Textbook Question
Draw the structure of the predominant form of
(e) a mixture of alanine, lysine, and aspartic acid at (iii) pH 2.
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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 4a
Draw the structure of the predominant form of
(a) isoleucine at pH 11.
Verified step by step guidance1
Step 1: Understand the structure of isoleucine. Isoleucine is an amino acid with the following functional groups: an amino group (-NH₂), a carboxylic acid group (-COOH), and a side chain containing a hydrocarbon group. Its molecular formula is C₆H₁₃NO₂.
Step 2: Recall the pH-dependent behavior of amino acids. At pH 11, the environment is basic, meaning the amino acid will lose protons from its acidic functional groups. Specifically, the carboxylic acid group (-COOH) will deprotonate to form a carboxylate ion (-COO⁻), and the amino group (-NH₂) will remain unprotonated as -NH₂.
Step 3: Identify the predominant form of isoleucine at pH 11. The carboxylic acid group will exist as -COO⁻, and the amino group will remain as -NH₂. The side chain does not have ionizable groups, so it remains unchanged.
Step 4: Draw the structure of isoleucine at pH 11. Represent the backbone with the amino group (-NH₂) on one end, the carboxylate group (-COO⁻) on the other end, and the side chain attached to the central carbon atom. Ensure the stereochemistry of isoleucine is correct, as it is a chiral molecule.
Step 5: Verify the structure. Double-check that the functional groups are correctly represented for the basic pH environment and that the stereochemistry of the molecule is accurate.
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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 are organic compounds that serve as the building blocks of proteins. Each amino acid has a central carbon atom bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R group). The structure of isoleucine, an essential amino acid, includes a branched-chain aliphatic side group, which influences its properties and behavior in biological systems.
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Charged Amino Acids
pH and Ionization
pH is a measure of the acidity or basicity of a solution, affecting the ionization state of functional groups in molecules. At higher pH levels, such as pH 11, amino acids tend to lose protons from their carboxyl groups and may also affect the amino group. Understanding how pH influences the ionization of amino acids is crucial for predicting their predominant forms in different environments.
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Zwitterion Formation
A zwitterion is a molecule that has both positive and negative charges but is overall neutral. In the case of amino acids, at physiological pH, they often exist as zwitterions, where the amino group is protonated (-NH3+) and the carboxyl group is deprotonated (-COO-). At pH 11, isoleucine will predominantly exist in a deprotonated form, affecting its solubility and reactivity in biological systems.
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Related Practice
Textbook Question
Most naturally occurring amino acids have chiral centers (the asymmetric α carbon atoms) that are named (S) by the Cahn–Ingold–Prelog convention (Section 5-3). The common naturally occurring form of cysteine has a chiral center that is named (R), however.
(a) What is the relationship between (R)-cysteine and (S)-alanine? Do they have the opposite three-dimensional configuration (as the names might suggest) or the same configuration?
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Textbook Question
The herbicide glyphosate (Roundup®) kills plants by inhibiting an enzyme needed for synthesis of phenylalanine. Deprived of phenylalanine, the plant cannot make the proteins it needs, and it gradually weakens and dies. Although a small amount of glyphosate is deadly to a plant, its human toxicity is quite low. Suggest why this powerful herbicide has little effect on humans.
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Textbook Question
Draw the structure of the predominant form of
(d) glutamic acid at pH 7.
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Textbook Question
Draw the structure of the predominant form of
(b) proline at pH 2.
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Textbook Question
Most naturally occurring amino acids have chiral centers (the asymmetric α carbon atoms) that are named (S) by the Cahn–Ingold–Prelog convention (Section 5-3). The common naturally occurring form of cysteine has a chiral center that is named (R), however.
(b) (S)-Alanine is an L-amino acid (Figure 24-2). Is (R)-cysteine a d-amino acid or an L-amino acid?
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