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 4d
Draw the structure of the predominant form of
(d) glutamic acid at pH 7.
Verified step by step guidance1
Identify the functional groups in glutamic acid: Glutamic acid contains an amino group (-NH₂), a carboxylic acid group (-COOH) attached to the alpha carbon, and a second carboxylic acid group (-COOH) in the side chain.
Determine the pKa values of the functional groups: The pKa values for glutamic acid are approximately 2.1 for the alpha carboxylic acid, 4.1 for the side chain carboxylic acid, and 9.7 for the amino group.
Compare the pH (7) to the pKa values to determine the protonation state of each group: At pH 7, the alpha carboxylic acid (pKa = 2.1) and the side chain carboxylic acid (pKa = 4.1) will both be deprotonated (-COO⁻), while the amino group (pKa = 9.7) will remain protonated (-NH₃⁺).
Draw the zwitterionic structure of glutamic acid: Represent the molecule with the alpha carbon bonded to the deprotonated alpha carboxylate group (-COO⁻), the protonated amino group (-NH₃⁺), the hydrogen atom, and the side chain containing the deprotonated carboxylate group (-COO⁻).
Verify the overall charge of the molecule: At pH 7, the molecule should have a net charge of -1, as there are two negatively charged carboxylate groups (-COO⁻) and one positively charged amino group (-NH₃⁺).
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Amino Acids
Amino acids are organic compounds that serve as the building blocks of proteins. Each amino acid contains 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 characteristics of the amino acid, including its polarity and charge, which are crucial for understanding its behavior in different pH environments.
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Standard Amino Acids
pH and Ionization
pH is a measure of the acidity or basicity of a solution, influencing the ionization state of molecules. At pH 7, which is neutral, amino acids can exist in a zwitterionic form, where the amino group is protonated (-NH3+) and the carboxyl group is deprotonated (-COO-). This ionization affects the overall charge and solubility of the amino acid, which is essential for drawing its predominant structure at this pH.
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Glutamic Acid Structure
Glutamic acid is a polar, negatively charged amino acid at physiological pH due to its side chain containing a carboxyl group. Its structure includes a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a side chain that is another carboxyl group. Understanding its structure is vital for predicting its behavior in biological systems and its role in protein synthesis and metabolism.
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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
The herbicide glyphosate (Roundup®) kills plants by inhibiting an enzyme needed for synthesis of phenylalanine. Deprived of phenylalanine, the plant cannot make the proteins it needs, and it gradually weakens and dies. Although a small amount of glyphosate is deadly to a plant, its human toxicity is quite low. Suggest why this powerful herbicide has little effect on humans.
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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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