Textbook Question
Explain the order of elution (with a buffer of pH 4) of the following pairs of amino acids through a column packed with Dowex 50:
a. aspartate before serine
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Ch. 21 - Amino Acids, Peptides, and Proteins
Bruice8th EditionOrganic ChemistryISBN: 9780135213711
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Table of contents
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 1)31
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 2)65
- Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry84
- Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure183
- Ch. 4 - Isomers: The Arrangement of Atoms in Space121
- Ch. 5 - Alkenes: Structure, Nomenclature, and an Introduction to Reactivity • Thermodynamics and Kinetics83
- Ch. 6 - The Reactions of Alkenes • The Stereochemistry of Addition Reactions175
- Ch. 7 - The Reactions of Alkynes • An Introduction to Multistep Synthesis119
- Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene148
- Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides212
- Ch. 10 - Reactions of Alcohols, Ethers, Epoxides, Amines, and Sulfur-Containing Compounds131
- Ch. 11 - Organometallic Compounds60
- Ch. 12 - Radicals90
- Ch. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis Spectroscopy62
- Ch. 14 - NMR Spectroscopy95
- Ch. 15 - Reactions of Carboxylic Acids and Carboxylic Acid Derivatives123
- Ch. 16 - Reactions of Aldehydes and Ketones • More Reactions of Carboxylic Acid Derivatives141
- Ch. 17 - Reactions at the Alpha-Carbon101
- Ch. 18 - Reactions of Benzene and Substituted Benzenes144
- Ch. 19 - More About Amines • Reactions of Heterocyclic Compounds49
- Ch. 20 - The Organic Chemistry of Carbohydrates80
- Ch. 21 - Amino Acids, Peptides, and Proteins57
- Ch. 22 - Catalysis in Organic Reactions and in Enzymatic Reactions35
- Ch. 23 - The Organic Chemistry of the Coenzymes—Compounds Derived from Vitamins1
- Ch. 28 - Pericyclic Reactions58
Chapter 22, Problem 13
a. What percentage of the a-amino group of lysine will be protonated at its pI?
<25%, 50%, >75%
b. Answer the same question for the e-amino group of lysine
Verified step by step guidance1
Identify the structure of lysine and its relevant functional groups: lysine has two amino groups (α-amino and ε-amino) and one carboxylic acid group. The α-amino group is attached to the α-carbon, while the ε-amino group is part of the side chain.
Understand the concept of the isoelectric point (pI): The pI is the pH at which the molecule has no net charge. For lysine, this occurs when the carboxylic acid group is deprotonated (-COO⁻), the α-amino group is protonated (-NH₃⁺), and the ε-amino group is also protonated (-NH₃⁺).
Recall the Henderson-Hasselbalch equation: \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \). This equation relates the pH, the pKa of a functional group, and the ratio of the deprotonated to protonated forms of that group.
Determine the protonation state of the α-amino group at the pI: At the pI, the pH is approximately the average of the pKa values of the α-amino group and the carboxylic acid group. Use the Henderson-Hasselbalch equation to calculate the ratio of protonated to deprotonated forms of the α-amino group, and then determine the percentage of the α-amino group that is protonated.
Repeat the process for the ε-amino group: At the pI, the ε-amino group is also influenced by its own pKa. Use the Henderson-Hasselbalch equation to calculate the ratio of protonated to deprotonated forms of the ε-amino group, and then determine the percentage of the ε-amino group that is protonated.
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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 like lysine, the pI is determined by the pKa values of the ionizable groups. At this pH, the positive and negative charges balance out, leading to a neutral overall charge, which is crucial for understanding the protonation state of amino groups.
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Definition of Isoelectric Point
Protonation and Deprotonation
Protonation refers to the addition of a proton (H+) to a molecule, while deprotonation is the removal of a proton. In the context of amino acids, the protonation state of functional groups, such as the amino groups in lysine, depends on the pH of the environment relative to their pKa values. At pH values below their pKa, groups are typically protonated, while at higher pH values, they tend to be deprotonated.
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Amino Acid Structure and Functional Groups
Amino acids consist of a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable R group (side chain). In lysine, there are two amino groups: the α-amino group and the ε-amino group. Understanding the structure and the specific pKa values of these groups is essential for predicting their protonation states at different pH levels, particularly at the isoelectric point.
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Related Practice
Textbook Question
A mixture of seven amino acids (glycine, glutamate, leucine, lysine, alanine, isoleucine, and aspartate) is separated by chromatography. Explain why only six spots show up when the chromatographic plate is coated with ninhydrin and heated.
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Textbook Question
a. Which amino acid has the lowest pI value?
b. Which amino acid has the highest pI value?
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Textbook Question
d. Which amino acid has a greater negative charge at pH = 6.20, glycine or methionine?
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Textbook Question
c. Which amino acid has the greatest amount of negative charge at pH = 6.20?
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Textbook Question
Explain why the pI of lysine is the average of the pKa values of its two protonated amino groups.
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