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Chapter 19 Amino Acids and Proteins

Chapter 19 Amino Acids and Proteins. 19.2 Amino Acids as Zwitterions. 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. Isoelectric Point (pI).

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Chapter 19 Amino Acids and Proteins

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  1. Chapter 19 Amino Acids and Proteins 19.2Amino Acids as Zwitterions

  2. 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

  3. 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

  4. 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.

  5. 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.

  6. pI, pH, and Charge

  7. Summary of pH, pI, and Ionization

  8. 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.

  9. 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

  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

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

  12. Learning Check Which structure represents: A. alanine at a pH above its pI? B. alanine at a pH below its pI?

  13. Solution Which structure represents: A. alanine at a pH above its pI? (2) B. alanine at a pH below its pI? (1)

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