CHE 242 Unit VIII The Structure, Properties, Reactions and Mechanisms of Carboxylic Acids and Their Derivatives CHAPTER TWENTY-FOUR

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CHE 242Unit VIIIThe Structure, Properties,
Reactions and Mechanisms of Carboxylic Acids and
Their DerivativesCHAPTER TWENTY-FOUR

• Terrence P. Sherlock
• Burlington County College
• 2004

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Structure of Proteins
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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) gt

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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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Zwitterion

• Amino acid exists as a dipolar ion.
• -COOH loses H, -NH2 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.

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Structure and pH
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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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Electrophoresis
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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. gt

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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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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. gt

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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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Pleated Sheet
Each carbonyl oxygen hydrogen bonds with an N-H
hydrogen on an adjacent peptide chain.
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Summary of Structure
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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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POWER POINT IMAGES FROM ORGANIC CHEMISTRY, 5TH
EDITIONL.G. WADEALL MATERIALS USED WITH
PERMISSION OF AUTHORPRESENTATION ADAPTED FOR
BURLINGTON COUNTY COLLEGEORGANIC CHEMISTRY
COURSEBYANNALICIA POEHLER STEFANIE LAYMAN
CALY MARTIN