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Ch.10 Proteins Workers of the Cell
Frost - General, Organic and Biological Chemistry 4th Edition
Frost4th EditionGeneral, Organic and Biological ChemistryISBN: 9780134988696Not the one you use?Change textbook
All textbooksFrost 4th EditionCh.10 Proteins Workers of the CellProblem 11b
Chapter 6, Problem 11b

Isoleucine has the zwitterion structure shown. Draw the structure and give the net charge of isoleucine that will predominate at the indicated pH values (pI = 6.0).
b. pH 6.0

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1
Understand the concept of a zwitterion: A zwitterion is a molecule that contains both positive and negative charges but is overall electrically neutral. Amino acids like isoleucine exist as zwitterions at their isoelectric point (pI). The pI is the pH at which the amino acid has no net charge.
Identify the functional groups in isoleucine: Isoleucine has an amino group (-NH3+), a carboxyl group (-COO−), and a side chain. At pH 6.0 (the pI), the amino group is protonated (+1 charge), and the carboxyl group is deprotonated (-1 charge). These charges balance each other, resulting in a net charge of 0.
Determine the behavior of isoleucine at pH 6.0: Since the pH matches the pI, the zwitterion form of isoleucine will predominate. This means the molecule will have no net charge, as the positive charge on the amino group and the negative charge on the carboxyl group cancel each other out.
Draw the structure of isoleucine at pH 6.0: Represent the zwitterion form with the amino group protonated (-NH3+) and the carboxyl group deprotonated (-COO−). Ensure the side chain is correctly depicted based on isoleucine's structure.
Summarize the net charge: At pH 6.0, isoleucine exists predominantly in its zwitterion form, and the net charge of the molecule is 0.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Zwitterion Structure

A zwitterion is a molecule that has both positive and negative charges but is overall neutral. In the case of amino acids like isoleucine, the amino group (–NH3+) carries a positive charge, while the carboxyl group (–COO–) carries a negative charge. This dual charge allows zwitterions to exist in a stable form in solution, particularly at specific pH levels.
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Isoelectric Point (pI)

The isoelectric point (pI) is the pH at which an amino acid exists predominantly in its zwitterionic form, resulting in a net charge of zero. For isoleucine, the pI is given as 6.0, meaning that at this pH, the positive and negative charges balance out. Understanding pI is crucial for predicting the behavior of amino acids in different pH environments.
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pH and Charge Relationship

The pH of a solution affects the ionization state of amino acids, influencing their net charge. At pH values below the pI, amino acids tend to be positively charged, while at pH values above the pI, they become negatively charged. This relationship is essential for determining the predominant charge of isoleucine at specific pH levels, such as the given pH of 6.0.
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Related Practice
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Draw the structure for the amino acid represented by each of the following abbreviations:

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