Ch.18 Amino Acids and Proteins

McMurry - Fundamentals of GOB 8th Edition

McMurry8th EditionFundamentals of GOBISBN: 9780134015187Not the one you use?Change textbook

McMurry 8th Edition

Ch.18 Amino Acids and Proteins

Problem 34a

Chapter 18, Problem 34a

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

Verified step by step guidance

Step 1: Understand the structure of valine (Val). Valine is an amino acid with the following structure: an amino group (-NH₂), a carboxyl group (-COOH), and a side chain (R group) that is isopropyl (-CH(CH₃)₂). At neutral pH, the amino group is protonated (-NH₃⁺), and the carboxyl group is deprotonated (-COO⁻).

Step 2: Analyze the behavior of valine at low pH (pH 1). At low pH, the environment is highly acidic, so all ionizable groups will be protonated. The amino group will remain as -NH₃⁺, and the carboxyl group will be protonated to -COOH. The side chain of valine does not ionize, so it remains unchanged.

Step 3: Draw the structure of valine at low pH. Represent the amino group as -NH₃⁺, the carboxyl group as -COOH, and the side chain as -CH(CH₃)₂. Ensure the correct connectivity of the groups to the central carbon (α-carbon).

Step 4: Analyze the behavior of valine at high pH (pH 14). At high pH, the environment is highly basic, so all ionizable groups will lose protons. The amino group will deprotonate to -NH₂, and the carboxyl group will remain deprotonated as -COO⁻. The side chain of valine does not ionize, so it remains unchanged.

Step 5: Draw the structure of valine at high pH. Represent the amino group as -NH₂, the carboxyl group as -COO⁻, and the side chain as -CH(CH₃)₂. Again, ensure the correct connectivity of the groups to the central carbon (α-carbon).

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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 consists of a central carbon atom, an amino group (–NH2), a carboxyl group (–COOH), a hydrogen atom, and a variable R group that determines the specific properties of the amino acid. Understanding the structure of amino acids is crucial for analyzing their behavior at different pH levels.

Ionization at Different pH Levels

The ionization state of amino acids and peptides changes with pH due to the protonation and deprotonation of functional groups. At low pH (acidic conditions), amino groups are typically protonated, while carboxyl groups are also protonated, leading to a positively charged species. Conversely, at high pH (basic conditions), carboxyl groups lose protons and become negatively charged, while amino groups may become neutral, affecting the overall charge and structure of the molecule.

Peptide Bond Formation

Peptide bonds are covalent bonds that link amino acids together to form dipeptides and tripeptides. This bond forms through a condensation reaction, where the carboxyl group of one amino acid reacts with the amino group of another, releasing a molecule of water. Understanding peptide bond formation is essential for drawing the structures of dipeptides and tripeptides, especially when considering their ionization states at varying pH levels.

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Related Practice

Textbook Question

Both α-keratin and tropocollagen have helical secondary structure. How do these molecules differ in (a) amino acid composition and (b) three-dimensional structure?

Textbook Question

Another endoprotease is trypsin. Trypsin hydrolyzes peptide bonds on the carboxyl side of lysine and arginine. If the following peptide sequence is hydrolyzed by trypsin, how many fragments will there be? Use the three-letter amino acid abbreviations to write the fragments out.

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

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.

d. Glu-Asp

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.

e. Gln-Ala-Asn

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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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

Verified video answer for a similar problem:

Key Concepts

Amino Acid Structure

Ionization at Different pH Levels

Peptide Bond Formation

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