Question

Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule. Which of the following are acidic?

(i)

$$\begin{array}{c}(\mathrm{CH}{\phantom{A}}_3){\phantom{A}}_2\mathrm{CH}-\mathrm{CH}-\mathrm{COOH}\\ \phantom{..}|\\ \phantom{.....}\mathrm{NH}{\phantom{A}}_2\end{array}$$

(ii)

$$\begin{array}{c}\mathrm{HOOC}-\mathrm{CH}{\phantom{A}}_2-\mathrm{CH}{\phantom{A}}_2-\mathrm{CH}-\mathrm{COOH}\\ \phantom{............}|\\ \phantom{...............}\mathrm{NH}{\phantom{A}}_2\end{array}$$

(iii)

$$\mathrm{H}{\phantom{A}}_2\mathrm{N}-\mathrm{CH}{\phantom{A}}_2-\mathrm{CH}{\phantom{A}}_2-\mathrm{CH}{\phantom{A}}_2-\mathrm{COOH}$$

(iv)

$$\begin{array}{c}\mathrm{HOOC}-\mathrm{CH}{\phantom{A}}_2-\mathrm{CH}-\mathrm{COOH}\\ \phantom{.....}|\\ \phantom{........}\mathrm{NH}{\phantom{A}}_2\end{array}$$

Answer 1

(ii)

$$\begin{array}{c}\mathrm{HOOC}-\mathrm{CH}{\phantom{A}}_2-\mathrm{CH}{\phantom{A}}_2-\mathrm{CH}-\mathrm{COOH}\\ \phantom{............}|\\ \phantom{...............}\mathrm{NH}{\phantom{A}}_2\end{array}$$

(iv)

$$\begin{array}{c}\mathrm{HOOC}-\mathrm{CH}{\phantom{A}}_2-\mathrm{CH}-\mathrm{COOH}\\ \phantom{.....}|\\ \phantom{........}\mathrm{NH}{\phantom{A}}_2\end{array}$$

Explanation:

Acidic amino acids are divided into two groups, one –COOH group against the other –NH₂ group. As result, the acidic amino acids (ii) and (iv) are the only ones among the supplied structures.