Textbook Question
Explain the difference in the pKa values of the carboxyl groups of alanine, serine, and cysteine
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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 63c
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
Verified step by step guidance1
Step 1: Understand the structure of the hexapeptide Lys-Ser-Asp-Cys-His-Tyr. Each amino acid has a side chain with specific functional groups that can gain or lose protons depending on the pH. Additionally, the N-terminus (amino group) and C-terminus (carboxyl group) of the peptide chain are ionizable.
Step 2: Determine the ionization state of the N-terminus and C-terminus at pH 7. The N-terminus (amino group) typically has a pKa around 9-10, meaning it will be protonated and positively charged at pH 7. The C-terminus (carboxyl group) has a pKa around 2-3, meaning it will be deprotonated and negatively charged at pH 7.
Step 3: Analyze the side chains of each amino acid in the hexapeptide at pH 7: Lysine (Lys) has a positively charged amino group (pKa ~10.5), Serine (Ser) has a neutral hydroxyl group, Aspartic acid (Asp) has a negatively charged carboxyl group (pKa ~3.9), Cysteine (Cys) has a neutral thiol group (pKa ~8.3, not ionized at pH 7), Histidine (His) has an imidazole group that is neutral at pH 7 (pKa ~6), and Tyrosine (Tyr) has a neutral phenol group (pKa ~10).
Step 4: Summarize the charges at pH 7: The N-terminus is positively charged, Lysine's side chain is positively charged, Aspartic acid's side chain is negatively charged, and the C-terminus is negatively charged. The side chains of Serine, Cysteine, Histidine, and Tyrosine are neutral at this pH.
Step 5: Assign the location and type of charge: Positive charges are located on the N-terminus and Lysine's side chain. Negative charges are located on the C-terminus and Aspartic acid's side chain. Neutral groups are present on Serine, Cysteine, Histidine, and Tyrosine.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Amino Acid Structure
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 side chain (R group). The properties of the side chain determine the amino acid's characteristics, including its charge at different pH levels. Understanding the structure of each amino acid in the hexapeptide is crucial for predicting their behavior in solution.
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11:14
Charged Amino Acids
pH and Charge
pH is a measure of the hydrogen ion concentration in a solution, influencing the ionization state of amino acids. At physiological pH (around 7), amino acids can exist in different charged forms depending on their side chains and the ionization of their amino and carboxyl groups. Knowing how pH affects the protonation and deprotonation of functional groups is essential for determining the overall charge of the hexapeptide.
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01:34Calculating formal and net charge.
Peptide Charge Calculation
The overall charge of a peptide is determined by the sum of the charges of its constituent amino acids. Each amino acid can be positively charged, negatively charged, or neutral depending on the pH. To calculate the charge of the hexapeptide Lys-Ser-Asp-Cys-His-Tyr at pH 7, one must evaluate the ionization states of the side chains and terminal groups of each amino acid, considering their pKa values.
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01:34Calculating formal and net charge.
Related Practice
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
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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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:
d. pH=12
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Textbook Question
What aldehydes are formed when the following amino acids are treated with ninhydrin?
b. leucine
c. arginine
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