.3 g/mol compare to 342.3 g/mol for anhydrous lactose. It is a white, odorless crystalline powder which is quite soluble in water but slightly soluble in ethanol. Another name for lactose monohydrate is D-Glucose, 4-O-β-D-galactopyranosyl-, hydrate (1:1); Lactose.
The gentle heating of lactose monohydrate at 418 K temperature results the removal of one molecule of water to form lactose anhydrous. The Infra red spectra of lactose monohydrate is as follows.