Table of contents

1. Matter and Measurements4h 31m
2. Atoms and the Periodic Table5h 23m
3. Ionic Compounds2h 18m
4. Molecular Compounds2h 18m
5. Classification & Balancing of Chemical Reactions3h 17m
6. Chemical Reactions & Quantities2h 27m
7. Energy, Rate and Equilibrium3h 46m
8. Gases, Liquids and Solids3h 25m
9. Solutions4h 10m
10. Acids and Bases3h 29m
11. Nuclear Chemistry56m
BONUS: Lab Techniques and Procedures1h 38m
BONUS: Mathematical Operations and Functions47m
12. Introduction to Organic Chemistry1h 34m
13. Alkenes, Alkynes, and Aromatic Compounds2h 12m
14. Compounds with Oxygen or Sulfur1h 6m
15. Aldehydes and Ketones1h 1m
16. Carboxylic Acids and Their Derivatives1h 11m
17. Amines39m
18. Amino Acids and Proteins1h 51m
19. Enzymes1h 37m
20. Carbohydrates1h 41m
21. The Generation of Biochemical Energy2h 8m
22. Carbohydrate Metabolism2h 22m
23. Lipids2h 26m
24. Lipid Metabolism1h 45m
25. Protein and Amino Acid Metabolism1h 37m
26. Nucleic Acids and Protein Synthesis2h 54m

18. Amino Acids and Proteins

Amino Acid Classifications

GOB Chemistry

18. Amino Acids and Proteins

Amino Acid Classifications

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Problem 13

Textbook Question

Draw the structure of glutamic acid at low pH, at high pH, and at the two forms that exist between low pH and high pH. Which of these structures represents the zwitterion?

Verified step by step guidance

Understand the structure of glutamic acid: Glutamic acid is an amino acid with the formula C₅H₉NO₄. It contains an amino group (-NH₂), a carboxylic acid group (-COOH), and a side chain with another carboxylic acid group (-CH₂-COOH).

At low pH (acidic conditions): All acidic and basic groups are protonated. The amino group (-NH₂) becomes -NH₃⁺, and both carboxylic acid groups (-COOH) remain in their protonated form. Draw the structure with these groups in their protonated states.

At high pH (basic conditions): All acidic and basic groups are deprotonated. The amino group (-NH₃⁺) loses a proton to become -NH₂, and both carboxylic acid groups (-COOH) lose protons to become -COO⁻. Draw the structure with these groups in their deprotonated states.

Between low and high pH (intermediate pH): Glutamic acid exists in two forms. The first form is the zwitterion, where the amino group is protonated (-NH₃⁺) and one carboxylic acid group is deprotonated (-COO⁻), while the other carboxylic acid group remains protonated (-COOH). The second form occurs when both carboxylic acid groups are deprotonated (-COO⁻), and the amino group remains protonated (-NH₃⁺). Draw both structures.

Identify the zwitterion: The zwitterion is the form of glutamic acid where the molecule has both a positive charge (on the -NH₃⁺ group) and a negative charge (on one -COO⁻ group), but the overall charge is neutral. Confirm this by examining the structure drawn in the previous step.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Amino Acid Structure

Amino acids, the building blocks of proteins, have a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable R group. The structure of an amino acid can change based on the pH of the environment, affecting the ionization of the amino and carboxyl groups.

Zwitterion

A zwitterion is a molecule that has both positive and negative charges but is overall electrically neutral. In the case of amino acids like glutamic acid, at a specific pH (usually around the isoelectric point), the amino group is protonated (positively charged) while the carboxyl group is deprotonated (negatively charged), resulting in a zwitterionic form.

pH and Ionization

pH is a measure of the acidity or basicity of a solution, influencing the ionization state of functional groups in amino acids. At low pH, amino acids tend to be fully protonated, while at high pH, they are mostly deprotonated. Understanding how pH affects the ionization of glutamic acid is crucial for drawing its structures at different pH levels.

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Related Practice

Textbook Question

Classify each amino acid in Problem 10.4 as polar (neutral, acidic, or basic) or nonpolar and as hydrophobic or hydrophilic.

a. leucine

b. glutamate

c. methionine

d. threonine

907

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Textbook Question

Isoleucine has the zwitterion structure shown. Draw the structure and give the net charge of isoleucine that will predominate at the indicated pH values (pI = 6.0).

b. pH 6.0

695

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Textbook Question

Glycine has the zwitterion structure shown. Draw the structure and give the net charge of glycine that will predominate at the indicated pH values (pI = 6.0).

b. pH 12.0

512

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Textbook Question

Consult Table 18.3 and draw alanine. Label the functional groups and give the three-letter abbreviation and the one-letter abbreviation. What group does the side chain fall into?

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Textbook Question

Name and draw the structures of the amino acids that fit the following descriptions:

a. Contains an isopropyl group

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Textbook Question

Name and draw the structures of the amino acids that fit the following descriptions:

a. Contains a thiol group

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Textbook Question

At neutral pH, which of the following amino acids has a net positive charge, which has a net negative charge, and which is neutral? (Hint: Draw the various charged forms of each amino acid before deciding.)

a. Asparagine

b. Lysine

c. Proline

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Textbook Question

Which of the following forms of aspartic acid would you expect to predominate at low pH, neutral pH, and high pH?

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