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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 54d
Chapter 22, Problem 54d

Draw the form of aspartate that predominates at the following pH values:
d. pH=11.0

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Determine the pKa values of the functional groups in aspartate. Aspartate has three ionizable groups: the α-carboxyl group (pKa ≈ 2.1), the α-amino group (pKa ≈ 9.8), and the side-chain carboxyl group (pKa ≈ 3.9).
Compare the given pH (11.0) to each pKa value to determine the protonation state of each functional group. If the pH is higher than the pKa, the group will be deprotonated. If the pH is lower than the pKa, the group will remain protonated.
At pH 11.0, the α-carboxyl group (pKa ≈ 2.1) will be deprotonated, forming a negatively charged carboxylate ion \( \text{COO}^- \).
At pH 11.0, the side-chain carboxyl group (pKa ≈ 3.9) will also be deprotonated, forming another negatively charged carboxylate ion \( \text{COO}^- \).
At pH 11.0, the α-amino group (pKa ≈ 9.8) will be deprotonated, losing its proton and becoming a neutral \( \text{NH}_2 \) group. Combine these observations to draw the predominant form of aspartate at pH 11.0.

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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, the building blocks of proteins, consist of a central carbon atom bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable R group (side chain). Aspartate, specifically, has a carboxyl group in its side chain, which influences its ionization state depending on the pH of the environment.
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pH and Ionization

pH is a measure of the acidity or basicity of a solution, affecting the ionization of functional groups in amino acids. At higher pH levels, such as pH 11.0, the environment is more basic, leading to the deprotonation of carboxyl groups and the potential protonation of amino groups, which alters the overall charge and form of the amino acid.
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Zwitterion Form

A zwitterion is a molecule that has both positive and negative charges but is overall neutral. At physiological pH, aspartate exists predominantly as a zwitterion, but at pH 11.0, the carboxyl groups are fully deprotonated, resulting in a negatively charged form. Understanding this concept is crucial for predicting the predominant form of aspartate at different pH levels.
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Related Practice
Textbook Question

Show the peptides that would result from cleavage by the indicated reagent: 

a. Val-Arg-Gly-Met-Arg-Ala-Ser by carboxypeptidase A

b. Ser-Phe-Lys-Met-Pro-Ser-Ala-Asp by cyanogen bromide

c. Arg-Ser-Pro-Lys-Lys-Ser-Glu-Gly by trypsin

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Textbook Question

Explain the difference in the pKa values of the carboxyl groups of alanine, serine, and cysteine

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Textbook Question

Explain why amino acids, unlike most amines and carboxylic acids, are insoluble in diethyl ether.

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Textbook Question

Which has a higher percentage of negative charge at physiological pH (7.4), leucine with pI = 5.98 or asparagine with pI = 5.43?

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Textbook Question

What aldehydes are formed when the following amino acids are treated with ninhydrin? 

b. leucine 

c. arginine

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Textbook Question

Draw the form of aspartate that predominates at the following pH values: c. pH=6.0

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