Textbook Question
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
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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 18a
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
Verified step by step guidance1
Understand the context: Dowex 50 is a cation-exchange resin, meaning it binds positively charged species (cations). The elution order of amino acids depends on their net charge at the given pH (pH 4 in this case). Amino acids with a more negative net charge will elute first because they interact less strongly with the resin.
Determine the pKa values of the relevant functional groups for each amino acid: Aspartate has a carboxylic acid side chain with a pKa around 3.9, while serine has a hydroxyl side chain, which is not ionizable under these conditions. Both amino acids also have an amino group (pKa ~9) and a carboxylic acid group (pKa ~2).
Calculate the net charge of each amino acid at pH 4: At pH 4, the carboxylic acid groups (pKa ~2) will be deprotonated (-1 charge), while the amino groups (pKa ~9) will remain protonated (+1 charge). For aspartate, the side chain carboxylic acid (pKa ~3.9) will also be partially deprotonated, contributing an additional negative charge. Serine, lacking an ionizable side chain, will have a net charge closer to neutral.
Compare the net charges: Aspartate will have a more negative net charge than serine at pH 4 due to its deprotonated side chain carboxylic acid. This means aspartate will interact less strongly with the cation-exchange resin and elute first.
Conclude the elution order: Based on their net charges at pH 4, aspartate will elute before serine from the Dowex 50 column.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Ion Exchange Chromatography
Ion exchange chromatography is a technique used to separate charged molecules based on their affinity to the charged stationary phase. In this method, amino acids are passed through a column containing charged resin, such as Dowex 50, which can attract and hold onto oppositely charged species. The order of elution depends on the charge of the amino acids at a given pH, influencing their interaction with the resin.
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Metal Ion Catalysis Concept 1
Amino Acid Charge at pH 4
The charge of an amino acid is determined by its side chain and the pH of the solution. At pH 4, aspartate, which has a carboxyl side chain, carries a negative charge, while serine, which has a neutral side chain, remains uncharged. This difference in charge affects their binding to the ion exchange resin, with aspartate being more negatively charged and thus interacting more strongly with the positively charged resin.
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Elution Order
The elution order in ion exchange chromatography is influenced by the strength of the interaction between the amino acids and the resin. Since aspartate is negatively charged at pH 4, it will bind more tightly to the positively charged Dowex 50 compared to serine, which is neutral. Consequently, aspartate will elute after serine, as it requires a higher concentration of salt or a change in pH to displace it from the resin.
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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
Why is excess ammonia used in the preceding reaction?
1545
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Textbook Question
What amino acid is formed using the N-phthalimidomalonic ester synthesis when the following alkyl halides are used in the third step?
a.
1359
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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:
b. serine before alanine
1162
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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.
2030
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