Chapter 3, Problem 9RE

MATHEMATICAL Predict the predominant forms of the amino acids from Question 8 at pH 10.

Interpretation Introduction

Interpretation:

The predominant forms of amino acids, histidine, asparagine, tryptophan, proline, and tyrosine, at pH 10 are to be predicted.

Concept introduction:

The name ‘amino acids’ itself implies that they have the amino group and carboxylic acid group. The central atom of the amino acid is called the alpha-carbon and that is why all the amino acids are called alpha-amino acids.

The naturally occurring amino acids are alpha-amino acids. The amino group attains the positive charge and the carboxyl group attains the negative charge at the neutral pH (potential of hydrogen).

Answer to Problem 9RE

Solution:

The imidazole ring of the histidine amino acid is deprotonated and the $\alpha$-amino group is also deprotonated at the pH 10, so the overall histidine amino acid becomes negatively charged because of the presence of the negative charge on the carboxyl group.

The $\alpha$-amino group of the asparagine amino acid is deprotonated at the pH 10, so the asparagine amino acid as a whole becomes negatively charged due to the presence of the negative charge on the carboxyl group.

The $\alpha$-amino group of the tryptophan amino acid is predominantly deprotonated, so the amino acid becomes negatively charged because the $\alpha$-carboxyl group loses the proton ion at the pH 10 leading to the development of the negative charge on the entire amino acid, making the amino acid anionic in nature.

The $\alpha$-amino group of proline amino acid becomes partially deprotonated at the pH 10, making the overall amino acid neutral in nature.

The $\alpha$-amino group of the tyrosine is predominantly deprotonated at the pH 10 and there is a hydroxyl group in the side chain in the structure of the amino acid. The phenolic hydroxyl group is a mixture of 50% protonated and 50% deprotonated forms, leading to the anionic nature of the tyrosine.

Explanation of Solution

The imidazole ring in the side chain of the histidine amino acid remains partially protonated in the physiological condition, which classifies the amino acid as the positively charged amino acid and keeps this amino acid in the polar amino acid category.

The histidine loses the proton ions from the imidazole ring and the $\alpha$-amino group of the amino acid. The carboxyl group of the amino acid also loses the proton ion and becomes negative. This negative charge contributes to the anionic nature of the amino acid at pH 10.

The $\alpha$-amino group of the asparagine remains protonated at the physiological pH. Deprotonation occurs when the pH is increased.

When the pH increases and reaches to 10, the $\alpha$-amino acid loses the proton and there is only one negative charge, which is present on the carboxyl group. This negative charge contributes to the anionic behavior of the asparagine at pH 10.

The $\alpha$-amino group of the tryptophan amino acid remains positively charged and the carboxyl group remains neutral, but as the pH is increased, deprotonation occurs, and both the amino group and carboxyl group lose the proton ions.

Then, the amino group becomes neutral and the carboxyl group attains the negative charge. The negative charge of the carboxyl group contributes to the anionic behavior of the tryptophan amino acid at the pH 10.

The proline remains positively-charged at the acidic pH as the amino group is having the proton ion, and as the pH increases, the deprotonation occurs and the carboxyl group loses the proton ion and becomes negatively-charged.

The positive charge and the negative charge cancel out the effect of each other and thus, the whole amino acid becomes neutral in nature at the pH 10.

The tyrosine has the phenyl group in the side chain and hydroxyl group is also present at the phenyl group. The phenolic hydroxyl is 50% mixture of the protonated and deprotonated forms.

The $\alpha$-amino group of the tyrosine is predominantly deprotonated at pH 10 and the carboxyl group is having a negative charge. So, the negative charge on the carboxyl group is responsible for the overall negative charge on the amino acid.

Conclusion

Therefore, it can be concluded that the histidine becomes anionic, asparagine becomes anionic, tryptophan becomes anionic, proline becomes neutral, and tyrosine becomes anionic at the pH 10.

(a)

Anion

(b)

Anion

(c)

Anion

(d)

Neutral

(e)

Anion

Explanation:

COOH COO^- COO^-

׀ ׀ ׀

H₃N⁺ — C — H H₃N⁺ — C — H H₃N⁺ — C — H

׀ ׀ ׀

CH₂ CH₂ CH₂

׀ ׀ ׀

At pH = 1 → At pH= 1.82 → At pH = 6

Charge = +2 Charge = +1 Charge = 0

↓

COO^-

H₃N— C — H

׀

CH₂

׀

At pH = 10 ← At pH = 9.71

Charge = -1 Charge = -1

Anionic form

Hence, the predominant form of histidine at pH 10 is anionic.

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(b)

Given information: Asparagine at pH 10

Explanation:

COOH COO^- COO^-

׀ ׀ ׀

H₃N⁺ — C — H H₃N⁺ — C — H H₂N — C — H

׀ | |

CH₂ CH₂ CH₂

׀ ׀ ׀

C = O C = O C = O

׀ ׀ ׀

NH₂ NH₂ NH₂

At pH = 1 → At pH= 2.02 → At pH = 8.80 → At pH = 10

Charge = +1 Charge = 0 Charge = -1 Charge = -1

Anionic

form

Hence, the predominant form of asparagine at pH 10 is anionic.

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(c)

Given information: Tryptophan at pH 10

Explanation:

COOH COO^- COO^-

׀ ׀ ׀

H₃N⁺ — C — H H₃N⁺ — C — H H₂N — C — H

׀ ׀ ׀

CH₂ CH₂ CH₂

׀ ׀ ׀

At pH = 1 → At pH= 2.38 → At pH = 9.39 → At pH = 10

Charge = +1 Charge = 0 Charge = -1 Charge = -1

(Anionic form)

Hence, the predominant form of tryptophan at pH 10 is anionic.

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(d)

Given information: Proline at pH 10.

Explanation:

At pH = 1 → At pH= 1.99 → At pH = 10

Charge = +1 Charge = 0 Charge = 0

Neutral form

Hence, the predominant form of proline at pH 10 is neutral.

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(e)

Given information: Tyrosine at pH 10.

Explanation:

COOH COO^- COO^-

׀ ׀ ׀

H₃N⁺ — C — H H₃N⁺ — C — H H₂N — C — H

׀ ׀ ׀

CH₂ CH₂ CH₂

׀ ׀ ׀

At pH = 1 → At pH= 2.20 → At pH = 9.11 → At pH = 10

Charge = +1 Charge = 0 Charge = -1 Charge = -1

(Anionic form)

Hence, the predominant form of tyrosine at pH 10 is anionic.