Chapter 29, Problem 29.36P

What is the predominant form of each of the following amino acids at $\text{pH}=1$? What is the overall charge on the amino acid at this $\text{pH}$? (a) threonine; (b) methionine; (c) aspartic acid; (d) arginine

Interpretation Introduction

(a)

Interpretation: The predominant form of threonine at $\text{pH}=1$ and the overall charge on it at this $\text{pH}$ is to be predicted.

Concept introduction: At isoelectric point, the amino acids exist in their neutral form. The amine groups exists as $-\stackrel{+}{\text{N}}{\text{H}}_3$ and carboxyl groups as $-{\text{COO}}^{-}$. The overall charge on an amino acid at isoelectric point is $0$.

Answer to Problem 29.36P

The predominant form of threonine at $\text{pH}=1$ is,

The overall charge on it is $+1$.

Explanation of Solution

At isoelectric point, the amino acids exist in their neutral form. The amine groups exists as $-\stackrel{+}{\text{N}}{\text{H}}_3$ and carboxyl groups as $-{\text{COO}}^{-}$. However, when the $\text{pH}$ of solution goes below isoelectric point, the amine group and carboxyl group exists as $-\stackrel{+}{\text{N}}{\text{H}}_{\text{3}}$ and $-\text{COOH}$, respectively.

The isoelectric point of threonine is $5.60$. Therefore, the predominant form of threonine at $\text{pH}=1$ is,

Figure 1

The overall charge on threonine at $\text{pH}=1$ is $+1$.

Conclusion

The predominant form of threonine at $\text{pH}=1$ is shown in Figure 1 and the overall charge on it is $+1$.

Interpretation Introduction

(b)

Interpretation: The predominant form of methionine at $\text{pH}=1$ and the overall charge on it at this $\text{pH}$ is to be predicted.

Concept introduction: At isoelectric point, the amino acids exist in their neutral form. The amine groups exists as $-\stackrel{+}{\text{N}}{\text{H}}_3$ and carboxyl groups as $-{\text{COO}}^{-}$. The overall charge on an amino acid at isoelectric point is $0$.

Answer to Problem 29.36P

The predominant form of methionine at $\text{pH}=1$ is

The overall charge on it is $+1$.

Explanation of Solution

At isoelectric point, the amino acids exist in their neutral form. The amine groups exists as $-\stackrel{+}{\text{N}}{\text{H}}_3$ and carboxyl groups as $-{\text{COO}}^{-}$. However, when the $\text{pH}$ of solution goes below isoelectric point, the amine group and carboxyl group exists as $-\stackrel{+}{\text{N}}{\text{H}}_{\text{3}}$ and $-\text{COOH}$, respectively.

The isoelectric point of methionine is $5.74$. Therefore, the predominant form of methionine at $\text{pH}=1$ is,

Figure 2

The overall charge on methionine at $\text{pH}=1$ is $+1$.

Conclusion

The predominant form of methionine at $\text{pH}=1$ is shown in Figure 2 and the overall charge on it is $+1$.

Interpretation Introduction

(c)

Interpretation: The predominant form of aspartic acid at $\text{pH}=1$ and the overall charge on it at this $\text{pH}$ is to be predicted.

Concept introduction: At isoelectric point, the amino acids exist in their neutral form. The amine groups exists as $-\stackrel{+}{\text{N}}{\text{H}}_3$ and carboxyl groups as $-{\text{COO}}^{-}$. The overall charge on an amino acid at isoelectric point is $0$.

Answer to Problem 29.36P

The predominant form of aspartic acid at $\text{pH}=1$ is,

The overall charge on it is $+1$.

Explanation of Solution

At isoelectric point, the amino acids exist in their neutral form. The amine groups exists as $-\stackrel{+}{\text{N}}{\text{H}}_3$ and carboxyl groups as $-{\text{COO}}^{-}$. However, when the $\text{pH}$ of solution goes below isoelectric point, the amine group and carboxyl group exists as $-\stackrel{+}{\text{N}}{\text{H}}_{\text{3}}$ and $-\text{COOH}$, respectively.

The isoelectric point of aspartic acid is $2.98$. Therefore, the predominant form of aspartic acid at $\text{pH}=1$ is,

Figure 3

The overall charge on aspartic acid at $\text{pH}=1$ is $+1$.

Conclusion

The predominant form of aspartic acid at $\text{pH}=1$ is shown in Figure 3 and the overall charge on it is $+1$.

Interpretation Introduction

(d)

Interpretation: The predominant form of arginine at $\text{pH}=1$ and the overall charge on it at this $\text{pH}$ is to be predicted.

Concept introduction: At isoelectric point, the amino acids exist in their neutral form. The amine groups exists as $-\stackrel{+}{\text{N}}{\text{H}}_3$ and carboxyl groups as $-{\text{COO}}^{-}$. The overall charge on an amino acid at isoelectric point is $0$.

Answer to Problem 29.36P

The predominant form of arginine at $\text{pH}=1$ is,

The overall charge on it is $+3$.

Explanation of Solution

At isoelectric point, the amino acids exist in their neutral form. The amine groups exists as $-\stackrel{+}{\text{N}}{\text{H}}_3$ and carboxyl groups as $-{\text{COO}}^{-}$. However, when the $\text{pH}$ of solution goes below isoelectric point, the amine group and carboxyl group exists as $-\stackrel{+}{\text{N}}{\text{H}}_{\text{3}}$ and $-\text{COOH}$, respectively.

The isoelectric point of arginine is $10.76$. Therefore, the predominant form of arginine at $\text{pH}=1$ is,

Figure 4

The overall charge on arginine at $\text{pH}=1$ is $+3$.

Conclusion

The predominant form of arginine at $\text{pH}=1$ is shown in Figure 4 and the overall charge on it is $+3$.