Carbohydrates

Chapter 16

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1. Types of carbohydrates

Carbohydrates consist of carbon, hydrogen, and oxygen only. are made by plants during photosynthesis. normally have the general formula (CH2O)n.

Carbohydrates are made up of saccharide (sugar) units. Monosaccharides consist of a single sugar unit. Disaccharides consist of two sugar units. Oligosaccharides consist of 3-10 sugar units. Polysaccharides consist of long chains, often branched, of sugar units.

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1. Types of carbohydrates

What to know from this section: Types of carbohydrates (16.13-14) General formula for a simple sugar Source of sugars (16.16)

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2. Monosaccharides

Monosaccharides have 3-7 carbons.

Aldoses have an aldehyde group.

Ketoses have a carbonyl group.

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2. Monosaccharides

Monosaccharides are further classified based on the number of carbons they contain.

triose, tetrose, pentose, hexose, heptose

Examples

A four-carbon monosaccharide with an aldehyde group is an aldotetrose.

A five-carbon monosaccharide with a ketone group is a ketopentose.

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2. Monosaccharides

Once a monosaccharide has been named as an aldose or a ketose, and the number of carbons has been designated, there are still several different isomeric forms for each.

Each specific monosaccharide has a unique name.

A prefix (D- or L-) is added to designate which of two possible isomeric forms is being referred to.

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2. Monosaccharides

Examples

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2. Monosaccharides

Draw two possible monosaccharide structures for the molecular formula C4H8O4.

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13.4. Reactions--addition

Addition of an alcohol to an aldehyde:

The product is called a hemiacetal (-OH and –OR attached to the same carbon).

Hemiacetals are very reactive. They react with an additional alcohol molecule, losing –OH and

adding another –OR.

H+

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13.4. Reactions--addition

The final product is an acetal (2 –OR groups attached to one carbon).

hemiacetal acetal

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13.4. Reactions--addition

Ketones undergo analogous addition reactions with alcohols.

The initial product is a reactive hemiketal (two –R groups, one –OH, and one –OR).

An additional –OR group is added to the hemiketal to produce a ketal.

hemiketal ketal

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13.4. Reactions--addition

13.4. Reactions

Hemiacetal, acetal, hemiketal, or ketal?

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13.4. Reactions

Monosaccharide addition reactions

1

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4

5

6 ]alcohol

aldehyde

D-glucose

]

14

1

23

4

5

6 Hemiacetal: one –H one –OH one –OR one -R

13.4. Reactions

Monosaccharide addition reactions

The cyclic form is more stable than the linear form and no further oxidation takes place in this case.

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2. Monosaccharides

What to know from this section Definitions of aldose and ketose (16.23) Simple sugar naming based on number of carbons (16.25-26) Naming based on aldose/ketose and number of carbons (16.27-28) Identify hemiacetals and hemiketals (16.29) Draw possible monosaccharide structures given molecular formula

(16.31-32)

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3. Stereoisomers and stereochemistry

Stereoisomers have the same molecular formulas. the same bonding of atoms to one another.

Stereoisomers differ in the spatial arrangement of the atoms in the molecule.

Enantiomers are stereoisomers that are nonsuperimposable mirror images of each other.

A molecule that can exist in enantiomeric forms is called a chiral molecule.

Simple enantiomers [link—bromochloroiodomethane]

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3. Stereoisomers and stereochemistry

A carbon atom that has four different groups bonded to it is called a chiral carbon atom.

Any molecule containing a chiral carbon can exist as a pair of enantiomers.

Larger biological molecules often have more than one chiral carbon.

Glyceraldehyde enantiomers

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3. Stereoisomers and stereochemistry

Plane polarized string analogy for plane polarized light.

jk

l

http://www.chemguide.co.uk/basicorg/isomerism/string4.GIF

demo

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3. Stereoisomers and stereochemistry

Enantiomers are also called optical isomers.

Enantiomers interact with plain polarized light.

They rotate the plane of the light in opposite directions.

This interaction with polarized light is called optical activity.

Optical activity distinguishes the isomers.

It is measured in a device called a polarimeter.

