Question

Write short notes on disaccharides.

Answer 1

Disaccharides:

• Disaccharides are sugars made up of two glucose or monosaccharide units.
• A glycosidic linkage is a carbon–an oxygen-carbon connection that connects two molecules.
• Maltose, lactose, and sucrose are a few examples of disaccharides.

Maltose $\left({\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}\right)$

• It is commonly known as malt sugar. Upon hydrolysis with dilute acids or with enzyme maltase present in yeast, it forms two molecules of $\mathrm{D}-(+)$glucose only

.${\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}+{\mathrm{H}}_2\mathrm{O}\underset{\mathrm{Maltase}}{\overset{{\mathrm{H}}^{+}}{\to }}{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$

• Maltose is composed of two $\mathrm{\alpha }-\mathrm{D}$ glucose units.
• It is also called reducing sugar.
• ${\mathrm{C}}_1$ of one of the glucose molecules are linked with ${\mathrm{C}}_4$ of the other glucose molecule through glycosidic linkage.
• The aldehydic group is free to be reduced.
• Maltose is present in foods like corn, barley etc

Lactose$\left({\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}\right)$

• Mammalian milk contains lactose.
• Upon hydrolysis, lactose gives equivalent amounts of $\mathrm{\beta }$-D glucose and $\mathrm{\beta }$-D galactose they are isomeric in nature.

${\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}+{\mathrm{H}}_2\mathrm{O}\stackrel{}{\underset{}{\overset{\mathrm{Hydrolysis}}{\to }}\mathrm{C}{}_6}{\mathrm{H}}_{12}{\mathrm{O}}_6+{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$

• In lactose, the glycosidic linkage is present between ${\mathrm{C}}_1$ of galactose and ${\mathrm{C}}_4$ of glucose.
• Lactose is a reducing sugar that undergoes mutarotation to produce both D-glucopyranose subunits of anomers.

Sucrose-$\left({\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}\right)$

• Sucrose is one of the most useful disaccharides.
• It is sweet in taste.
• Upon hydrolysis, it forms an equimolar mixture of D (+)glucose and D (-) fructose.

${\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}\left(\mathrm{S}\right)+{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{Hydrolysis}}{\to {\mathrm{C}}_6}{\mathrm{H}}_{12}{\mathrm{O}}_6\left(\mathrm{aq}\right)+{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\left(\mathrm{aq}\right)$

• An $\mathrm{\alpha }-1,\mathrm{\beta }-2-$glycosidic (head-to-head) connection binds the anomeric carbons of glucose and fructose together.
• Sucrose is incapable of mutarotation and only exists in one form in both the solid and solution states.
• Furthermore, sucrose does not undergo the reactions that aldehydes and ketones do. As a result, sucrose is a sugar that does not reduce.