Question

Assume the perticular amino acid has an isoelectric point of $\mathrm{6.0.}$ In solution around pH $1.0$, which of the following species will predominate?

• A. $R|H_3\stackrel{\oplus }{N}{CHCO}_{2}H$
• B. $R|H_{2}N{CHCO}_{2}H$
• C. $R|H_3\stackrel{\oplus }{N}CHC{O}_2^{\ominus }$
• D. $R|H_{2}NCHC{O}_2^{\ominus }$

Answer 1

The correct option is A $R|H_3\stackrel{\oplus }{N}{CHCO}_{2}H$

At isoelectric point, amino acid is present as Zwitter ions (bipolar ion) i.e. with both $+ve$ and $-ve$ end.

$H_3{N}^{⨁}-\stackrel{\stackrel{R}{|}}{\underset{H}{\underset{|}{C}}}-CO{O}^{\ominus }Zwitterion$

In acidic medium or low $pH$, only cationic specie predominates as,

$H_{3}N-\stackrel{\stackrel{R}{|}}{\underset{H}{\underset{|}{C}}}-CO{O}^{\ominus }\stackrel{H^{+}}{\to }{H}_3{N}^{⨁}-\stackrel{\stackrel{R}{|}}{\underset{H}{\underset{|}{C}}}-COOH$