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Polarimeter

Compounds that rotate light in a clockwise direction are dextrorotatory, designated by (+) before the angle.

Compounds that rotate light in a counterclockwise direction are levorotatory, designated by (-) before the angle.

3. Stereoisomers and stereochemistry

Home-made polarimeter

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3. Stereoisomers and stereochemistry

Fischer projections are two-dimensional drawings that represent a three-dimensional molecule with one or more chiral carbons.

The intersection of two lines represents a chiral carbon.

Horizontal lines represent bonds projecting outward.

Vertical lines represent bonds projecting backward.

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3. Stereoisomers and stereochemistry

Bromochlorofluoromethane

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3. Stereoisomers and stereochemistry

Conventions for drawing monosaccharides as Fischer projections :

The most oxidized carbon is closest to the top.

The carbons are numbered from the top.

The chiral carbon with the highest number determines the D or L designation.

If the OH is to the right, the sugar is D.

If the OH is to the left, the sugar is L.

Most common sugars are in the D form.

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3. Stereoisomers and stereochemistry

Determine whether each of the following monosaccharides is D- or L–.

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3. Stereoisomers and stereochemistry

What to know from this section: Definitions (bold-face terms) Explanation of plane polarized light, and relationship to

stereoisomers (16.37-38) How to identify chiral carbons in molecules (16.45-46) How to interpret Fischer projections How to identify D- and L- sugars (16.44) How to draw the mirror image of a molecule (16.42)

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4. Monosaccharides: glucose

Glucose is an aldohexose (C6H12O6).

D- form

k

j

l

n

m

o

most oxidized carbon

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4. Monosaccharides: glucose

Under physiological conditions, glucose exists almost entirely in a cyclic hemiacetal form.

The C-5 hydroxyl reacts with the C-1 aldehyde group. C-1 becomes a chiral carbon.

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4. Monosaccharides: glucose

When the ring forms, the –OH on C-1 can be below (α-) or above (β-) the ring.

Isomers that differ in the arrangement of bonds around a hemiacetal carbon are called anomers.

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4. Monosaccharides: glucose

The ring forms are represented as Haworth projections on the previous slide.

Groups on the left of the Fischer projection are above the ring.

Groups on the right of the Fischer projection are below the ring.

For the cyclic forms of D-sugars, the -CH2OH group is always up.

If the –OH on C-1 is cis to the -CH2OH group, it is a β-D-sugar.

If the –OH on C-1 is trans to the -CH2OH group, it is an α-D-sugar.

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4. Monosaccharides: glucose

Fischer and Haworth projections for D-glucose:

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4. Monosaccharides: fructose

Fructose, the sweetest of all sugars, is a ketohexose. Cyclization of D-fructose produces a hemiketal.

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4. Monosaccharides: reducing sugars

Benedict’s test* is used to distinguish between reducing and non-reducing sugars.

A reducing sugar can be oxidized. The substance reduced is Cu+2.

+ Cu+2

+ Cu2O

*Benedict’s reagent = a basic buffer solution plus Cu+2 ions

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4. Monosaccharides: reducing sugars

In general, Benedict’s reagent is used to distinguish between aldehydes (e.g., aldoses) and ketones.

Ketoses, however, can convert to aldoses in basic solution.

D-fructose enediol D-glucose

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4. Monosaccharides

What to know from this section: Be able to relate the open chain form of a monosaccharide to its

cyclic form. Understand the relationship between Fischer projections and

Haworth projections. Understand the ring-forming reactions that yield hemiacetals or

hemiketals. Identify α and β anomers. Know the composition of Benedict’s reagent. Understand what characteristics a monosaccharide needs to be a

reducing sugar (react with Benedict’s reagent). Identify the enediol reaction that converts a ketose to an aldose.

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5. Disaccharides

In biological systems, monosaccharides exist in the cyclic form, as hemiacetals or hemiketals.

When a hemiacetal reacts with an alcohol, the product is an acetal.

When a hemiketal reacts with an alcohol, the product is a ketal.

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5. Disaccharides

A disaccharide is formed when the hemiacetal or hemiketal group on one monosaccharide reacts with one of the hydroxyl groups on another monosaccharide.

