Chapter 19 Amino Acids and Proteins 19 2

Zwitterions and Isoelectric Points A zwitterion § has an equal number of —NH 3+

Isoelectric Point (p. I) The isoelectric points (p. I) § are the p. H

Zwitterions in Acidic Solutions In solutions that are more acidic than the p. I,

Zwitterions in Basic Solutions In solutions that are more basic than the p. I,

p. I, p. H, and Charge General, Organic, and Biological Chemistry Copyright © 2010

Summary of p. H, p. I, and Ionization General, Organic, and Biological Chemistry Copyright

Ionized Forms of Polar (Acidic) and Polar (Basic) Amino Acids Polar (acidic) and polar

Zwitterions of Aspartic Acid Aspartic acid, a polar (acidic) amino acid, § has a

Electrophoresis: Separation of Amino Acids In electrophoresis, an electric current is used to separate

Electrophoresis With an electric current, a mixture of lysine, aspartate, and valine are separated.

Learning Check Which structure represents: A. alanine at a p. H above its p.

Solution Which structure represents: A. alanine at a p. H above its p. I?

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Zwitterions and Isoelectric Points A zwitterion § has an equal number of —NH 3+ and COO– groups § forms when the H from —COOH in an amino acid transfers to the —NH 2 General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 2

Isoelectric Point (p. I) The isoelectric points (p. I) § are the p. H at which zwitterions have an overall zero charge § of nonpolar and polar (neutral) amino acids exist at p. H values from 5. 1 to 6. 3 General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 3

Zwitterions in Acidic Solutions In solutions that are more acidic than the p. I, § the COO– in the zwitterion accepts a proton § the amino acid has a positive charge Glycine, with a p. I of 6. 0, has a 1+ charge in solutions that have a p. H below p. H 6. 0. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 4

Zwitterions in Basic Solutions In solutions that are more basic than the p. I, § the NH 3+ in the zwitterion loses a proton § the amino acid has a negative charge Glycine, with a p. I of 6. 0, has a 1– charge in solutions that have a p. H above p. H 6. 0. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 5

Ionized Forms of Polar (Acidic) and Polar (Basic) Amino Acids Polar (acidic) and polar (basic) amino acids § also ionize the —COO and —NH 3+ in their polar R groups § Zwitterions of polar (acidic) amino acids exist at p. H values from 2. 8 to 3. 2. § Zwitterions of polar (basic) amino acids exist at p. H values from 7. 6 to 10. 8. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 8

Zwitterions of Aspartic Acid Aspartic acid, a polar (acidic) amino acid, § has a p. I of 2. 8 § forms a zwitterion at p. H 2. 8 § forms negative ions with charges 1– and 2– at p. H values greater than p. H 2. 8 General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 9

Electrophoresis: Separation of Amino Acids In electrophoresis, an electric current is used to separate a mixture of amino acids, and § the positively charged amino acids move toward the negative electrode § the negatively charged amino acids move toward the positive electrode § an amino acid at its p. I does not migrate § the amino acids are identified as separate bands on the filter paper or thin layer plate General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 10

Learning Check Which structure represents: A. alanine at a p. H above its p. I? B. alanine at a p. H below its p. I? General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 12

Solution Which structure represents: A. alanine at a p. H above its p. I? B. alanine at a p. H below its p. I? (2) (1) General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 13