Chapter 29, Problem 29.36P

To calculate the isoelectric point of amino acids having other ionizable functional groups, we must also take into account the $\text{p}{K}_{\text{a}}$ of the additional functional group in the side chain.

For an acidic amino acid (one with an additional acidic $\text{OH}$ group): $\text{p}I=\frac{\text{p}{K}_a\left(\text{α-COOH}\right)+\text{p}{K}_a\left(\text{second}\text{COOH}\right)}{2}$

For an basic amino acid (one with an additional basic $\text{NH}$ group): $\text{p}I=\frac{\text{p}{K}_a\left({\text{α-NH}}_3{}^{+}\right)+\text{p}{K}_a\left(\text{side}\text{chain}\text{NH}\right)}{2}$

a. Indicate, which $\text{p}{K}_{\text{a}}$ values must be used to calculate the $\text{p}I$ of each of the following amino acids: $\left[1\right]$ glutamic acid; $\left[2\right]$ lysine; $\left[3\right]$ arginine.

b. In general, how does the $\text{p}I$ of an acidic amino acid compare to that of a neutral amino acid?

c. In general, how does the $\text{p}I$ of a basic amino acid compare to the $\text{p}I$ of a neutral amino acid?

Definition Definition Group of atoms that shape the chemical characteristics of a molecule. The behavior of a functional group is uniform in undergoing comparable chemical reactions, regardless of the other constituents of the molecule. Functional groups aid in the classification and anticipation of reactivity of organic molecules.

Expert Solution

Interpretation Introduction

(a)

Interpretation: The $\text{p}{K}_{\text{a}}$ values that must be used to calculate the $\text{p}I$ of each of the following amino acids: $\left[1\right]$ glutamic acid; $\left[2\right]$ lysine; $\left[3\right]$ arginine are to be indicated.

Concept introduction: The chemical compounds in which carbon is bonded with acidic and basic group along with hydrocarbon side chain are known as amino acids. The amino acids are classified into acidic, basic, polar, non-polar, essential, and non-essential categories. The three word abbreviation or one word abbreviation is used for amino acids.

Answer to Problem 29.36P

As per respective formulae, the $\text{p}{K}_{\text{a}}$ values that must be used to calculate the $\text{p}I$ of $\left[1\right]$ glutamic acid are $2.10$, $4.07$; $\left[2\right]$ lysine are $8.95$, $10.53$; $\left[3\right]$ arginine are $9.04$ and $12.48$.

Explanation of Solution

Glutamic acid has acidic side chain. Lysine has basic side chain. Arginine has basic side chain.

The $\text{p}I$ value for an acidic amino acid (one have an addition acidic $\text{OH}$ group) is calculated by the formula,

$\text{p}I=\frac{\text{p}{K}_{\text{a}}\left(\text{α-COOH}\right)+\text{p}{K}_{\text{a}}\left(\text{second}\text{COOH}\right)}{2}$

The $\text{p}I$ for a basic amino acid is calculated by the formula,

$\text{p}I=\frac{\text{p}{K}_{\text{a}}\left({\text{α-NH}}_3{}^{+}\right)+\text{p}{K}_{\text{a}}\left(\text{side}\text{chain}\text{NH}\right)}{2}$

The $\text{p}{K}_{\text{a}}$ values for the ionizable functional group of $\left[1\right]$ glutamic acid; $\left[2\right]$ lysine; $\left[3\right]$ arginine are as follows:

$$$\begin{array}{ccccc}\text{Amino}\text{acid}& \text{α-COOH}& {\text{α-NH}}_3{}^{+}& \text{Side}\text{chain}& \text{p}I\\ \text{Glutamic acid}& 2.10& 9.47& 4.07& 3.08\\ \text{Lysine}& 2.18& 8.95& 10.53& 9.74\\ \text{Arginine}& 2.01& 9.04& 12.48& 10.76\end{array}$

As per respective formulae, the $\text{p}{K}_{\text{a}}$ values that must be used to calculate the $\text{p}I$ of $\left[1\right]$ glutamic acid are $2.10$, $4.07$; $\left[2\right]$ lysine are $8.95$, $10.53$; $\left[3\right]$ arginine are $9.04$ and $12.48$.

