BRCC CHM 102 Class Notes Chapter 20 Page 1 of 12

Chapter 20 Carbohydrates The major classes of organic compounds common to living systems are lipids, proteins, nucleic acids, and carbohydrates. Many carbohydrates are what we call Asugars.@

• Carbohydrates are the most abundant organic compounds in the plant world.

• They make up more than half of the total organic carbon in the world.

• Carbohydrates have many uses in both plants and animals:

o store chemical energy (glucose, starch, glycogen) o act as structural support (cellulose in wood, chitin in mollusks, and connective tissue in animals) o are components of nucleic acids (RNA)

Where do the terms used in carbohydrate chemistry come from? saccharum - Latin for Asugar@ saccharide - term used in classifying carbohydrates monosaccharide - simple carbohydrate - cannot be broken into smaller carbohydrates disaccharide - can be cleaved by hydrolysis into 2 monosaccharides

ex. sucrose (table sugar) on hydrolysis yields 1 molecule of glucose and 1 molecule of fructose oligosaccharide - (oligos is Greek for Afew@) a carbohydrate made of 3 to 10 monosaccharide units polysaccharide - a carbohydrate made of more than 10 monosaccharide units - ex. cellulose Monosaccharides can be grouped as polyhydroxy aldehydes, aldoses, or as polyhydroxy ketones, ketoses.

OH OH OH OH OH | | | | |

glucose CH2 - CH - CH - CH - CH - CHO » aldehyde group carbohydrates are properly defined as polyhydroxy aldehydes or ketones Green plants synthesize simple carbohydrates (mainly glucose) through photosynthesis whereby CO2 from the air is combined with water to produce sugar. Plants use carbohydrates for storing energy and for structure. Animals eat plants and convert carbohydrates to energy and other stuff.

Oxygen Carbon Dioxide Cycle carbohydrates + O2 » High Energy 8 9 CO2 + H2O » Low Energy

BRCC CHM 102 Class Notes Chapter 20 Page 2 of 12

Fischer Projections: Fischer projections - a shorthand way to draw stereomolecules for non-artists (ie. chemists and students) - proposed by Emil Fischer - they are used to determine if 2 molecules are identical or different and whether 2 formulas are mirror images.

Rules: 1. To test 2 molecules, you cannot take the drawings out

of the plane of the paper when you turn them. 2. You can rotate the drawings 180 degrees, but not 90

degree.

Examples: Are these mirror images superimposable?

H3CH2C

CH3

H

Cl

H

CH3

CH2CH3

Cl

A

B

OH

CH3

H

CH3

H

CH3

OH

CH3

C D Answers: A & B are not superimposable. C & D are superimposable. Molecules may have more than one chiral carbon. Consider 2,3-dihydroxybutanal as an example. Here are the Fischer projections:

H

CHO

OH

CH3

H OH

OH

CHO

H

CH3

OH H

H

CHO

OH

CH3

OH H

OH

CHO

H

CH3

H OH

A

B

C

D In this example, A and B are a pair of enantiomers. C and D are another pair of enantiomers. They are 2 sets of mirror images.

BRCC CHM 102 Class Notes Chapter 20 Page 3 of 12

• Recall that all physical and chemical properties of enantiomers are identical except:

o they rotate polarized light in opposite directions o they react with other chiral molecules at different rates (major biochemical consequences).

• Enantiomers have identical melting points, boiling points, densities, and rates of reaction with achiral

molecules. What are the relationships between A and C or B and D? They are NOT enantiomers, but they ARE stereoisomers. However, all physical and chemical properties of A and B are different from C and D. Stereoisomers that are not enantiomers are called diastereomers. A and B are diastereomers of C and D. D- and L- Monosaccharides

• The R,S system is the most widely accepted modern standard for designating configuration.

• The configurations of carbohydrates and amino acids are usually designated by an older system proposed by Fischer called the D,L system.

o He made an arbitrary assignment of dextrorotary or D to glyceraldehyde, naming it D-

glyceraldehyde. The enantiomer was named L-glyceraldehyde.

o These are the reference points for the assignment of relative configuration to all other aldoses and ketoses.

o To make an assignment of D or L, look for the chiral carbon (stereocenter) farthest from the

carbonyl group. This is called the penultimate carbon. Which way is the OH group pointing?

D points to the right L points to the left

• The 3 most abundant hexoses in the biological world are D-glucose, D-galactose, and D-fructose.

H

CHO

OH

HHO

OHH

OHH

CH2OH

H

CHO

OH

HHO

HHO

OHH

CH2OH

CH2OH

O

HO H

H O

H O

CH2OH

H

H

D-glucose

D-galactose

D-fructose

BRCC CHM 102 Class Notes Chapter 20 Page 4 of 12

Monosaccharides

• Monosaccharides are the basic building blocks of carbohydrates.

