Question

Amino acids generally exist in the form of Zwitterions. This means they contain:
A. basic $- {\text{N}}{{\text{H}}_{\text{2}}}$ and acidic $- {\text{COOH}}$ group
B. the basic $- {{\text{N}}^ + }{{\text{H}}_3}$ and acidic $- {\text{CO}}{{\text{O}}^ - }$ group
C. basic $- {\text{N}}{{\text{H}}_{\text{2}}}$ and acidic $- {{\text{H}}^ + }$ group
D. basic $- {\text{CO}}{{\text{O}}^ - }$ an acidic $- {{\text{N}}^ + }{{\text{H}}_3}$ group

Answer 1

To answer this question we should talk about amino acids. The amino acids are made mainly of carbon, hydrogen, nitrogen and oxygen. The amino acids have amine group, carboxylic acid and alkyl chain. In water, the carboxylic acid loses proton and amine gains proton. The formed structure is known as Zwitterion. We will check the behaviour of ions in water to determine which one is acidic and which one is basic.

Complete step-by-step answer:
In all amino acids two functional groups are present. One is carboxylic and the second is an amine group. The carboxylic acid donate proton so it behaves as acid and amine has lone pair so it can accept the proton so it behaves as base.

As both acidic part and basic part are present in amino acid so, amino acid behaves as an acid as well as a base in water. It’s carboxylic part donates proton in water simultaneously the basic part gain the proton so, one side of amine acid (having carboxylic acid) gains negative charge after losing proton and another side of amino acid (having amine group) gains positive charge after accepting proton.

This structure of amino acid having oppositely charged sides is known as Zwitterion.

So, the Zwitter on contain carboxylate ion $- {\text{CO}}{{\text{O}}^ - }$ and $- {{\text{N}}^ + }{{\text{H}}_3}$ ion. As the carboxylate ion $- {\text{CO}}{{\text{O}}^ - }$ has negative charge so, it can accept proton so, it is basic and $- {{\text{N}}^ + }{{\text{H}}_3}$ ion has positive charge so, it can lose proton so, it behaves as base.

So, amino acids generally exist in the form of Zwitterions. This means they contain basic $- {\text{CO}}{{\text{O}}^ - }$ an acidic $- {{\text{N}}^ + }{{\text{H}}_3}$ group.

Therefore, option (D) basic $- {\text{CO}}{{\text{O}}^ - }$ an acidic $- {{\text{N}}^ + }{{\text{H}}_3}$ group, is correct.

Note: Zwitterion contains two poles of opposite charge, so it is also known as dipolar ion. As Zwitterion contains positive as well negative charge, so overall charge is canceled out so Zwitterions are neutral. The isoelectronic point is the term used for amine acid based on the behaviour of amino acids in acidic or basic medium. In the acidic medium, the basic part amine of amino acid gets protonated and on passing current, the protonated positive charged amino acid migrates towards the cathode. In the basic medium, acidic part amine of amino acid lose proton and becomes negatively charged and on passing current, the deprotonated negatively charged amino acid migrates towards the anode. The pH at which amino acid does not migrate anywhere is known as isoelectronic point.