CHE 242
Unit VIII
The Structure, Properties,
Reactions and Mechanisms of
Carboxylic Acids and Their
Derivatives
CHAPTER TWENTY-FOUR
Terrence P. Sherlock
Burlington County College
2004

Structure of Proteins
Chapter 24
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2

Stereochemistry of

-Amino Acids
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Essential Amino Acids
• Arginine (Arg)
• Threonine (Thr)
• Lysine (Lys)
• Valine (Val)
• Phenylalanine (Phe)
• Tryptophan (Trp)
• Methionine (Met)
• Histidine (His)
• Leucine (Leu)
• Isoleucine (Ile)
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4 Chapter 24

Complete Proteins
• Provide all the essential amino acids.
• Examples: those in meat, fish, milk, eggs.
• Plant proteins are generally incomplete.
• Vegetarians should eat many different kinds of plants, or supplement diet with milk or eggs.
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Rare Amino Acids
• 4-Hydroxyproline, 5-hydroxylysine found in collagen.
• D -Glutamic acid in cell walls of bacteria
• D -Serine in earthworms
• 
-Aminobutyric acid, a neurotransmitter
• 
-Alanine, constituent of the vitamin pantothenic acid.
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6

Zwitterion
• Amino acid exists as a dipolar ion.
• -COOH loses H + , -NH
2 gains H + .
• Actual structure depends on pH.
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Properties of Amino Acids
• High melting points, over 200 
C
• More soluble in water than in ether.
• Larger dipole moments than simple acids or simple amines.
• Less acidic than most carboxylic acids, less basic than most amines.
O
+ _
H
3
N CH C O p K a
= 10
R p K b
= 12
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Structure and pH
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9

Isoelectric Point
• pH at which amino acids exist as the zwitterion (neutral).
• Depends on structure of the side chain.
• Acidic amino acids, isoelectric pH ~3.
• Basic amino acids, isoelectric pH ~9.
• Neutral amino acids, isoelectric pH is slightly acidic, 5-6.
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=>
10

Electrophoresis
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11

Reaction with Ninhydrin
• Used to visualize spots or bands of amino acids separated by chromatography or electrophoresis.
• Deep purple color formed with traces of any amino acid. =>
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Structure of Peptide
• The peptide bond is an amide bond.
• Amides are very stable and neutral.
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Peptide Bond Formation
• The amino group of one molecule condenses with the acid group of another.
• Polypeptides usually have molecular weight less than 5000.
• Protein molecular weight 6000-40,000,000.
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Classification of Proteins
• Simple: hydrolyze to amino acids only.
• Conjugated: bonded to a nonprotein group, such as sugar, nucleic acid, or lipid.
• Fibrous: long, stringy filaments, insoluble in water, function as structure.
• Globular: folded into spherical shape, function as enzymes, hormones, or transport proteins.
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15

Levels of
Protein Structure
• Primary: the sequence of the amino acids in the chain and the disulfide links.
• Secondary: structure formed by hydrogen bonding. Examples are

helix and pleated sheet.
• Tertiary: complete 3-D conformation.
• Quaternary: association of two or more peptide chains to form protein. =>
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Alpha Helix
Each carbonyl oxygen can hydrogen bond with an N-H hydrogen on the next turn of the coil.
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17

Pleated Sheet
Each carbonyl oxygen hydrogen bonds with an N-H hydrogen on an adjacent peptide chain.
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18

Summary of Structure
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19

Denaturation
• Disruption of the normal structure of a protein, such that it loses biological activity.
• Usually caused by heat or changes in pH.
• Usually irreversible. A cooked egg cannot be “uncooked.”
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Chapter 24 21

POWER POINT IMAGES FROM
“ORGANIC CHEMISTRY, 5 TH EDITION”
L.G. WADE
ALL MATERIALS USED WITH PERMISSION OF AUTHOR
PRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGE
ORGANIC CHEMISTRY COURSE
BY:
ANNALICIA POEHLER STEFANIE LAYMAN CALY MARTIN
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