Amino Acids Proteins, and Enzymes

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

has an equal number of NH3+ and COO groups forms when the H from COOH in an amino acid transfers to the NH2 General, Organic, and Biological Chemistry Copyright 2010 Pearson Education, Inc. 2 Isoelectric Point (pI) 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 General, Organic, and Biological Chemistry Copyright 2010 Pearson Education, Inc. 3 Zwitterions in Acidic Solutions 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. General, Organic, and Biological Chemistry Copyright 2010 Pearson Education, Inc. 4 Zwitterions in Basic Solutions In solutions that are more basic than the pI, the NH3+ 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. General, Organic, and Biological Chemistry Copyright 2010 Pearson Education, Inc. 5 pI, pH, and Charge General, Organic, and Biological Chemistry

Copyright 2010 Pearson Education, Inc. 6 Summary of pH, pI, and Ionization General, Organic, and Biological Chemistry Copyright 2010 Pearson Education, Inc. 7

Ionized Forms of Polar (Acidic) and Polar (Basic) Amino Acids Polar (acidic) and polar (basic) amino acids also ionize the COO and NH3+ 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 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 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 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 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. 10 Electrophoresis With an electric current, a mixture of lysine, aspartate, and valine are separated. General, Organic, and Biological Chemistry

Copyright 2010 Pearson Education, Inc. 11 Learning Check Which structure represents: A. alanine at a pH above its pI? B. alanine at a pH below its pI? General, Organic, and Biological Chemistry

Copyright 2010 Pearson Education, Inc. 12 Solution Which structure represents: A. alanine at a pH above its pI? B. alanine at a pH below its pI? General, Organic, and Biological Chemistry (2)

(1) Copyright 2010 Pearson Education, Inc. 13

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