Disaccharides

1. Disaccharides Disaccharides ( di - means two) are double sugars. One hydrolysis, they yield two simple sugars . Disaccharides 2 monosaccharides   Three common disaccharides 1. Sucrose 2. Maltose 3. Lactose * All of which are isomers with the molecular formula  

2. General Reaction O A disaccharide Sucrose Maltose Lactose A monosaccharide Glucose Glucose Glucose A monosaccharide Fructose Glucose Galactose

3. In terms of Solubility Sucrose – very soluble in water Maltose – fairly soluble Lactose – only slightly soluble The disaccharides, just like monosaccharide are white crystalline, sweet solids. The disaccharides are also optically active; they rotate the plane of polarized light. However, even though they are soluble in water, they are too large to pass through cell membranes.

4. Disaccharide Formation and Structure Disaccharides are formed when two monosaccharides are joined by dehydration synthesis reaction.

5. Disaccharide Formation and Structure CH 2 OH H O H OH O H CH 2 OH H O H HO OH + H 2 CH 2 OH H O H OH CH 2 OH H O H OH O α - GLUCOSE α - GLUCOSE MALTOSE

6. Disaccharide Structure Sucrose Maltose

7. Consider the combination of a molecule of a-glucose with a molecule of b-glucose. The products such as a reaction are b-maltose The linkage in b-maltose is between carbon 1 of one glucose and carbon 4 of the other glucose. Such a linkage is caalled a-1,4 linkage; -maltose has an –OH on carbon 1 of the second glucose molecule below the plane rather than above. Both - & -maltose exist, but the predominant form is the . B-maltose

8. if a molecule of -glucose combines with another molecule of -glucose, cellobiose , a compound with a 1-4 glycosidic linkage is formed. Enzymes are specific in the type of glycosidic linkage ( or ) whose hydrolysis they can catalyze. Conversly , an enzyme that contains catalys \ zes the hydrolysis of glycosidic linkages catalyzes the hydrolysis of cellobiose . Such - glycosidic linkage hydrolysis enzymes are not found in the human digestive system. That is why human cannot digest cellulose and cellobiose , both of which have glycosidic linkages.

9. Reducing properties In maltose The aldehyde groups are at carbon 1 in each of the original glucose molecules. Since the linkage is 1.4, one free aldehyde group remains. Therefore, maltose acts as a reducing sugar. In sucrose The glucose part had the aldehyde at carbon 1, and the fructose part had the ketone group at carbon 2. since the linkage is 1,2, neither group is free. Therefore, sucrose is not a reducing sugar. In lactose Which has a 1,4-linkage, acts as a reducing sugar because both of the original aldehyde group wre on carbon 1, and one of them is free to react.

10. Fermentation Sucrose and maltose will ferment when yeast is added because yeast contains the enzyme sucrase and maltase. Lactose will not ferment because yeast does not contain lactase.

11. Testing for disaccharides The chemical reactions of these sugars can be used to distinguish them in the laboratory. If you have 2 test tubes containing a disaccharide,C12H22O11.To determine if it is sucrose lactose or maltose. We can use the alkaline Cu complex reaction of glucose and the principle of fermentation.

12. Sucrose Is a sugar used at home Also known as the cane sugar When hydrolyzed, it forms a mixture of glucose and fructose. This 50:50 mixture of glucose and fructose is called invert sugar because it reverses the rotation of polarized light.

13. Maltose Commonly known as malt sugar. Present in germinating grain Produced commercially by hydrolysis of starch.

14. Lactose Commercially known as milk sugar. Of animal origin Bacteria cause fermentation of lactose forming lactic acid. When these reaction occur ,it changes the taste to a sour one.