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Ch. 21 - Amino Acids, Peptides, and Proteins
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 21 - Amino Acids, Peptides, and ProteinsProblem 16
Chapter 22, Problem 16

A mixture of seven amino acids (glycine, glutamate, leucine, lysine, alanine, isoleucine, and aspartate) is separated by chromatography. Explain why only six spots show up when the chromatographic plate is coated with ninhydrin and heated.

Verified step by step guidance
1
Understand the role of ninhydrin in detecting amino acids: Ninhydrin reacts with the amino group (-NH2) of amino acids to produce a colored compound, typically purple or blue, which allows visualization of the spots on the chromatographic plate.
Identify the amino acids in the mixture: Glycine, glutamate, leucine, lysine, alanine, isoleucine, and aspartate are all amino acids. However, consider whether any of these amino acids might lack a free amino group under the conditions of the experiment.
Recognize the unique property of proline: Although proline is not explicitly listed in the mixture, it is important to note that proline (if present) reacts differently with ninhydrin due to its secondary amine structure, producing a yellow color instead of the typical purple/blue. This is not directly relevant here but is a useful concept for understanding ninhydrin reactions.
Focus on the amino acids in the mixture: Among the listed amino acids, all except one (glycine) have side chains that do not interfere with the ninhydrin reaction. Glycine, being the simplest amino acid, reacts normally with ninhydrin. However, consider whether any amino acid might be missing a free amino group due to its structure or chemical state.
Conclude why only six spots appear: One of the amino acids in the mixture, glutamate or aspartate, may exist in a zwitterionic form or bound state under the experimental conditions, preventing it from reacting with ninhydrin. Alternatively, experimental error or incomplete reaction could explain the absence of one spot. This highlights the importance of understanding the chemical behavior of amino acids under specific 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 and Properties

Amino acids are organic compounds that serve as the building blocks of proteins. Each amino acid has a central carbon atom, an amino group, a carboxyl group, a hydrogen atom, and a variable R group that determines its unique properties. The polarity and charge of the R group influence how amino acids interact with each other and with solvents, which is crucial for their separation in chromatography.
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Chromatography Principles

Chromatography is a technique used to separate components of a mixture based on their different affinities for a stationary phase and a mobile phase. In this case, the amino acids will travel different distances on the chromatographic plate depending on their interactions with the coating and the solvent. The separation allows for the visualization of individual components, which can be detected using ninhydrin.
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Ninhydrin Reaction with Amino Acids

Ninhydrin is a chemical reagent that reacts with amino acids to produce a colored compound, allowing for their detection on chromatographic plates. However, not all amino acids react equally; for instance, proline does not produce a color change with ninhydrin due to its unique cyclic structure. In this case, if one of the seven amino acids does not react, it would result in only six visible spots on the chromatogram.
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Related Practice
Textbook Question

a. What percentage of the a-amino group of lysine will be protonated at its pI?

<25%, 50%, >75%

b. Answer the same question for the e-amino group of lysine

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Textbook Question

Explain the order of elution (with a buffer of pH 4) of the following pairs of amino acids through a column packed with Dowex 50:

a. aspartate before serine

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Textbook Question

Why is excess ammonia used in the preceding reaction?

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Textbook Question

d. Which amino acid has a greater negative charge at pH = 6.20, glycine or methionine?

3
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Textbook Question

Explain the order of elution (with a buffer of pH 4) of the following pairs of amino acids through a column packed with Dowex 50:

b. serine before alanine

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Textbook Question

Explain why the pI of lysine is the average of the pKa values of its two protonated amino groups.

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