Textbook Question
Which of the following forms of aspartic acid would you expect to predominate at low pH, neutral pH, and high pH?
a.
b.
c.
Ch.18 Amino Acids and Proteins
McMurry8th EditionFundamentals of General, Organic, and Biological ChemistryISBN: 9780134015187
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Table of contents
- Ch.1 Matter and Measurements27
- Ch.2 Atoms and the Periodic Table20
- Ch.3 Ionic Compounds8
- Ch.4 Molecular Compounds23
- Ch.5 Classification & Balancing of Chemical Reactions12
- Ch.6 Chemical Reactions: Mole and Mass Relationships28
- Ch.7 Chemical Reactions: Energy, Rate and Equilibrium64
- Ch.8 Gases, Liquids and Solids24
- Ch.9 Solutions52
- Ch.10 Acids and Bases25
- Ch.11 Nuclear Chemistry22
- Ch.12 Introduction to Organic Chemistry: Alkanes57
- Ch.13 Alkenes, Alkynes, and Aromatic Compounds47
- Ch.14 Some Compounds with Oxygen, Sulfur, or a Halogen50
- Ch.15 Aldehydes and Ketones45
- Ch.16 Amines42
- Ch.17 Carboxylic Acids and Their Derivatives57
- Ch.18 Amino Acids and Proteins82
- Ch.19 Enzymes and Vitamins63
- Ch.20 Carbohydrates44
- Ch.21 The Generation of Biochemical Energy65
- Ch.22 Carbohydrate Metabolism45
- Ch.23 Lipids42
- Ch.24 Lipid Metabolism33
- Ch.25 Protein and Amino Acid Metabolism24
- Ch.26 Nucleic Acids and Protein Synthesis45
McMurry8th EditionFundamentals of General, Organic, and Biological ChemistryISBN: 9780134015187Not the one you use?Change textbook
Chapter 18, Problem 52
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
Verified step by step guidance1
Step 1: Understand the problem. At neutral pH (approximately pH 7), amino acids exist in their zwitterionic form, where the amino group is protonated (NH₃⁺) and the carboxyl group is deprotonated (COO⁻). The net charge of the amino acid depends on the side chain (R group). Analyze the side chains of each amino acid to determine their charge at neutral pH.
Step 2: Analyze asparagine. Asparagine has a side chain with an amide group (-CONH₂), which does not ionize at physiological pH. Therefore, asparagine is neutral at pH 7.
Step 3: Analyze lysine. Lysine has a side chain with an amino group (-NH₂) that can accept a proton to become positively charged (-NH₃⁺) at neutral pH. Therefore, lysine has a net positive charge at pH 7.
Step 4: Analyze proline. Proline has a cyclic structure where the amino group is part of the ring, but its side chain does not ionize at physiological pH. Therefore, proline is neutral at pH 7.
Step 5: Summarize the results. At neutral pH, asparagine is neutral, lysine has a net positive charge, and proline is neutral. Drawing the structures of each amino acid in their zwitterionic forms can help visualize these charges.
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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 are organic compounds that serve as the building blocks of proteins. Each amino acid has a central carbon atom, an amino group (–NH2), a carboxyl group (–COOH), a hydrogen atom, and a variable side chain (R group). The properties of the side chain determine the amino acid's characteristics, including its charge at different pH levels.
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Ionization of Amino Acids
At different pH levels, amino acids can gain or lose protons, leading to changes in their charge. At neutral pH (around 7), amino acids exist in a zwitterionic form, where the amino group is protonated (positively charged) and the carboxyl group is deprotonated (negatively charged). This ionization affects the overall net charge of the amino acid.
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Net Charge of Amino Acids
The net charge of an amino acid at a given pH is determined by the balance of its ionized groups. For example, lysine has a positively charged side chain at neutral pH, giving it a net positive charge, while asparagine is neutral, and proline also remains neutral due to its side chain structure. Understanding these charges is crucial for predicting amino acid behavior in biological systems.
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Related Practice
Textbook Question
Which form of aspartic acid in Problem 18.54 is the zwitterion? What is the pI for the zwitterion?
a.
b.
c.
Textbook Question
Draw leucine and identify any chiral carbon atoms with arrows.
1
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Textbook Question
Proteins are usually least soluble in water at their isoelectric points. Explain.
Textbook Question
Is phenylalanine hydrophilic or hydrophobic? Explain why.
Textbook Question
Name and draw the structures of the amino acids that fit the following descriptions:
a. Contains an isopropyl group