Question

Draw the structure of the predominant form of

(a)Isoleucine at pH 11 (b) Proline at pH 2

(c)Arginine at pH 7 (d) Glutamic acid at pH 7

A mixture of alanine, lysine, and aspartic acid at 1). pH 6 ; 2). pH 11; 3). pH 2

Short answer

(a)

Isoleucine at pH 11

(b)

Proline at pH 2

(c)

Arginine at pH 7

(d)

Glutamic acid at pH 7

(e)

(i)

Alanine at pH 6

(ii)

Lysine at pH 11

(iii)

Aspartic acid at pH 2

Step-by-step solution

Amino acids

Amino acids are defined as compounds containing at least one amino group and one carboxylic group. The pH at which a particular amino acid does not migrate under the influence of an electric field is known as the isoelectric point.

Isoelectric point

Amino acids exist in their neutral form at the isoelectric point. The amine group exists as $-\stackrel{+}{{\mathrm{NH}}_3}$, and the carboxyl groups exist as $-{\mathrm{COO}}^{-}$. However, when the pH rises above the isoelectric point, the amine group becomes $-{\mathrm{NH}}_2$, and the carboxyl groups remain the same as $-{\mathrm{COO}}^{-}$. Also, when the pH falls below the isoelectric point, the amine group becomes $-\stackrel{+}{{\mathrm{NH}}_3}$, and the carboxyl groups become $-\mathrm{COOH}$.

(a)

The isoelectric point of isoleucine is $6.02$. Therefore, the predominant form of isoleucine at $\mathrm{pH}=11$can be represented as shown below:

Isoleucine at pH 11

(b)

The isoelectric point of proline is $6.30$. Therefore, the predominant form of proline at $\mathrm{pH}=2$can be represented as shown below:

Proline at pH 2

(c)

The isoelectric point of arginine is $10.8$. Therefore, the predominant form of arginine at $\mathrm{pH}=7$can be represented as shown below:

Arginine at pH 7

(d)

The isoelectric point of glutamic acid is $3.2$. Therefore, the predominant form of glutamic acid at $\mathrm{pH}=7$can be represented as shown below:

Glutamic acid at pH 7

(e)

The isoelectric point of alanine is $6.1$, that of lysine is $9.8$, and that of aspartic acid is $3.0$. Therefore, their predominant form at $\mathrm{pH}=6$ can be represented as shown below:

Alanine at pH 6

(ii)

The isoelectric point of alanine is $6.1$, that of lysine is $9.8$, and that of aspartic acid is $3.0$. Therefore, their predominant form at $\mathrm{pH}=11$ can be represented as shown below:

Lysine at pH 11

(iii)

The isoelectric point of alanine is $6.1$, that of lysine is $9.8$, and that of aspartic acid is $3.0$. Therefore, their predominant form at $\mathrm{pH}=2$ can be represented as shown below:

Aspartic acid at pH 2