Chapter 27, Problem 27.10P

Interpretation Introduction

(a)

Interpretation:

The peptide $\text{Gly}-\text{Leu}-\text{Val}$, is to be classified as the acidic, basic, or neutral. The net charge on the peptide at $\text{pH}=6$ is to be stated.

Concept introduction:

Amino acids are classified as acidic, basic and neutral according to the functional group they possesses. Acidic amino acids are those which have one more than one carboxylic acid group in its side chain and basic amino acids are those which have one or more amino group present in their side chain. Isoelectric point of the amino acids is the pH of the dilute aqueous solution of the amino acid at which the total charge on all molecules of amino acid is zero.

Answer to Problem 27.10P

The peptide $\text{Gly}-\text{Leu}-\text{Val}$ is neutral as shown below.

There will be no charge on the peptide, $\text{Gly}-\text{Leu}-\text{Val}$ at $\text{pH}=6$.

Explanation of Solution

Peptides are classified as acidic, basic, or neutral according to the amino acids they contain. If they contain unbalanced basic amino acid then they are considered as basic peptide and if they contain unbalanced acidic amino acid then they are considered as basic peptides. If the basic and acidic amino acids are balanced or not at all present in the peptide then it is considered as neutral peptide.

The given peptide $\text{Gly}-\text{Leu}-\text{Val}$ is shown below.

Figure 1

This tripeptide contains one amino group and one carboxylic group. So, this amino acid is neutral in nature. The amine group of glycine having ${\text{pK}}_{\text{a}}$ equals to $9.60$ remains in the protonated form at $\text{pH}=6$. On the other hand, carboxylic group of valine having ${\text{pK}}_{\text{a}}$ equals to $2.32$ remains in the carboxylate form at $\text{pH}=6$ as shown below.

Figure 2

Therefore, the net charge on the peptide, $\text{Gly}-\text{Leu}-\text{Val}$ remains zero at $\text{pH}=6$.

Conclusion

The peptide $\text{Gly}-\text{Leu}-\text{Val}$ is neutral and no net charge is present on this peptide.

Interpretation Introduction

(b)

Interpretation:

The peptide, $\text{Leu}-\text{Trp}-\text{Lys}-\text{Gly}-\text{Lys}$, is to be classified as the acidic, basic, or neutral. The net charge on the peptide at $\text{pH}=6$ is to be stated.

Concept introduction:

Amino acids are classified as acidic, basic and neutral according to the functional group they possesses. Acidic amino acids are those which have one more than one carboxylic acid group in its side chain and basic amino acids are those which have one or more amino group present in their side chain. Isoelectric point of the amino acids is the pH of the dilute aqueous solution of the amino acid at which the total charge on all molecules of amino acid is zero.

Answer to Problem 27.10P

The peptide, $\text{Leu}-\text{Trp}-\text{Lys}-\text{Gly}-\text{Lys}$ is basic as shown below.

The net charge on the peptide, $\text{Leu}-\text{Trp}-\text{Lys}-\text{Gly}-\text{Lys}$ is $+3$ at $\text{pH}=6$.

Explanation of Solution

Peptides are classified as acidic, basic, or neutral according to the amino acids they contain. If they contain unbalanced basic amino acid then they are considered as basic peptide and if they contain unbalanced acidic amino acid then they are considered as basic peptides. If the basic and acidic amino acids are balanced or not at all present in the peptide then it is considered as neutral peptide.

The given peptide, $\text{Leu}-\text{Trp}-\text{Lys}-\text{Gly}-\text{Lys}$ is shown below.

Figure 3

This given peptide contains three amino group in its chain and one carboxylic group. So, this amino acid is basic in nature. All the amine group of the given peptide having value of ${\text{pK}}_{\text{a}}$ greater than $\text{pH}=6$, so, the amine group remains in the protonated form. On the other hand, carboxylic group of lysine having ${\text{pK}}_{\text{a}}$ equals to $2.18$ remains in the carboxylate form at $\text{pH}=6$ as shown below.

Figure 4

Therefore, the net charge of $+3$ is present on the peptide, $\text{Leu}-\text{Trp}-\text{Lys}-\text{Gly}-\text{Lys}$ at $\text{pH}=6$.

Conclusion

The peptide, $\text{Leu}-\text{Trp}-\text{Lys}-\text{Gly}-\text{Lys}$ is basic in nature and $+3$ charge is present on this peptide.

Interpretation Introduction

(c)

Interpretation:

The peptide, $N-\text{acetyl}-\text{Asp}-\text{Val}-\text{Ser}-\text{Arg}-\text{Arg}$, is to be classified as the acidic, basic, or neutral. The net charge on the peptide at $\text{pH}=6$ is to be stated.

