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(i)
Draw the structure of the predominant form of
(e) a mixture of alanine, lysine, and aspartic acid at (i) pH 6;
Verified step by step guidance1
Step 1: Understand the problem. You are tasked with determining the predominant form of alanine, lysine, and aspartic acid at pH 6. This involves analyzing the ionization states of their functional groups (amino, carboxyl, and side chains) based on their pKa values.
Step 2: Recall the pKa values of the relevant functional groups. For alanine, the amino group has a pKa of approximately 9.7, and the carboxyl group has a pKa of approximately 2.3. For lysine, the amino group has a pKa of approximately 9.0, the carboxyl group has a pKa of approximately 2.2, and the side chain amino group has a pKa of approximately 10.5. For aspartic acid, the amino group has a pKa of approximately 9.8, the carboxyl group has a pKa of approximately 2.1, and the side chain carboxyl group has a pKa of approximately 3.9.
Step 3: Determine the ionization state of each functional group at pH 6. At this pH, groups with pKa values below 6 will be deprotonated (negative charge for carboxyl groups), and groups with pKa values above 6 will remain protonated (positive charge for amino groups). For alanine, the amino group will be protonated (+1), and the carboxyl group will be deprotonated (-1). For lysine, the amino group will be protonated (+1), the carboxyl group will be deprotonated (-1), and the side chain amino group will remain protonated (+1). For aspartic acid, the amino group will be protonated (+1), the carboxyl group will be deprotonated (-1), and the side chain carboxyl group will be deprotonated (-1).
Step 4: Combine the charges to determine the net charge of each amino acid at pH 6. Alanine will have a net charge of 0 (+1 from the amino group and -1 from the carboxyl group). Lysine will have a net charge of +1 (+1 from the amino group, -1 from the carboxyl group, and +1 from the side chain amino group). Aspartic acid will have a net charge of -1 (+1 from the amino group, -1 from the carboxyl group, and -1 from the side chain carboxyl group).
Step 5: Draw the structures of alanine, lysine, and aspartic acid at pH 6, showing the ionization states of their functional groups. Ensure that the amino groups are protonated where appropriate, the carboxyl groups are deprotonated, and the side chains reflect their correct ionization states. Label each structure clearly to indicate the amino acid and its net charge.
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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 a specific pH, amino acids can exist in different forms: cationic, zwitterionic, or anionic. At pH 6, which is near the physiological pH, amino acids like alanine, lysine, and aspartic acid will have distinct charges that affect their overall structure and interactions.
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Zwitterion Formation
A zwitterion is a molecule that has both positive and negative charges but is overall neutral. At physiological pH, amino acids typically exist as zwitterions, where the amino group is protonated (-NH3+) and the carboxyl group is deprotonated (-COO-). This form is crucial for understanding the solubility and reactivity of amino acids in biological systems.
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Related Practice
Textbook Question
Draw the structure of the predominant form of
(e) a mixture of alanine, lysine, and aspartic acid at (iii) pH 2.
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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.
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Textbook Question
Draw the structure of the predominant form of
(b) proline at pH 2.
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Textbook Question
Draw the electrophoretic separation of Trp, Cys, and His at pH 6.0.
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