Textbook Question
Although tryptophan contains a heterocyclic amine, it is considered a neutral amino acid. Explain why the indole nitrogen of tryptophan is more weakly basic than one of the imidazole nitrogens of histidine.
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Ch. 24 - Amino Acids, Peptides, and Proteins
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
Chapter 24, Problem 4e(iii)
Draw the structure of the predominant form of
(e) a mixture of alanine, lysine, and aspartic acid at (iii) pH 2.
Verified step by step guidance1
Step 1: Understand the problem. At pH 2, the environment is highly acidic. This means that the amino acids will predominantly exist in their protonated forms. Amino acids have functional groups that can gain or lose protons depending on the pH, such as the amino group (-NH₂), carboxylic acid group (-COOH), and side chain groups.
Step 2: Analyze the structure of alanine. Alanine has a simple side chain (-CH₃). At pH 2, the amino group (-NH₂) will be protonated to form -NH₃⁺, and the carboxylic acid group (-COOH) will remain protonated.
Step 3: Analyze the structure of lysine. Lysine has a side chain with an amino group (-NH₂). At pH 2, the amino group in the side chain will also be protonated to form -NH₃⁺, along with the main amino group (-NH₃⁺). The carboxylic acid group (-COOH) will remain protonated.
Step 4: Analyze the structure of aspartic acid. Aspartic acid has a side chain with a carboxylic acid group (-COOH). At pH 2, both the main carboxylic acid group and the side chain carboxylic acid group will remain protonated, and the amino group (-NH₂) will be protonated to form -NH₃⁺.
Step 5: Combine the information. Draw the structures of alanine, lysine, and aspartic acid with their respective protonated forms at pH 2. Ensure that all amino groups are protonated (-NH₃⁺) and all carboxylic acid groups are in their protonated form (-COOH). This represents the predominant form of the mixture at pH 2.
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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 bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R group) that determines its unique properties. Understanding the structure of amino acids is essential for predicting their behavior in different pH environments.
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Charged Amino Acids
pH and Ionization
pH is a measure of the acidity or basicity of a solution, influencing the ionization state of amino acids. At low pH (like pH 2), amino acids tend to be protonated, meaning the carboxyl group (-COOH) remains protonated, while the amino group (-NH2) is also protonated to form -NH3+. This affects the overall charge and structure of the amino acids in solution.
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Zwitterions
Zwitterions are molecules that have both positive and negative charges but are overall neutral. At physiological pH, many amino acids exist as zwitterions, where the amino group is positively charged and the carboxyl group is negatively charged. Understanding zwitterionic forms is crucial for predicting the predominant species of amino acids in different pH conditions, such as the acidic environment at pH 2.
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Related Practice
Textbook Question
Draw the electrophoretic separation of Ala, Lys, and Asp at pH 9.7.
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Textbook Question
Draw the structure of the predominant form of
(d) glutamic acid at pH 7.
1
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Textbook Question
Draw the structure of the predominant form of
(b) proline at pH 2.
2
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Textbook Question
Draw the structure of the predominant form of
(e) a mixture of alanine, lysine, and aspartic acid at (i) pH 6;
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Textbook Question
Draw the structure of the predominant form of
(a) isoleucine at pH 11.
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