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CHE 242 Unit VIII The Structure, Properties, Reactions and Mechanisms of Carboxylic Acids and Their Derivatives CHAPTER - PowerPoint PPT Presentation

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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. =>. Stereochemistry of - Amino Acids. =>. Arginine (Arg) Threonine (Thr)

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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


Structure of proteins l.jpg
Structure of Proteins


Chapter 24

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Stereochemistry of-Amino Acids


Chapter 24

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Arginine (Arg)

Threonine (Thr)

Lysine (Lys)

Valine (Val)

Phenylalanine (Phe)

Tryptophan (Trp)

Methionine (Met)

Histidine (His)

Leucine (Leu)

Isoleucine (Ile) =>

Essential Amino Acids

Chapter 24

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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. =>

Chapter 24

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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. =>

Chapter 24

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  • Amino acid exists as a dipolar ion.

  • -COOH loses H+, -NH2 gains H+.

  • Actualstructure depends on pH. =>

Chapter 24

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pKb = 12

pKa = 10


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.

Chapter 24

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Structure and pH


Chapter 24

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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. =>

Chapter 24

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Chapter 24

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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. =>

Chapter 24

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Structure of Peptide

  • The peptide bond is an amide bond.

  • Amides are very stable and neutral.

Chapter 24

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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. =>

Chapter 24

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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. =>

Chapter 24

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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. =>

Chapter 24

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Alpha Helix

Each carbonyl oxygen can hydrogen bond with an N-H hydrogen on the next turn of the coil.

Chapter 24

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Pleated Sheet

Each carbonyl oxygen hydrogen

bonds with an N-H hydrogen on an

adjacent peptide chain.

Chapter 24

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Summary of Structure


Chapter 24

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  • 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.” =>

Chapter 24

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Chapter 24