• The general formula for a monosaccharide is CnH2nOn where n = 3 to 9.

• The most important carbohydrate is D-glucose. C6H12O6 o The major constituent of polysaccharides. o It is also called dextrose, grape sugar, or blood sugar. o Exists mainly in the cyclic form.

HO

OH

H

HO

H

OHOH

H

H

HO

• This is the chair conformation.

• Represents the stereochemistry of the glucose molecule.

• Hard to draw routinely.

It is much easier to draw a Haworth projection, whereby the ring is represented as a planar pentagon or hexagon.

aldehyde form cyclic forms

• Because carbon 1 (C1) is attached to an –OH and an –OR, this makes a hemiacetal.

• Though hemiacetals are usually unstable, they are much more stable in a ring. In fact, glucose only exists in the hemiacetal form as a solid.

• When dissolved in water, the ring can open and close.

• The aldehyde form is much less stable than the hemiacetal form, so it will reclose into a ring.

• There are 2 ways it can close

o with the –OH at C1 pointing down (called the α form) o with the –OH at C1 pointing up (called the β form)

• The C1 carbon is called an anomeric carbon and the alpha and beta forms are called anomers.

• The aldehyde form cannot be obtained as a solid.

• Both ring forms can be obtained as a solid.

BRCC CHM 102 Class Notes Chapter 20 Page 5 of 12

• All 3 forms are optically active.

• When the alpha and beta form is dissolved in water, the amount of rotation changes because it

becomes a mixture of the 3 forms (racemic mixture). This is called mutarotation.

• Alpha and beta forms are not enantiomers because they are not mirror images – they are diastereomers.

H O

OH

H

OH

H

OHH

OH

CH2OH

» OH on C1 points down so this is the α anomer

H O

OH

OH

H

H

OHH

OH

CH2OH

» OH on C1 points up so this is the β anomer

α–D–glucose β–D–glucose Aldohexoses

• The open chain form of glucose has 4 chiral carbons. Therefore there are 24 or 16 isomers.

• These isomers have the same structure, but different conformations at the 4 chiral carbons.

• Together, they are called aldohexoses – “aldo” from aldehyde, “hex” from 6 carbons, and “ose” as the ending for carbohydrates.

• The 16 aldohexoses are made up of 8 pairs of enantiomers. The common aldohexoses are –D sugars.

• Aldohexoses differ in whether the –OH groups point to the left or right in the Fischer

projection or up or down in the Haworth projection.

You can have an aldose with just 4 carbons, such as D–Erythrose.

H

CHO

OH

CH2OH

H OH

BRCC CHM 102 Class Notes Chapter 20 Page 6 of 12

You can also have an aldose with 5 carbons, such as D–Ribose.

H

CHO

OH

H O

CH2OH

H O

H

H

Ketoses are similar to aldoses, except that there is a ketone group on carbon number 2 instead of an aldehyde on carbon number 1. An important 6-carbon ketose (ketohexose) is D-fructose or fruit sugar. It is much sweeter than glucose or sucrose (table sugar).

CH2OH

O

HO H

H O

H O

CH2OH

H

H

D–fructose

Properties of Monosaccharides Physical Properties:

• Taste – sweet • Solids at room temperature • Solubility – extremely soluble in water because of all the OH (hydroxyl) groups – very

concentrated solutions are thick (ex. honey, molasses, etc) • Optical rotation – monosaccharides show optical activity – all 3 forms of D–glucose rotate

the plane of light to the right.

Note: Not all D series monosaccharides rotate the plane of light to the right – there is no simple relationship between conformation and rotation. ex. D-fructose rotates light to the left.

Chemical Properties:

• All monosaccharides with at least 5 carbons exist mainly in the cyclic form.

BRCC CHM 102 Class Notes Chapter 20 Page 7 of 12

o 6 membered rings are called pyranose rings (ex. glucose) o 5 membered rings are called furanose rings (ex. fructose)

• All cyclic forms show mutarotation.

• All aldoses have an aldehyde group and can be easily oxidized to carboxylic acids. For this reason, they are called reducing sugars.

• Ketoses have a hydroxyl group on the carbon next to the carbonyl group, which can be oxidized. They are also called reducing sugars.

• Benedict’s Solution is used to test for reducing sugars. All monosaccharide aldoses and ketoses give a positive test.

Glycoside Formation * Aldehydes react with alcohols to form hemiacetals and acetals. D-glucose is a hemiacetal.

H O

OH

H

OH

H

OHH

OH

CH2OH

<-------- hemiacetal at Carbon #1

* If the hemiacetal in D-glucose reacts with an alcohol, an acetal forms, which is called a

glycoside. Glycoside rings do not open when dissolved in water. The ring is locked.

