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4C Focus on Health & Medicine: Monitoring Glucose Levels

4C Focus on Health & Medicine: Monitoring Glucose Levels

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626



CARBOHYDRATES



20.5 DISACCHARIDES

Disaccharides are carbohydrates composed of two monosaccharides. Disaccharides are

acetals, compounds that contain two alkoxy groups (OR groups) bonded to the same carbon.

Recall from Section 16.8 that reaction of a hemiacetal with an alcohol forms an acetal.

hemiacetal



O

OH



+



acetal



O



CH3OH



OCH3



+



H2O



In a similar fashion, a disaccharide results when a hemiacetal of one monosaccharide reacts with

a hydroxyl group of a second monosaccharide to form an acetal. The new C O bond that joins

the two rings together is called a glycosidic linkage.

hemiacetal

5



glycosidic linkage

5



O



4



1

3



OH



+



HO



2



O



O



4



1

3



OH



O

1



O



OH



4



+



H2O



2

general structure

of a disaccharide

[The acetal O’s are labeled in red.]



The two monosaccharide rings may be five-membered or six-membered. All disaccharides

contain at least one acetal that joins the rings together. Each ring is numbered beginning at the

anomeric carbon, the carbon in each ring bonded to two oxygen atoms.

The glycosidic linkage that joins the two monosaccharides in a disaccharide can be oriented in

two different ways, shown with Haworth projections in structures A and B.

H

4

O



H



H

4



H

1

O



OH



O



H

OH



O



H



The glycoside bond is down.

1



4-α-glycosidic linkage

A



O



H

OH



1

H



OH



𝛂 glycoside



Maltose gets its name from malt, the

liquid obtained from barley used in

the brewing of beer.



O



𝛃 glycoside

The glycoside bond is up.

1



4-β-glycosidic linkage

B



• An 𝛂 glycoside has the glycosidic linkage oriented down, below the plane of the ring that

contains the acetal joining the monosaccharides.

• A 𝛃 glycoside has the glycosidic linkage oriented up, above the plane of the ring that

contains the acetal joining the monosaccharides.

glycosidic linkage



Numbers are used to designate which ring atoms are joined in the disaccharide. Disaccharide A

has a 1→4-𝛂-glycosidic linkage since the glycoside bond is oriented down and joins C1 of one

ring to C4 of the other. Disaccharide B has a 1→4-𝛃-glycosidic linkage since the glycoside bond

is oriented up and joins C1 of one ring to C4 of the other.



maltose



smi26573_ch20.indd 626



Sample Problem 20.6 illustrates these structural features in the disaccharide maltose. Maltose,

which is formed by the hydrolysis of starch, is found in grains such as barley. Maltose is formed

from two molecules of glucose.



12/16/08 11:44:43 AM



DISACCHARIDES



627



SAMPLE PROBLEM 20.6



(a) Locate the glycosidic linkage in maltose. (b) Number the carbon atoms in both rings.

(c) Classify the glycosidic linkage as α or β, and use numbers to designate its location.

glucose



CH2OH

O



H



H

OH



H



O



H



H



H

OH



O



OH

H



glucose



CH2OH

H



H

OH



H



OH



OH



maltose



ANALYSIS AND SOLUTION



a. and b. The glycosidic linkage is the acetal C O bond that joins the two monosaccharides,

labeled in red. Each ring is numbered beginning at the anomeric carbon, the carbon

bonded to two oxygen atoms.

acetal carbon

CH2OH

5

O H

H

H

H

4

OH

H

1 O 4

OH

2

3

H

OH



CH2OH

5

O

H

OH

H

2

OH



3

H



H

1

OH



c. Maltose has an α glycosidic linkage since the C O bond is drawn down. The glycosidic

linkage joins C1 of one ring to C4 of the other ring, so maltose contains a 1→4-α-glycosidic

linkage.



PROBLEM 20.17



(a) Locate the glycosidic linkage in cellobiose. (b) Number the carbon atoms in both rings.

(c) Classify the glycosidic linkage as α or β, and use numbers to designate its location.

CH2OH

H

CH2OH

O



H



H

OH



H



H



O



OH

H



H



OH



O

H

OH



OH



H

H



OH

cellobiose



The hydrolysis of a disaccharide cleaves the C O glycosidic linkage and forms two monosaccharides. For example, hydrolysis of maltose yields two molecules of glucose.

Cleave the glycosidic linkage.

CH2OH

H



O

H

OH



H



OH

H



CH2OH

H



H

O



O

H

OH



maltose



+



H

OH



H



OH



CH2OH



OH



H



H

H



OH



O

H

OH



CH2OH

+



H



OH



OH

H



OH

glucose



H



H



O

H

OH



H



H



HO



OH

H



OH



glucose



Water is added in hydrolysis.



smi26573_ch20.indd 627



12/16/08 11:44:45 AM



628



CARBOHYDRATES



SAMPLE PROBLEM 20.7



Draw the products formed when the following disaccharide is hydrolyzed with water.

