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 7
Draw the electrophoretic separation of Ala, Lys, and Asp at pH 9.7.
Verified step by step guidance1
Determine the pKa values of the functional groups in each amino acid (Ala, Lys, and Asp). For Ala, consider the amino group (pKa ~9.7) and carboxyl group (pKa ~2.3). For Lys, include the amino group (pKa ~9.7), carboxyl group (pKa ~2.2), and side-chain amino group (pKa ~10.5). For Asp, include the amino group (pKa ~9.7), carboxyl group (pKa ~2.1), and side-chain carboxyl group (pKa ~3.9).
Evaluate the net charge of each amino acid at pH 9.7. Compare the pH to the pKa values of each functional group to determine whether each group is protonated or deprotonated. For example, if pH > pKa, the group is deprotonated; if pH < pKa, the group is protonated.
Assign the overall charge to each amino acid based on the protonation states of its functional groups. For instance, at pH 9.7, the amino group of Ala will be neutral, and the carboxyl group will be negatively charged, resulting in a net charge of -1. Repeat this process for Lys and Asp.
Predict the direction of migration for each amino acid during electrophoresis at pH 9.7. Amino acids with a net negative charge will migrate toward the anode (positive electrode), while those with a net positive charge will migrate toward the cathode (negative electrode). Neutral amino acids will not migrate significantly.
Draw the electrophoretic separation by placing Ala, Lys, and Asp on a diagram. Indicate their starting positions and the direction of their migration based on their net charges. Label the anode and cathode to show the orientation of the electric field.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Isoelectric Point (pI)
The isoelectric point (pI) is the pH at which a molecule, such as an amino acid, carries no net electrical charge. For amino acids, this is crucial because it determines their solubility and behavior in an electric field. At pH values below their pI, amino acids are positively charged, while at pH values above their pI, they are negatively charged.
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Definition of Isoelectric Point
Electrophoresis
Electrophoresis is a technique used to separate charged particles, such as amino acids, based on their size and charge under an electric field. When an electric current is applied, molecules migrate towards the electrode of opposite charge. The rate of migration depends on the charge-to-mass ratio of the molecules, allowing for their separation in a gel or solution.
Amino Acid Properties
Amino acids have unique side chains that influence their charge at different pH levels. For instance, at pH 9.7, the amino acid alanine (Ala) is neutral, lysine (Lys) is positively charged due to its basic side chain, and aspartic acid (Asp) is negatively charged because of its acidic side chain. Understanding these properties is essential for predicting their behavior during electrophoresis.
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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 Trp, Cys, and His at pH 6.0.
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Textbook Question
Show how you would use bromination followed by amination to synthesize the following amino acids.
(b) leucine
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Textbook Question
Show how the following amino acids might be formed in the laboratory by reductive amination of the appropriate α-ketoacid.
(a) alanine
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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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