H O

OH

OCH3

H

H

OHH

OH

CH2OH

H O

OH

H

OH

H

OHH

OH

CH2OH

+ CH3OH

H O

OH

H

OCH3

H

OHH

OH

CH2OH

glycosides (locked rings)

* Notice that there are two possible products. The form shown on top is beta and the form shown

on the bottom is alpha.

* This particular reaction does not appear very useful, however, the formation of glycoside linkages is how monosaccharides link together to make bigger sugars as we shall see.

BRCC CHM 102 Class Notes Chapter 20 Page 8 of 12

Monosaccharide Derivatives * If one or more functional groups of a monosaccharide have been chemically changed, it is called

a derivative. These are important in biochemistry.

Reduction:

H

CHO

OH

HHO

OHH

OHH

CH2OH

+ H2 ---------->

H

CH2OH

OH

HHO

OHH

OHH

CH2OH

<--- carbonyl group reduced to an alcohol

D-glucose D-sorbitol

Oxidation: 2 kinds of oxidation

H

CHO

OH

HHO

OHH

OHH

CH2OH

H

COOH

OH

HHO

OHH

OHH

CH2OH

<----- oxidized to a carboxylic acid D-gluconic acid

--------------> | | | | | | | | ------->

H

CHO

OH

HHO

OHH

OHH

COOH

D-glucuronic acid <----- oxidized to a carboxylic acid

Disaccharides * Monosaccharides can be converted to glycosides by reactions with alcohols. When the alcohol

is from another monosaccharide, they can form disaccharides.

BRCC CHM 102 Class Notes Chapter 20 Page 9 of 12

example 1

* The glycoside bond is from the C-1 carbon of one glucose to the C-4 carbon of another glucose.

This is an alpha α(1→4) linkage.

* The first glucose ring has an acetal functional group. The ring can no longer open and close. It is locked. The second glucose ring can still open and close. It still shows mutarotation.

example 2

β(1→4) linkage Which ring is locked? Which ring is unlocked?

example 3

H O

OH

H

O

H

OHH

OH

CH2OH

O

CH2OH

OH

HOH2C

OH

Sucrose – D-glucose and fructose linked together Both rings are locked. This is a non-reducing sugar. α(1→2) linkage.

BRCC CHM 102 Class Notes Chapter 20 Page 10 of 12

Blood Groups * ABO Blood Group System – possible blood types are A, B, AB, or O

* The blood type depends on the oligosaccharides attached to the surface of the red blood cells

(RBCs) as markers.

* These are genetically determined. A person can have the enzyme that puts A type saccharides on the surface of RBCs, or the enzyme that puts B type saccharides on RBCs, or neither.

* Blood carries antibodies against foreign substances, identified by markers on the outside of cells. Some bacteria, like those living in the intestine, have similar markers to A & B.

* If a person receives the wrong type of blood, antibodies will cause the blood to clump, which can be fatal.

Blood Table Donor’s Blood Type

Person’s Type O A B AB O A B AB * Rh factor – If the Rh factor is present, then it is called Rh positive. If the Rh vactor is not present,

then it is called Rh negative.

* If you give someone with Rh negative blood some Rh positive blood, then this will cause an antibody response. This is a problem for mothers who are Rh negative and have babies who are Rh positive.

Acidic Polysaccharides * Acidic Polysaccharides are a group of polysaccharides important in connective tissues. See text.

* Hyaluronic acid – macromolecule containing 300 – 100,000 repeating units abundant in

embryonic tissues and also maintains the retina in the eye, and makes up the synovial fluid in the joints as lubricant.

* Heparin – an abundant acidic polysaccharide found in many body organs and has many functions. One important function is preventing blood clotting.

BRCC CHM 102 Class Notes Chapter 20 Page 11 of 12

BRCC CHM 102 Class Notes Chapter 20 Page 12 of 12

* Amylopectin is a macromolecule. The straight chain portions have 20 – 25 glucose units

between branch points and may have 100,000 – 1,000,000 glucose units.

* Starch is a mixture of amylose and amylopectin.

* The test for starch is the iodine test. Iodine atoms assemble inside the amylose helixes to give intense purple color.

* Glycogen, sometimes called animal starch, is similar to amylopectin. It is a macromolecule with a greater degree of branching. Animals make glycogen when glucose is abundant.

* Cellulose is a linear polymer of D-glucose units linked in a β(1→4) linkage.

* Cellulose is the most abundant molecule in living tissue. 50% of organic carbon in the

biosphere is cellulose. Wood is made of 50% cellulose and cotton is made of 95% cellulose.

* It provides plants with strength and rigidity.

* Molecules of cellulose align side by side into water insoluble lattices or fibers. The OH groups H-bond with neighboring chains to hold the chains together.

* Cellulose provides dietary fiber for animals – nondigestible material to provide bulk in the diet.