CH2OH

H



O

H

OH



H

CH2OH



H



OH



O



O

H



H

OH



OH



OH



H

H



ANALYSIS

SOLUTION



OH



Locate the glycosidic linkage, the acetal C O bond that joins the two monosaccharides. Cleave

the C O bond by adding the elements of H2O across the bond.

Two monosaccharides are formed by cleaving the glycosidic linkage.



CH2OH

H



O

H

OH



CH2OH

H



H



OH



O

H



OH



glycosidic linkage

CH2OH

O

H

OH



H



H



OH



H



O

H

OH



+



H



OH



OH

H



Add OH onto the ring.

CH2OH



H



OH



H



H



+ HO



OH



O

H

OH



H



H



OH



H



OH



H



PROBLEM 20.18



H



H



Add H to the O atom.



H

OH



What monosaccharides are formed when cellobiose (Problem 20.17) is hydrolyzed with water?



20.5A



FOCUS ON HEALTH & MEDICINE

LACTOSE INTOLERANCE



Lactose is the principal disaccharide found in milk from both humans and cows. Unlike many

mono- and disaccharides, lactose is not appreciably sweet. Lactose consists of one galactose ring

and one glucose ring, joined by a 1→4-β-glycoside bond from the anomeric carbon of galactose

to C4 of glucose.



glucose

CH2OH

H

CH2OH



Individuals who are lactose intolerant

can drink lactose-free milk. Tablets

that contain the lactase enzyme can

also be taken when ice cream or

other milk products are ingested.



OH



O

H

OH



H

H



OH

H



H

OH



H



H



O



H

galactose



O

H

OH



OH

β glycoside bond



lactose



Lactose is digested in the body by first cleaving the 1→4-β-glycoside bond using the enzyme

lactase. Individuals who are lactose intolerant no longer produce this enzyme, and so lactose

cannot be properly digested, causing abdominal cramps and diarrhea. Lactose intolerance is

especially prevalent in Asian and African populations whose diets have not traditionally included

milk beyond infancy.



smi26573_ch20.indd 628



12/16/08 11:44:45 AM



DISACCHARIDES



629



PROBLEM 20.19



Lactose contains both an acetal and a hemiacetal. Label both functional groups. What products

are formed when lactose is hydrolyzed with water?



20.5B



FOCUS ON HEALTH & MEDICINE

SUCROSE AND ARTIFICIAL SWEETENERS



Sucrose, the disaccharide found in sugarcane and the compound generally referred to as “sugar,”

is the most common disaccharide in nature. It contains one glucose ring and one fructose ring.

Unlike maltose and lactose, which contain only six-membered rings, sucrose contains one sixmembered and one five-membered ring.

CH2OH

O



H



H

OH



H



H



OH

H

OH

O

CH2OH O

H

HO

H

CH2OH

sugarcane



TABLE 20.1 Relative

Sweetness of Some

Carbohydrates and

Artificial Sweeteners

Compound



Relative Sweetness



Sorbitol



0.60



Glucose



0.75



Sucrose



1.00



Fructose



1.75



Aspartame



150



Saccharin



350



Sucralose



600



smi26573_ch20.indd 629



two varieties of refined sugar



OH

H

sucrose



Sucrose’s pleasant sweetness has made it a widely used ingredient in baked goods, cereals, bread,

and many other products. It is estimated that the average American ingests 100 lb of sucrose

annually. Like other carbohydrates, however, sucrose contains many calories. To reduce caloric

intake while maintaining sweetness, a variety of artificial sweeteners have been developed. These

include aspartame, saccharin, and sucralose (Figure 20.3). These compounds are much sweeter

than sucrose, so only a small amount of each compound is needed to achieve the same level of

perceived sweetness.

A relative sweetness scale ranks the sweetness of carbohydrates and synthetic sweeteners, as

shown in Table 20.1.

Aspartame (trade name: Nutrasweet) is used in the artificial sweetener Equal and many diet

beverages. While harmless to most individuals, aspartame is hydrolyzed in the body to the amino

acid phenylalanine (and other products). Infants afflicted with phenylketonuria cannot metabolize this amino acid, so it accumulates, causing mental retardation. When this condition is identified early, a diet limiting the consumption of phenylalanine (and compounds like aspartame that

are converted to it) can make a normal life possible.



PROBLEM 20.20



Unlike lactose, which contains one acetal and one hemiacetal, sucrose contains two acetals.

Locate the acetals in sucrose. Classify each alcohol as 1°, 2°, or 3°.



PROBLEM 20.21



Sucralose is prepared from sucrose. Locate the acetals in sucralose. Label each alkyl halide as

1°, 2°, or 3°.



PROBLEM 20.22



Identify the functional groups in aspartame.



12/16/08 11:44:50 AM



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