Question

For alpha-amino acids having the structure $R-CH(N{{H}_{2}})-COOH$
Which of the following statements are true
(A) Water solubility is maximum at a pH when concentrations of anions and cations are equal
(B) They give ninhydrin test
(C) On reacting with nitrous acid give off ${{N}_{2}}$
A. All
B. B and C
C. A and B
D. A

Answer 1

Hint: Protein is a biological polymer whose monomer is amino acid. Amino acids are aliphatic carboxylic acids having one basic amino group and one acidic carboxylic group. All alpha amino acids have $L$ structures.

Complete Step by Step Solution:
Amino acids contain both acidic and basic functional groups which exist as zeiter ions. At about pH $5.6$ the concentration of the cation is equal to the concentration of the anion. But this has no influence on the solubility of amino acids.

Ninhydrin test of alpha amino acids is the characteristic test of alpha amino acids. The amino group of free amino acid undergoes a chemical reaction with a compound known as ninhydrin, which always behaves as an oxidising agent. When exposed to ninhydrin, the amino acid gives carbon dioxide, ammonia, water along with an aldehyde along with hydrindantin.

Alpha amino acid on treatment with nitrous acid gives alcoholic type compounds along with water and nitrogen gas.
The reaction is as follows:
$R-CH(N{{H}_{2}})-COOH+HN{{O}_{2}}\to R-CH(OH)-COOH+{{H}_{2}}O+{{N}_{2}}$
Thus the correct option is $B$.

Note: Alpha amino acids give ninhydrin test as a characteristic test of amino acid. It gives off nitrogen gas on treatment with nitrous acid. Amino acids are the building blocks of the protein connected by peptide links.