Chapter 16, Problem 16.25UKC

(a)

Interpretation Introduction

Interpretation:

Amino acid in ball and stick model below has to be determined.

Concept Introduction:

Amino acids are the biomolecules that contain an amino group, carboxylic acid group, and an alkyl chain. Beside basic amino acid glycine, all amino acids contain chirality centers. Amino acids can exist in different forms depend on value of $\text{pH}$ of aqueous solution. These forms can be explained as follows:

(1) Neutral amino acids exist in their zwitterionic form at $\text{pH}$ value 6.

(2) Amino acids exist with net $+1$ charge after proton is accepted by carboxylate anion if $\text{pH}$ is lowered to 2 or less than 2.

(3) Amino acids exist with net $-1$ charge after proton is donated by ammonium cation if $\text{pH}$ is raised to 10 or higher than 10.

(4) At physiological $\text{pH}$ 7.4, neutral amino acids are mainly exist in their zwitterionic forms.

(5) Value of $\text{pH}$ that has neutral form of amino acid is known as isoelectric point of that amino acid. Isoelectric point of neutral amino acids is around 6.

(6) Isoelectric point of amino acids with additional carboxylic group is around 3.

(7) Isoelectric point of amino acids with additional basic nitrogen atom is around $7.6-10.8$.

Explanation of Solution

In ball and stick model different atoms are as follows:

Amino acid can be drawn as follows:

(b)

Interpretation Introduction

Interpretation:

Three-letter and one-letter abbreviations for amino acid in ball and stick model below have to be determined.

Concept Introduction:

Refer to part (a).

Explanation of Solution

Structure of given amino acid as follows:

Since amino acid has $-{\text{CH}}_2{\text{CH}}_{\text{2}}{\text{SCH}}_{\text{3}}$ as side chain thus this amino acid is methionine. Therefore, three-letter and one-letter abbreviations for methionine are Met and M respectively.

(c)

Interpretation Introduction

Interpretation:

The form that presents at isoelectric point for amino acid represents below has to be determined.

Concept Introduction:

Refer to part (a).

Explanation of Solution

Structure of given amino acid as follows:

Since amino acid has $-{\text{CH}}_2{\text{CH}}_{\text{2}}{\text{SCH}}_{\text{3}}$ as side chain thus this amino acid is methionine. Neutral amino acids exist in their zwitterionic form at $\text{pH}$ value 6. Thus at isoelectric point, methionine exists as follows: