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 8
Draw the electrophoretic separation of Trp, Cys, and His at pH 6.0.
Verified step by step guidance1
Understand the concept of electrophoresis: Electrophoresis is a technique used to separate molecules based on their charge and size. Amino acids like Trp (Tryptophan), Cys (Cysteine), and His (Histidine) have different charges depending on the pH of the solution due to their ionizable groups.
Determine the ionization state of each amino acid at pH 6.0: At pH 6.0, the amino acids will have specific charges based on the pKa values of their functional groups. For example, Histidine has an imidazole side chain with a pKa around 6.0, making it partially protonated and positively charged. Cysteine has a thiol group with a pKa around 8.3, which will be neutral at pH 6.0. Tryptophan has a non-ionizable side chain and will be neutral at this pH.
Predict the movement of each amino acid in an electrophoretic field: In electrophoresis, positively charged molecules move toward the cathode (negative electrode), negatively charged molecules move toward the anode (positive electrode), and neutral molecules remain stationary. Based on their charges at pH 6.0, His will move toward the cathode, while Trp and Cys will remain stationary.
Draw the electrophoretic separation: Represent the electrophoretic setup with a gel or medium, and indicate the starting positions of Trp, Cys, and His. Show the movement of His toward the cathode, while Trp and Cys remain stationary. Label the electrodes and the amino acids clearly.
Explain the reasoning behind the separation: Highlight how the pH affects the ionization state of the amino acids and their movement in the electrophoretic field. Emphasize the importance of understanding pKa values and the relationship between charge and pH in predicting electrophoretic behavior.
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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 in determining their behavior in electrophoresis, as they will migrate towards the electrode of opposite charge when the pH is below or above their pI. Understanding the pI helps predict the migration direction and speed of Trp, Cys, and His at a specific pH.
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Definition of Isoelectric Point
Amino Acid Structure and Properties
Amino acids have distinct side chains (R groups) that influence their charge and solubility at different pH levels. Tryptophan (Trp) is non-polar, cysteine (Cys) contains a thiol group that can be deprotonated, and histidine (His) has an imidazole side chain that can be positively charged at lower pH. Recognizing these properties is essential for predicting their behavior during electrophoresis at pH 6.0.
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Electrophoresis
Electrophoresis is a technique used to separate charged molecules in a gel or solution based on their size and charge. When an electric field is applied, molecules migrate towards the electrode of opposite charge. The rate of migration depends on the net charge of the molecules, which is influenced by the pH of the environment, making it vital to understand how Trp, Cys, and His will behave at pH 6.0.
Related Practice
Textbook Question
Draw the electrophoretic separation of Ala, Lys, and Asp at pH 9.7.
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Textbook Question
Show how you would use a Strecker synthesis to make phenylalanine.
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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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