Disaccharides

Disaccharides

The carbohydrates which on hydrolysis give two monosaccharide units are known as disaccharides.

Example: Sucrose, maltose, lactose etc.,

1. Maltose:

Properties of Maltose:

a.  Maltose or malt sugar is formed as an intermediate product in the acid hydrolysis of starch.

b.  It is also produced during the course of digestion of starch by pancreatic amyalse.

c.   It is a reducing disaccharide.

d.  Maltose is composed of two α-D-glucose units held together by α(1-4) glycosidic linkage.

e.   It is hydrolyzed by dilute acids (or) enzyme maltase into two α - D - glucose units.

f.    Maltose is readily fermented by yeast.

Structure of Maltose

2. Lactose :

a.  Lactose is formed by the mammary glands. It is milk sugar.

b.  It is a reducing sugar, forms osazone.

c.   It is hydrolysed by acids and enzyme lactase into one molecule of α- D- galactose and one molecule of α - D - glucose.

d.  It is fermented by yeast.

e.   In the lactose, the galactose and glucose units are held together by α(1-4) linkage.

Structure of Lactose:

3. Sucrose:

Properties of sucrose:

·     When heated to 200°C it loses water to form a brown mass called caramel. On strong heating, it gives pure carbon with a burnt smell.

·     Concentrated sulphuric acid dehydrates sucrose into carbon. This on further oxidation by H₂SO₄ gives CO₂

C₁₂H₂₂O₁₁+H₂SO₄‚--> C + H₂O +SO₂

·     When boiled with HCl, sucrose gives laevulic acid.

·     Concentrated nitric acid oxidises cane sugar (sucrose) to oxalic acid.

C₁₂H₂₂O₁₁+9O₂ --> 6(COOH)₂+ 5H₂O

·     Sucrose is fermented by invertase into glucose and fructose which are converted to ethanol by zymase. Both these enzymes are available in yeast.

·     Sucrose on acetylation gives octa-acetyl derivative.

·     Sucrose on methylation gives octa-o-methyl derivative.

·     Sucrose does not react with HCN, NH₂OH, phenyl hydrazine, Tollen’s reagent and Fehling’s solution.

·     Controlled reduction of sucrose gives a mixture of sorbitol and mannitol.

·     It reacts with lime water Ca(OH)₂ to give calcium sucrate.

Hydrolysis of sucrose:

Sucrose is hydrolysed by dilute acids or enzymes like sucrase or invertase into an equimolar mixture of glucose and fructose.

Sucrose is dextrorotatory. But the hydrolysed product is laevorotatory. Since the direction of rotation is reversed, this phenomenon is known as inversion of cane sugar. The mixture of sugars formed on hydrolysis is known as invert sugar.

According to Hudson, sucrose is first split into α-D(+) glucopyranose and β-D(+) fructofuranose, both are dextro rotatory. However, the less stable β-D(+) fructofuranose then sets up an equilibirum with its more stable isomer, α-D(–) fructopyranose which is strongly laevorotatory. Thus, the invert sugar gives a specific rotation of –28.2°.

Structure of Sucrose:

Table 5.2 Differences between glucose, fructose and galactose

5.3 Differences between sucrose, lactose and maltose