Chapter 24, Problem 24.27SP

1. a. The isoelectric point (pl) of phenylalanine is pH 5.5. Draw the structure of the major form of phenylalanine at pH values of 1, 5.5, and 11.
2. b. The isoelectric point of histidine is pH 7 6. Draw the structures of the major forms of histidine at pH values of 1, 4, 7.6, and 11. Explain why the nitrogen in the histidine ring is a weaker base than the α-amino group.
3. c. The isoelectric point of glutamic acid is pH 3.2. Draw the structures of the major forms of glutamic acid at pH values of 1,3.2, 7, and 11. Explain why the side-chain carboxylic acid is a weaker acid than the acid group next to the α-carbon atom.

Interpretation Introduction

Interpretation:

The structures of the major form of phenylalanine at $\text{pH}$ values of $1,5.5,\text{and}11$ are to be drawn.

Concept introduction:

The more acidic strength increases the formation of the hydrogen ions and decreases the $\text{pH}$ value of the solution. The major factors that contribute to the acidic strength are $\text{p}{K}_{\text{a}}$ value, inductive effect and polarity. The value of $\text{pH}$ is calculated by the use of concentration of ${\text{H}}^{+}$ and ${\text{OH}}^{-}$ ions in the solution. $\text{pH}$ is the numeric value which defines acidity and basicity of the solution. The charges of the compounds get balanced at isoelectronic point of solution

Answer to Problem 24.27SP

The structures of the major form of phenylalanine at $\text{pH}$ values of $1,5.5,\text{and}11$ are shown in Figure 1.

Explanation of Solution

The structures of the major forms of phenylalanine at $\text{pH}$ values of $1,5.5,\text{and}11$ are shown in Figure 1.

Figure 1

It is conferred from the above reaction that phenylalanine at $\text{pH}$ value of $1\text{and}11$ is fully protonated and deprotonated respectively, whereas charges are balanced at $\text{pH}$ value of $5.5$ due to an isoelectronic point.

Interpretation Introduction

Interpretation:

The structures of the major forms of histidine at $\text{pH}$ values of $1,4,7.6\text{and}11$ are to be shown and an explanation corresponding to the fact that nitrogen in the histidine ring is a weaker base than the $\text{α}$ amino group is to be explained.

Concept introduction:

The more acidic strength increases the formation of the hydrogen ions and decreases the $\text{pH}$ value of the solution. The major factors that contribute to the acidic strength are $\text{p}{K}_{\text{a}}$ value, inductive effect and polarity. The value of $\text{pH}$ is calculated by the use of concentration of ${\text{H}}^{+}$ and ${\text{OH}}^{-}$ ions in the solution. $\text{pH}$ is the numeric value which defines acidity and basicity of the solution. The charges of the compounds get balanced at isoelectronic point of solution

Answer to Problem 24.27SP

The structures of the major forms of histidine at $\text{pH}$ values of $1,4,7.6\text{and}11$ are shown in Figure 2. Nitrogen atom in the histidine ring is a weaker base as compared to the $\text{α}$ amino group because aromatic amines such as pyridines are weaker base than aliphatic amines.

Explanation of Solution

The structures of the major forms of histidine at $\text{pH}$ values of $1,4,7.6\text{and}11$ are shown in Figure 2.

Figure 2

It is conferred from the above reaction that histidine at $\text{pH}$ value of $1\text{and}11$ is fully protonated and deprotonated respectively, whereas charges are balanced at $\text{pH}$ value of $7.6$ due to an isoelectronic point. Only carboxylic group is deprotonated at $\text{pH}$ value of $4$ due to more acidic strength of a compound. Nitrogen atom in the histidine ring is a weaker base as compared to the $\text{α}$ amino group because aromatic amines such as pyridines are weaker base as compared to aliphatic amines and the imidazole ring has two nitrogen atoms out of which one nitrogen atom participates in aromaticity, which decreases the basic strength of the compound.

Interpretation Introduction

Interpretation:

The structures of the major forms of glutamic acid at $\text{pH}$ values of $1,3.2,7\text{and}11$ and an explanation corresponding to the fact that the side-chain carboxylic acid is a weakera cid group next to the $\text{α}$- carbon atom are to be stated.

Concept introduction:

The more acidic strength increases the formation of the hydrogen ions and decreases the $\text{pH}$ value of the solution. The major factors that contribute to the acidic strength are $\text{p}{K}_{\text{a}}$ value, inductive effect and polarity. The value of $\text{pH}$ is calculated by the use of concentration of ${\text{H}}^{+}$ and ${\text{OH}}^{-}$ ions in the solution. $\text{pH}$ is the numeric value which defines acidity and basicity of the solution. The charges of the compounds get balanced at isoelectronic point of solution.

Answer to Problem 24.27SP

The structures of the major forms of glutamic acid at $\text{pH}$ values of $1,3.2,7\text{and}11$ are shown in Figure 3. The side-chain carboxylic acid is a weaker acid group as compared to the $\text{α}$- carbon atom due to more distance of ${\text{NH}}_3{}^{+}$ group from side-chain carboxylic acid.

Explanation of Solution

The structures of the major forms of glutamic acid at $\text{pH}$ values of $1,3.2,7\text{and}11$ are shown in Figure 3.

Figure 3

It is conferred from the above reaction that glutamic acid at $\text{pH}$ value of $1\text{and}11$ is fully protonated and deprotonated respectively, whereas charges are balanced at $\text{pH}$ value of $3.2$ due to an isoelectronic point. Two carboxylic groups and one amino group are deprotonated and protonated are respectively at $\text{pH}$ value of $7$ due to more basic strength of a compound at this point. The side-chain carboxylic acid is a weaker acid group next to the $\text{α}$- carbon atom due to more distance of ${\text{NH}}_3{}^{+}$ group from side-chain carboxylic acid.