Concept introduction:

Amino acids are classified as acidic, basic and neutral according to the functional group they possesses. Acidic amino acids are those which have one more than one carboxylic acid group in its side chain and basic amino acids are those which have one or more amino group present in their side chain. Isoelectric point of the amino acids is the pH of the dilute aqueous solution of the amino acid at which the total charge on all molecules of amino acid is zero.

Answer to Problem 27.10P

The peptide $N-\text{acetyl}-\text{Asp}-\text{Val}-\text{Ser}-\text{Arg}-\text{Arg}$ is neutral as shown below.

There will be no charge on the peptide, $N-\text{acetyl}-\text{Asp}-\text{Val}-\text{Ser}-\text{Arg}-\text{Arg}$ at $\text{pH}=6$.

Explanation of Solution

Peptides are classified as acidic, basic, or neutral according to the amino acids they contain. If they contain unbalanced basic amino acid then they are considered as basic peptide and if they contain unbalanced acidic amino acid then they are considered as basic peptides. If the basic and acidic amino acids are balanced or not at all present in the peptide then it is considered as neutral peptide.

The given peptide $N-\text{acetyl}-\text{Asp}-\text{Val}-\text{Ser}-\text{Arg}-\text{Arg}$ is shown below.

Figure 5

This given peptide contains two amino groups in the side chain and two carboxylic groups. So, this amino acid is neutral in nature. All the amine groups of this peptide having ${\text{pK}}_{\text{a}}$ greater than $\text{pH}=6$, so, amine groups remain protonated. On the other hand, carboxylic group of arginine and aspartic acids having ${\text{pK}}_{\text{a}}$ smaller than $\text{pH}=6$ and remains in the carboxylate form as shown below.

Figure 6

Therefore, the net charge on the peptide, $N-\text{acetyl}-\text{Asp}-\text{Val}-\text{Ser}-\text{Arg}-\text{Arg}$ remains zero at $\text{pH}=6$.

Conclusion

The peptide $N-\text{acetyl}-\text{Asp}-\text{Val}-\text{Ser}-\text{Arg}-\text{Arg}$ is neutral and no net charge is present on this peptide.

Interpretation Introduction

(d)

Interpretation:

The peptide $\text{Glu}-\text{Lys}-\text{Asp}-\text{Ala}-\text{Phe}-\text{Ile}$, is to be classified as the acidic, basic, or neutral. The net charge on the peptide at $\text{pH}=6$ is to be stated.

Concept introduction:

Amino acids are classified as acidic, basic and neutral according to the functional group they possesses. Acidic amino acids are those which have one more than one carboxylic acid group in its side chain and basic amino acids are those which have one or more amino group present in their side chain. Isoelectric point of the amino acids is the pH of the dilute aqueous solution of the amino acid at which the total charge on all molecules of amino acid is zero.

Answer to Problem 27.10P

The peptide $\text{Glu}-\text{Lys}-\text{Asp}-\text{Ala}-\text{Phe}-\text{Ile}$ is acidic in nature as shown below.

The charge present on the peptide, $\text{Glu}-\text{Lys}-\text{Asp}-\text{Ala}-\text{Phe}-\text{Ile}$ is $-2$ at $\text{pH}=6$.

Explanation of Solution

Peptides are classified as acidic, basic, or neutral according to the amino acids they contain. If they contain unbalanced basic amino acid then they are considered as basic peptide and if they contain unbalanced acidic amino acid then they are considered as basic peptides. If the basic and acidic amino acids are balanced or not at all present in the peptide then it is considered as neutral peptide.

The given peptide $\text{Glu}-\text{Lys}-\text{Asp}-\text{Ala}-\text{Phe}-\text{Ile}$ is shown below.

Figure 7

This given peptide contains two amino groups in the side chain and four carboxylic groups. So, this amino acid is acidic in nature. All the amine groups of this peptide having ${\text{pK}}_{\text{a}}$ greater than $\text{pH}=6$, so, amine groups remain protonated. On the other hand, carboxylic group of the peptide is having ${\text{pK}}_{\text{a}}$ value smaller than $\text{pH}=6$ and remains in the carboxylate form as shown below.

Figure 8

Therefore, the net charge on the peptide, $\text{Glu}-\text{Lys}-\text{Asp}-\text{Ala}-\text{Phe}-\text{Ile}$ is $-2$ at $\text{pH}=6$.

Conclusion

The peptide $\text{Glu}-\text{Lys}-\text{Asp}-\text{Ala}-\text{Phe}-\text{Ile}$ is acidic and $-2$ net charge is present on this peptide.