1 Chapter 13: Carbohydrates Chem 20 El Camino College

2 Carbohydrates Carbohydrates have the formulas of C n (H 2 O) n Glucose is formed by photosynthesis: 6 CO H 2 O → C 6 (H 2 O) 6 + 6O 2

3 D and L The first carbon (C #1) is the carbon on the top In the L version, the OH on the last chiral carbon is on the left In the D version, the OH on the last chiral carbon is on the right

4 Examples of D and L Isomers of Monosaccharides

5 Cyclic Structure of Glucose The carbonyl group of D-glucose can react with the –OH on carbon 5 to form a ring of six atoms. It has one more chiral center (carbon 1) than the open chain.

6 Drawing the Cyclic (Haworth) Structure 1. Starting with the aldehyde carbon, number the carbons 2. Draw the structure laying on the right side Make sure you don’t change the locations of the OH groups

7 Drawing the Cyclic Structure for Glucose STEP 1 Number the carbon chain and turn on its side to the right

8 Cyclic Structures 3. Draw the following cyclic structure Note that there is an oxygen in the ring Always draw the structure in the exact same way

9 Cyclic Structures 4. In your straight chain drawing, note if the OH is down or up on carbons 2, 3, and 4 Put those OH groups down or up on the ring CH 2 OH is up, OH on C1 can go up or down Fill in the rest with hydrogens

10 Cyclic Structures

11 Cyclic Structure of Glucose In the Haworth structure, we suppose that the ring is flat. the group -CH 2 OH is above the ring. The –OH group on carbon 1 is written down in the alpha form and up in the beta form.

12 Cyclic Structure of Glucose Solid glucose is α-D-glucose. In aqueous solution, there is equilibrium of three forms: 36% α-D-glucose, 64% β-D-glucose and trace of open chain.

13 Cyclic Structure, D-Galactose

14 Cyclic Structure, D-Galactose The cyclic structure of galactose is similar to glucose, except that the –OH on carbon 4 is up.

15 Cyclic Structure D-Galactose The cyclic D-galactose can be in alpha and beta forms

16 Cyclic Structure

17 Cyclic Structure Fructose The hydroxyl group on carbon 5 reacts with the ketone group on carbon 2 to form a five-atom ring. The cyclic D-fructose can be in alpha and beta forms

18 Chemical Properties Reduction of monosaccharides: The carbonyl group (-C=O) can be reduced to –OH group. For example D-glucose is reduced to sorbitol (D- glucitol)

19 Oxidation of Monosaccharides Aldoses are called “reducing sugars” The aldehyde group (-CHO) in aldose can be oxidized by silver ammonia ion Ag(NH 3 ) 2 + of Tollens’ reagent to form silver In the Benedict test, the aldehyde group is oxidized to a carboxylic acid group by Cu(II) citrate complex and a brick-red ppt (Cu 2 O) forms Fructose is also a reducing sugar since it is converted to glucose in basic solution.

20 Benedict Test of D-Glucose [O]

21 Disaccharides Disaccharides can be broken down into two monosaccharides Maltose + H + + H 2 O  glucose + glucose Lactose + H + + H 2 O  glucose + galactose Sucrose + H + + H 2 O  glucose + fructose

22 Alpha and Beta Linkages There are different types of linkages in disaccharides In  -linkages, both OH groups involved in bonding are pointing down In  -linkages, one OH involved in bonding is up and the other is down.

23 Formation of alpha lactose from glucose + galactose (formation of ether bridge)

24 Formation of Beta Lactose

25 Formation of Alpha Maltose

26 Sucrose from glucose + fructose

27 Polysaccharides Long chains of monosaccharides are called polysaccharides Amylose and amylopectin are plant starches Glycogen is animal starch—it provides storage for glucose in muscles

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29 Polysaccharides Cellulose (in wood and cotton) is a structural material for plants People don’t have the enzymes to digest the  - linkages in cellulose, but cows do!

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