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Ch.18 Amino Acids and Proteins
McMurry - Fundamentals of General, Organic, and Biological Chemistry 8th Edition
McMurry8th EditionFundamentals of General, Organic, and Biological ChemistryISBN: 9780134015187Not the one you use?Change textbook
All textbooksMcMurry 8th EditionCh.18 Amino Acids and ProteinsProblem 34e
Chapter 18, Problem 34e

Draw the structure of the following amino acids, dipeptides, and tripeptides at low pH (pH 1) and high pH (pH 14). At each pH, assume that all functional groups that might do so are ionized.
e. Gln-Ala-Asn

Verified step by step guidance
1
Step 1: Understand the problem. You are tasked with drawing the structure of the tripeptide Gln-Ala-Asn (Glutamine-Alanine-Asparagine) at two different pH levels: low pH (pH 1) and high pH (pH 14). At each pH, you need to account for the ionization states of the functional groups in the amino acids.
Step 2: Recall the ionizable groups in amino acids. Amino acids have an amino group (-NH2), a carboxyl group (-COOH), and sometimes ionizable side chains. At low pH, the amino group is protonated (-NH3⁺), and the carboxyl group is not ionized (-COOH). At high pH, the amino group is deprotonated (-NH2), and the carboxyl group is ionized (-COO⁻).
Step 3: Analyze the specific amino acids in the tripeptide. Glutamine (Gln) and Asparagine (Asn) have side chains that are not ionizable under physiological pH ranges, so their side chains remain neutral. Alanine (Ala) also has a non-ionizable side chain (a methyl group). Focus on the terminal amino and carboxyl groups of the peptide and their ionization states at the given pH levels.
Step 4: Draw the structure at low pH (pH 1). At this pH, the N-terminal amino group of Gln will be protonated (-NH3⁺), the C-terminal carboxyl group of Asn will be in its non-ionized form (-COOH), and the peptide bonds between the amino acids will remain neutral. Ensure that the side chains of Gln, Ala, and Asn are drawn in their neutral forms.
Step 5: Draw the structure at high pH (pH 14). At this pH, the N-terminal amino group of Gln will be deprotonated (-NH2), the C-terminal carboxyl group of Asn will be ionized (-COO⁻), and the peptide bonds will remain neutral. Again, ensure that the side chains of Gln, Ala, and Asn are drawn in their neutral forms, as they are not ionizable under these conditions.

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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 composed of a central carbon atom, an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R group). The structure of amino acids can change based on pH, affecting the ionization of the amino and carboxyl groups, which is crucial for understanding their behavior in different environments.
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Amino Acid Catabolism: Amino Group Example 2

Peptide Bonds

Peptide bonds are covalent bonds formed between the carboxyl group of one amino acid and the amino group of another, releasing a molecule of water in a condensation reaction. This bond links amino acids together to form dipeptides and tripeptides, which are essential for building proteins and understanding their structure at varying pH levels.
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Ionization at Different pH Levels

The ionization state of amino acids and peptides is influenced by the pH of the environment. At low pH (acidic), amino groups are protonated, while carboxyl groups are mostly in their non-ionized form. Conversely, at high pH (basic), carboxyl groups are deprotonated, and amino groups may lose a proton, affecting the overall charge and structure of the molecules.
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Related Practice
Textbook Question

If the same peptide found in Problem 18.32 is subjected to acid hydrolysis, how many fragments will result? Why?

Ala-Phe-Lys-Cys-Gly-Asp-Arg-Leu-Leu-Phe-Gly-Ala

Textbook Question

Draw the structure of the following amino acids, dipeptides, and tripeptides at low pH (pH 1) and high pH (pH 14). At each pH, assume that all functional groups that might do so are ionized.

a. Val

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Textbook Question

Draw the structure of the following amino acids, dipeptides, and tripeptides at low pH (pH 1) and high pH (pH 14). At each pH, assume that all functional groups that might do so are ionized.

d. Glu-Asp

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Textbook Question

Interactions of amino acids on the interior of proteins are key to the shapes of proteins. In group (a), which pairs of amino acids form hydrophobic interactions? In group (b), which pairs form ionic interactions? Which pairs in group (c) form hydrogen bonds?

a. 1 Pro . . . Phe

   2 Lys . . . Ser

   3 Thr . . . Leu

   4 Ala . . . Gly

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Textbook Question

Draw the hexapeptide Asp-Gly-Phe-Leu-Glu-Ala in linear form showing all of the atoms, and show (using dotted lines) the hydrogen bonding that stabilizes this structure if it is part of an α-helix.

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Textbook Question

Compare and contrast the characteristics of fibrous and globular proteins. Consider biological function, water solubility, amino acid composition, secondary structure, and tertiary structure. Give examples of three fibrous and three globular proteins. (Hint: Make a table.)

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