The acetal or ketal formed is called a glycoside. The C-O-C bond is called a glycosidic bond.

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5. Disaccharides

Maltose is formed from α-D-glucose and a second D-glucose (α- or β-).

In this example, carbon-1 on the α-D-glucose links to carbon-4 on the β-D-glucose.

The linking oxygen atom is α to (below) the left ring. The connection is called an α(14) glycosidic linkage.

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5. Disaccharides

Maltose

The hemiacetal hydroxyl group on C-1 will react with Benedict’s reagent, because the ring can open at that point to form an aldehyde (reducing group).

The product is named β-maltose because of the position of this hydroxyl group.

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5. Disaccharides

Lactose is formed from β-D-galactose and α- or β-D-glucose.

In this example, carbon-1 on the β-D-galactose links to carbon-4 on the β-D-glucose.

The linking oxygen atom is β to (above) the galactose ring. The connection is called a β(14) glycosidic linkage.

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5. Disaccharides

The first step in digestion of lactose is its hydrolysis to re-form galactose and glucose.

Glucose is readily metabolized.

If the enzyme lactase is not present, lactose can’t be hydrolyzed before it is eliminated.

This condition is called lactose intolerance. It can be remedied by ingesting lactase when eating lactose-

containing foods.

An enzyme-catalyzed process makes galactose usable by the body.

Galactosemia is a condition in which one or more of the enzymes is missing.

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5. Disaccharides

Sucrose is formed from α-D-glucose and β-D-fructose.

Carbon-1 on the glucose links to carbon-2 on the fructose (a ketose).

The linking oxygen atom is α to (below) the glucose ring and β to (above) the fructose ring..

The connection is called an (α1β2) glycosidic linkage.

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5. Disaccharides

In sucrose, the anomeric carbons of both glucose and fructose are linked.

There is no hemiacetal or hemiketal group that can be oxidized.

Sucrose is not a reducing sugar and will not react with Benedict’s reagent.

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5. Disaccharides

What to know from this section: Identify hemiacetal and hemiketal monosaccharides and the acetal

and ketal disaccharides that can be formed from them. Identify glycosidic linkages of various types. Know why maltose and lactose are reducing sugars, while sucrose

is not. Understand the chemical origins of lactose intolerance and

galactosemia.

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6. Polysaccharides

Starch is a heterogeneous mixture of two polymers of glucose.

Amylose (about 20% of plant starch) is a linear polymer of α-D-glucose units connected by α(14) glycosidic bonds.

Amylose chains can contain up to 4,000 glucose units.

Amylose coils into a helix that repeats every six glucose units.

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6. Polysaccharides

Starch is a heterogeneous mixture of two polymers of glucose.

Amylopectin consists of an amylose backbone with chains of [α(14) glycoside-bonded] glucose units branching off the C-6 hydroxyl groups by α(16) glycosidic bonds.

Each of the many branches contains 20-25 glucose units.

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6. Polysaccharides

Digestion of starch:

Two enzymes are produced in the pancreas and the salivary glands. α-Amylase cleaves the α(14) glycosidic bonds randomly along the

amylose chain to make shorter polysaccharides.

β-Amylase sequentially cleaves pairs of glucose units (the disaccharide maltose) from the reducing ends of amylose chains.

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6. Polysaccharides

Glycogen is the main glucose storage molecule.

Its structure is identical to amylopectin’s, except the branches are shorter, there are many more branches.

amylopectin

glycogen

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6. Polysaccharides

Cellulose is a straight-chain polymer of β-D-glucose units linked by β(14) glycosidic bonds.

Typical cellulose molecules contain about 3,000 glucose units. Cellulose is part of the structure of plant cell walls.

Humans (and all but a few animals) can’t digest cellulose because they lack the enzyme cellulase that hydrolyzes the β(14) glycosidic bond.

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6. Polysaccharides

What to know from this section: For amylose, amylopectin, glycogen, and cellulose:

the arrangement of the glucose units the type(s) of linkages

The functions of α-amylase and β-amylase Why humans can’t digest cellulose

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