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CHE 242

Unit VIIIThe Structure, Properties,

Reactions and Mechanisms of

Carboxylic Acids and TheirDerivatives

CHAPTER TWENTY-FOUR

Terrence P. Sherlock

Burlington County College2004

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Chapter 24 2

Structure of Proteins

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Chapter 24 3

Stereochemistry of

-Amino Acids

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Chapter 24 4

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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Chapter 24 5

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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Chapter 24 6

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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Chapter 24 7

Zwitterion

Amino acid exists as a dipolar ion.

-COOH loses H+, -NH2 gains H+.

Actualstructure depends on pH.

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Chapter 24 8

Properties of Amino Acids

High melting points, over 200C

More soluble in water than in ether.

Larger dipole moments than simpleacids or simple amines.

Less acidic than most carboxylic acids,

less basic than most amines.H3N CH

R

C

O

O+ _

pKa = 10 pKb = 12

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Chapter 24 9

Structure and pH

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Chapter 24 10

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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Chapter 24 11

Electrophoresis

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Chapter 24 12

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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Chapter 24 13

Structure of Peptide

The peptide bond is an amide bond. Amides are very stable and neutral.

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Chapter 24 14

Peptide Bond Formation

The amino group of one molecule condenseswith the acid group of another.

Polypeptides usually have molecular weight

less than 5000.

Protein molecular weight 6000-40,000,000.

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Chapter 24 15

Classification

of Proteins Simple: hydrolyze to amino acids only.

Conjugated: bonded to a nonproteingroup, such as sugar, nucleic acid, or

lipid.

Fibrous: long, stringy filaments, insolublein water, function as structure.

Globular: folded into spherical shape,function as enzymes, hormones, ortransport proteins.

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Chapter 24 16

Levels of

Protein Structure Primary: the sequence of the amino

acids in the chain and the disulfide links.

Secondary: structure formed byhydrogen 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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Chapter 24 17

Alpha Helix

Each carbonyl oxygen can hydrogenbond with an N-H hydrogen on the next

turn of the coil.

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Chapter 24 18

Pleated SheetEach carbonyl oxygen hydrogen

bonds with an N-H hydrogen on an

adjacent peptide chain.

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Chapter 24 19

Summary of Structure

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Chapter 24 20

Denaturation

Disruption of the normal structure of aprotein, such that it loses biological

activity. Usually caused by heat or changes in pH.

Usually irreversible. A cooked eggcannot be uncooked.

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Chapter 24 21

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Chapter 24 22

POWER POINT IMAGES FROM

ORGANIC CHEMISTRY, 5THEDITION

L.G. WADE

ALL MATERIALS USED WITH PERMISSION OF AUTHOR

PRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGE

ORGANIC CHEMISTRY COURSEBY:

ANNALICIA POEHLER STEFANIE LAYMAN CALY MARTIN