General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.1 Chapter 19 Amino Acids and Proteins 19.2 Amino Acids as Zwitterions.

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Presentation transcript:

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.1 Chapter 19 Amino Acids and Proteins 19.2 Amino Acids as Zwitterions

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.2 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 Zwitterions and Isoelectric Points

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.3 The isoelectric points (pI)  are the pH at which zwitterions have an overall zero charge  of nonpolar and polar (neutral) amino acids exist at pH values from 5.1 to 6.3 Isoelectric Point (pI)

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.4 In solutions that are more acidic than the pI,  the COO – in the zwitterion accepts a proton  the amino acid has a positive charge Glycine, with a pI of 6.0, has a 1+ charge in solutions that have a pH below pH 6.0. Zwitterions in Acidic Solutions

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.5 In solutions that are more basic than the pI,  the NH 3 + in the zwitterion loses a proton  the amino acid has a negative charge Glycine, with a pI of 6.0, has a 1 – charge in solutions that have a pH above pH 6.0. Zwitterions in Basic Solutions

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.6 pI, pH, and Charge

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.7 Summary of pH, pI, and Ionization

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 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 pH values from 2.8 to 3.2.  Zwitterions of polar (basic) amino acids exist at pH values from 7.6 to

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. Zwitterions of Aspartic Acid Aspartic acid, a polar (acidic) amino acid,  has a pI of 2.8  forms a zwitterion at pH 2.8  forms negative ions with charges 1 – and 2 – at pH values greater than pH 2.8 9

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.10 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 pI 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.11 Electrophoresis With an electric current, a mixture of lysine, aspartate, and valine are separated.

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

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