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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 49a
Chapter 22, Problem 49a

Glycine has pKa values of 2.34 and 9.60. At what pH does glycine exist in the forms shown?
a. Chemical structures of glycine showing zwitterionic and anionic forms, each labeled with 50% representation.

Verified step by step guidance
1
Step 1: Identify the forms of glycine shown in the image. The left structure represents the zwitterionic form with a neutral amino group (-NH2) and a deprotonated carboxyl group (-COO⁻). The right structure represents the protonated form with a positively charged amino group (-NH3⁺) and a deprotonated carboxyl group (-COO⁻).
Step 2: Recognize that the pKa values provided (2.34 and 9.60) correspond to the ionization of the carboxyl group and the amino group, respectively. The pKa of 2.34 is for the carboxyl group losing a proton, and the pKa of 9.60 is for the amino group losing a proton.
Step 3: Understand that at a pH equal to the pKa of a functional group, the molecule exists in equilibrium between the protonated and deprotonated forms of that group. For the amino group (pKa = 9.60), at pH 9.60, the molecule will exist as 50% -NH3⁺ and 50% -NH2.
Step 4: Analyze the image. The structures shown indicate that the amino group is in equilibrium between its protonated (-NH3⁺) and neutral (-NH2) forms, while the carboxyl group remains deprotonated (-COO⁻). This suggests the pH is near the pKa of the amino group, which is 9.60.
Step 5: Conclude that the pH at which glycine exists in the forms shown (50% -NH3⁺ and 50% -NH2) is approximately equal to the pKa of the amino group, which is 9.60.

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Key Concepts

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

Zwitterion

A zwitterion is a molecule that has both positive and negative charges but is overall neutral. In the case of glycine, at a specific pH, it exists as a zwitterion where the amino group is protonated (positively charged) and the carboxyl group is deprotonated (negatively charged). This form is significant in biological systems and affects the molecule's solubility and reactivity.
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pKa and pH Relationship

The pKa value of a compound indicates the pH at which half of the species are protonated and half are deprotonated. For glycine, with pKa values of 2.34 and 9.60, the pH at which it exists in equal amounts of its zwitterionic and deprotonated forms can be determined using the Henderson-Hasselbalch equation. Understanding this relationship is crucial for predicting the behavior of amino acids in different pH environments.
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Amino Acid Ionization

Amino acids can exist in different ionic forms depending on the pH of the solution. At low pH, amino acids are fully protonated, while at high pH, they are fully deprotonated. The pH at which an amino acid exists in its zwitterionic form is critical for understanding its role in proteins and biological processes, as it influences interactions and stability.
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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

Three peptides were obtained from a trypsin digestion of two different polypeptides. In each case, indicate the possible sequences from the given data and tell what further experiment should be carried out in order to determine the primary structure of the polypeptide.

a. polypeptide I:

1. Val-Gly-Asp-Lys

2. Leu-Glu-Pro-Ala-Arg

3. Ala-Leu-Gly-Asp

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Textbook Question

How would a protein that resides in the nonpolar interior of a membrane fold compared with the water-soluble protein just discussed?

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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

Indicate the peptides produced from cleavage by the indicated reagent:

a. His-Lys-Leu-Val-Glu-Pro-Arg-Ala-Gly-Ala by trypsin

b. Leu-Gly-Ser-Met-Phe-Pro-Tyr-Gly-Val by chymotrypsin

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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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