Conclusion

As per respective formulae, the $\text{p}{K}_{\text{a}}$ values that must be used to calculate the $\text{p}I$ of $\left[1\right]$ glutamic acid are $2.10$, $4.07$; $\left[2\right]$ lysine are $8.95$, $10.53$; $\left[3\right]$ arginine are $9.04$ and $12.48$.

Expert Solution

Interpretation Introduction

(b)

Interpretation: The comparison between $\text{p}I$ of an acidic amino acid to the $\text{p}I$ of a neutral amino acid is to be stated.

Concept introduction: The chemical compounds in which carbon is bonded with acidic and basic group along with hydrocarbon side chain are known as amino acids. The amino acids are classified into acidic, basic, polar, non-polar, essential, and non-essential categories. The three word abbreviation or one word abbreviation is used for amino acids.

Answer to Problem 29.36P

The $\text{p}I$ of an acidic amino acid is lower than the $\text{p}I$ of a neutral amino acid.

Explanation of Solution

The $\text{pH}$ value determines the charge of an $\text{α}$-amino acid.

The $\text{pH}$ value at which an amino acid stay mainly in its neutral form is called isoelectric point. Amino acids that possess acidic side chain have lower $\text{p}I$ value than that of neutral amino acids. This is due to genration of extra negative charge in the acidic side chain of an acidic amino acid. Under acidic conditions this negative charge is neutralized.

Thus, the $\text{p}I$ of an acidic amino acid is lower than the $\text{p}I$ of a neutral amino acid.

Conclusion

The $\text{p}I$ of an acidic amino acid is lower than the $\text{p}I$ of a neutral amino acid.

Expert Solution

Interpretation Introduction

(c)

Interpretation: The comparison between $\text{p}I$ of a basic amino acid to the $\text{p}I$ of a neutral amino acid is to be stated.

Concept introduction: The chemical compounds in which carbon is bonded with acidic and basic group along with hydrocarbon side chain are known as amino acids. The amino acids are classified into acidic, basic, polar, non-polar, essential, and non-essential categories. The three word abbreviation or one word abbreviation is used for amino acids.

Answer to Problem 29.36P

The $\text{p}I$ of a basic amino acid is higher than the $\text{p}I$ of a neutral amino acid.

Explanation of Solution

The $\text{pH}$ value determines the charge of an $\text{α}$-amino acid.

The $\text{pH}$ value at which an amino acid stay mainly in its neutral form is called isoelectric point. Amino acids that possess basic side chain have higher $\text{p}I$ value than that of neutral amino acids. This is due to genration of extra positive charge in the basic side chain of a basic amino acid. Under basic conditions this positive charge is neutralized.

Thus, the $\text{p}I$ of a basic amino acid is higher than the $\text{p}I$ of a neutral amino acid.

Conclusion

The $\text{p}I$ of a basic amino acid is higher than the $\text{p}I$ of a neutral amino acid.

Interpretation Introduction

(a)

Interpretation: The $\text{p}{K}_{\text{a}}$ values that must be used to calculate the $\text{p}I$ of each of the following amino acids: $\left[1\right]$ glutamic acid; $\left[2\right]$ lysine; $\left[3\right]$ arginine are to be indicated.

Concept introduction: The chemical compounds in which carbon is bonded with acidic and basic group along with hydrocarbon side chain are known as amino acids. The amino acids are classified into acidic, basic, polar, non-polar, essential, and non-essential categories. The three word abbreviation or one word abbreviation is used for amino acids.

Answer to Problem 29.36P

As per respective formulae, the $\text{p}{K}_{\text{a}}$ values that must be used to calculate the $\text{p}I$ of $\left[1\right]$ glutamic acid are $2.10$, $4.07$; $\left[2\right]$ lysine are $8.95$, $10.53$; $\left[3\right]$ arginine are $9.04$ and $12.48$.

Explanation of Solution

Glutamic acid has acidic side chain. Lysine has basic side chain. Arginine has basic side chain.

