Question

Within the list shown below, the correct pair of structures of alanine in $pH$ ranges $2-4$ and $9-11$ is
(I) $H_3N^+ - CH(CH_3)CO_2H$
(II) $H_2N - CH(CH_3)CO_2^ -$
(II) $H_3N^+ - CH(CH_3)CO_2 ^-$
(IV) $H_2N - CH(CH_3)CO_2H$

• A. I and II
• B. I, and III
• C. II and III
• D. III and IV

Answer 1

Answer: (A)

Isoelectric point for neutral amino acids ranges from $pH 5.5$ to 6.3. Acidic amino acids have isoelectric point around 3 while basic amino acids have ranges from $pH 7.6$ to $10.8$.
At isoelectric point, amino acids exist as Zwitter ions. At low $pH$, it exists as cation and at high $pH$, it exists as anion. Hence, alanine (pH range $2-4$) will exist as cation while alanine (pH range $9-11$ ) will exist as anion.
Acidic medium:
$H _{3} N ^{+}- CH \left( CH _{3}\right) CO _{2} H$
Basic medium:
$H _{2} N - CH \left( CH _{3}\right) CO _{2}^{-}$