Chapter 17, Problem 110QP

(a)

Interpretation Introduction

Interpretation:

The structure that corresponds to tripeptide $\text{Leu-Ser-Ala}$ is to be identified. The other tripeptides in the given structures are to be identified.

Explanation of Solution

The simplest amino acid is alanine after glycine because the side chain in alanine is the methyl group $\left(-{\text{CH}}_3\right)$ that is connected to the $\alpha \text{-}$ carbon atom. Leucine is the amino acid that consists of glycine in which the hydrogen atom is replaced by an isobutyl group $\left(-{\text{CH}}_2-\text{CH}-{\left({\text{CH}}_3\right)}_2\right)$ . Serine is the amino acid that consists of side chain that has hydroxymethyl group $\left(-{\text{CH}}_2\text{OH}\right)$ . In the structure D, the first amino acid has a side chain $\left(-{\text{CH}}_2-\text{CH}-{\left({\text{CH}}_3\right)}_2\right)$ . Thus, it is leucine $\left(\text{Leu}\right)$ . The second amino acid has a side chain $\left(-{\text{CH}}_2\text{OH}\right)$ , so the second amino acid is serine $\left(\text{Ser}\right)$ and the side chain in the third amino acid is $\left(-{\text{CH}}_3\right)$ , so it is alanine. Thus, the structure represents the tripeptide $\text{Leu-Ser-Ala}$ .

In the structure A, the first amino acid has the side chain $\left(-\text{CH}{\left({\text{CH}}_3\right)}_2\right)$ , this means that first amino acid is valine $\left(\text{Val}\right)$ . The second amino acid has a side chain $\left(-{\text{CH}}_2\text{SH}\right)$ , so, the second amino acid is cysteine $\left(\text{Cys}\right)$ . The third amino acid has methyl group $\left(-{\text{CH}}_3\right)$ in its side chain, this means the third amino acid is alanine $\left(\text{Ala}\right)$ . Therefore, the structure A represents the tripeptide $\text{Val-Cys-Ala}$ .

(b)

Interpretation Introduction

Interpretation:

The other tripeptides in the given structures are to be identified.

Explanation of Solution

In the structure B, the first amino acid has methyl group $\left(-{\text{CH}}_3\right)$ in its side chain. This means that the first amino acid is alanine $\left(\text{Ala}\right)$ . The second amino acid has hydroxymethyl group $\left(-{\text{CH}}_2\text{OH}\right)$ in its side chain. Thus, the second amino acid is serine $\left(\text{Ser}\right)$ . The third amino acid has isobutyl group $\left(-{\text{CH}}_2-\text{CH}-{\left({\text{CH}}_3\right)}_2\right)$ in its side chain. Thus, the third amino acid is Leucine $\left(\text{Leu}\right)$ . Therefore, the structure represents the tripeptide $\text{Ala-Ser-Leu}$ .

(c)

Interpretation Introduction

Interpretation:

The other tripeptides in the given structures are to be identified.

Explanation of Solution

In the structure C, the first amino acid has hydrogen atom and it is not replaced by any substituent. So, the first amino acid is glycine $\left(\text{Gly}\right)$ . The second amino acid has $\left(-{\text{CH}}_2\text{OH}\right)$ in its side chain. Thus, the second amino acid is serine $\left(\text{Ser}\right)$ . The third amino acid has methyl group $\left(-{\text{CH}}_3\right)$ in its side chain. Thus, the third amino acid is alanine $\left(\text{Ala}\right)$ . Therefore, the structure C represents the peptide $\text{Gly-Ser-Ala}$ .

(d)

Interpretation Introduction

Interpretation:

The other tripeptides in the given structures are to be identified.

Explanation of Solution

In the structure D, the first amino acid has a side chain $\left(-{\text{CH}}_2-\text{CH}-{\left({\text{CH}}_3\right)}_2\right)$ . Thus, it is leucine $\left(\text{Leu}\right)$ . The second amino acid has a side chain $\left(-{\text{CH}}_2\text{OH}\right)$ , so the second amino acid is serine $\left(\text{Ser}\right)$ and the side chain in the third amino acid is $\left(-{\text{CH}}_3\right)$ , so it is alanine. Thus, the structure represents the tripeptide $\text{Leu-Ser-Ala}$ .

(e)

Interpretation Introduction

Interpretation:

The other tripeptides in the given structures are to be identified.

Explanation of Solution

In the structure E, first amino acid has isobutyl group $\left(-{\text{CH}}_2-\text{CH}-{\left({\text{CH}}_3\right)}_2\right)$ in its side chain. Thus, the first amino acid is Leucine $\left(\text{Leu}\right)$ . The second amino acid has $\left(-\text{CH}\left(\text{OH}\right){\text{CH}}_3\right)$ in its side chain. This means that the second amino acid is threonine $\left(\text{Thr}\right)$ . The third amino acid has a methyl group $\left(-{\text{CH}}_3\right)$ in its side chain. Thus, the third amino acid is alanine $\left(\text{Ala}\right)$ . Thus, the structure E represents the tripeptide $\text{Leu-Thr-Ala}$ .