The $\text{p}I$ value for an acidic amino acid (one have an addition acidic $\text{OH}$ group) is calculated by the formula,

$\text{p}I=\frac{\text{p}{K}_{\text{a}}\left(\text{α-COOH}\right)+\text{p}{K}_{\text{a}}\left(\text{second}\text{COOH}\right)}{2}$

The $\text{p}I$ for a basic amino acid is calculated by the formula,

$\text{p}I=\frac{\text{p}{K}_{\text{a}}\left({\text{α-NH}}_3{}^{+}\right)+\text{p}{K}_{\text{a}}\left(\text{side}\text{chain}\text{NH}\right)}{2}$

The $\text{p}{K}_{\text{a}}$ values for the ionizable functional group of $\left[1\right]$ glutamic acid; $\left[2\right]$ lysine; $\left[3\right]$ arginine are as follows:

$$$\begin{array}{ccccc}\text{Amino}\text{acid}& \text{α-COOH}& {\text{α-NH}}_3{}^{+}& \text{Side}\text{chain}& \text{p}I\\ \text{Glutamic acid}& 2.10& 9.47& 4.07& 3.08\\ \text{Lysine}& 2.18& 8.95& 10.53& 9.74\\ \text{Arginine}& 2.01& 9.04& 12.48& 10.76\end{array}$

As per respective formulae, the $\text{p}{K}_{\text{a}}$ values that must be used to calculate the $\text{p}I$ of $\left[1\right]$ glutamic acid are $2.10$, $4.07$; $\left[2\right]$ lysine are $8.95$, $10.53$; $\left[3\right]$ arginine are $9.04$ and $12.48$.

Conclusion

As per respective formulae, the $\text{p}{K}_{\text{a}}$ values that must be used to calculate the $\text{p}I$ of $\left[1\right]$ glutamic acid are $2.10$, $4.07$; $\left[2\right]$ lysine are $8.95$, $10.53$; $\left[3\right]$ arginine are $9.04$ and $12.48$.

Interpretation Introduction

(b)

Interpretation: The comparison between $\text{p}I$ of an acidic amino acid to the $\text{p}I$ of a neutral amino acid is to be stated.

Concept introduction: The chemical compounds in which carbon is bonded with acidic and basic group along with hydrocarbon side chain are known as amino acids. The amino acids are classified into acidic, basic, polar, non-polar, essential, and non-essential categories. The three word abbreviation or one word abbreviation is used for amino acids.

Answer to Problem 29.36P

The $\text{p}I$ of an acidic amino acid is lower than the $\text{p}I$ of a neutral amino acid.

Explanation of Solution

The $\text{pH}$ value determines the charge of an $\text{α}$-amino acid.

The $\text{pH}$ value at which an amino acid stay mainly in its neutral form is called isoelectric point. Amino acids that possess acidic side chain have lower $\text{p}I$ value than that of neutral amino acids. This is due to genration of extra negative charge in the acidic side chain of an acidic amino acid. Under acidic conditions this negative charge is neutralized.

Thus, the $\text{p}I$ of an acidic amino acid is lower than the $\text{p}I$ of a neutral amino acid.

Conclusion

The $\text{p}I$ of an acidic amino acid is lower than the $\text{p}I$ of a neutral amino acid.

Interpretation Introduction

(c)

Interpretation: The comparison between $\text{p}I$ of a basic amino acid to the $\text{p}I$ of a neutral amino acid is to be stated.

Concept introduction: The chemical compounds in which carbon is bonded with acidic and basic group along with hydrocarbon side chain are known as amino acids. The amino acids are classified into acidic, basic, polar, non-polar, essential, and non-essential categories. The three word abbreviation or one word abbreviation is used for amino acids.

Answer to Problem 29.36P

The $\text{p}I$ of a basic amino acid is higher than the $\text{p}I$ of a neutral amino acid.

Explanation of Solution

The $\text{pH}$ value determines the charge of an $\text{α}$-amino acid.

The $\text{pH}$ value at which an amino acid stay mainly in its neutral form is called isoelectric point. Amino acids that possess basic side chain have higher $\text{p}I$ value than that of neutral amino acids. This is due to genration of extra positive charge in the basic side chain of a basic amino acid. Under basic conditions this positive charge is neutralized.

Thus, the $\text{p}I$ of a basic amino acid is higher than the $\text{p}I$ of a neutral amino acid.

Conclusion

The $\text{p}I$ of a basic amino acid is higher than the $\text{p}I$ of a neutral amino acid.