Textbook Question
α-Amino acids can be prepared by treating an aldehyde with ammonia/trace acid, followed by hydrogen cyanide, followed by acid-catalyzed hydrolysis.
a. Draw the structures of the two intermediates formed in this reaction.
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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 63d
Identify the location and type of charge on the hexapeptide Lys-Ser-Asp-Cys-His-Tyr at each of the following pH values:
d. pH=12
Verified step by step guidance1
Step 1: Identify the ionizable groups in the hexapeptide Lys-Ser-Asp-Cys-His-Tyr. These include the N-terminal amino group, the C-terminal carboxyl group, and the side chains of Lys (lysine), Asp (aspartic acid), Cys (cysteine), His (histidine), and Tyr (tyrosine).
Step 2: Determine the pKa values of the ionizable groups. Approximate pKa values are: N-terminal amino group (~9), C-terminal carboxyl group (~2), Lys side chain (~10.5), Asp side chain (~3.9), Cys side chain (~8.3), His side chain (~6), and Tyr side chain (~10).
Step 3: Compare the pH (12) to the pKa of each ionizable group to determine its protonation state. At pH > pKa, the group will be deprotonated, and at pH < pKa, the group will be protonated. For example, the Lys side chain (pKa ~10.5) will be deprotonated and neutral at pH 12, while the Asp side chain (pKa ~3.9) will be deprotonated and negatively charged.
Step 4: Assign the charge to each ionizable group based on its protonation state. For instance, the N-terminal amino group will be deprotonated and neutral, the C-terminal carboxyl group will be deprotonated and negatively charged, and so on for the side chains of Lys, Ser, Asp, Cys, His, and Tyr.
Step 5: Sum up the charges of all ionizable groups to determine the overall charge of the hexapeptide at pH 12. Ensure that you account for each group’s individual charge contribution to the total charge.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Amino Acid Structure and Properties
Amino acids, the building blocks of peptides, have a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable R group. The properties of the R group determine the amino acid's characteristics, including its charge at different pH levels. Understanding the structure and properties of the amino acids in the hexapeptide is essential for predicting their behavior in solution.
Recommended video:
Guided course
04:10
The 7 Ionizable Amino Acids
pH and Charge States
pH is a measure of the acidity or basicity of a solution, which affects the ionization of functional groups in amino acids. At high pH values, such as pH 12, amino groups tend to lose protons and become neutral, while carboxyl groups are deprotonated and carry a negative charge. This shift in charge states is crucial for determining the overall charge of the hexapeptide at a given pH.
Recommended video:
Guided course
03:08Ground vs. Excited States
Peptide Charge Calculation
The overall charge of a peptide is determined by the sum of the charges of its constituent amino acids at a specific pH. Each amino acid in the hexapeptide Lys-Ser-Asp-Cys-His-Tyr will contribute to the total charge based on its ionization state at pH 12. By analyzing the charge of each amino acid, one can accurately determine the net charge of the hexapeptide in that environment.
Recommended video:
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01:34Calculating formal and net charge.
Related Practice
Textbook Question
Explain the difference in the pKa values of the carboxyl groups of alanine, serine, and cysteine
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Textbook Question
After the polypeptide shown below was treated with maleic anhydride, it was hydrolyzed by trypsin. (After a polypeptide is treated with maleic anhy- dride, trypsin will cleave the polypeptide only on the C-side of arginine.)
Gly-Ala-Asp-Ala-Leu-Pro-Gly-Ile-Leu-Val-Arg-Asp-Val-Gly-Lys-Val-Glu-Val-Phe-Glu-Ala-Gly- Arg-Ala-Glu-Phe-Lys-Glu-Pro-Arg-Leu-Val-Met-Lys-Val-Glu-Gly-Arg-Pro-Val-Gly-Ala-Gly-Leu-Trp
a. After a polypeptide is treated with maleic anhydride, why does trypsin no longer cleave it on the C-side of lysine?
b. How many fragments are obtained from the polypeptide?
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Textbook Question
Identify the location and type of charge on the hexapeptide Lys-Ser-Asp-Cys-His-Tyr at each of the following pH values:
c. pH=7
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Textbook Question
Explain why amino acids, unlike most amines and carboxylic acids, are insoluble in diethyl ether.
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Textbook Question
Draw the product obtained when a lysine side chain in a polypeptide reacts with maleic anhydride.
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