Amino Acids - Practice Questions & MCQ

Amino acids have amino(-NH₂) and carboxyl(-COOH) functional groups. Based on the relative position of the two functional groups in the alkyl chain, the amino acids are categorised as α, β, ?, δ and so on. On the hydrolysis of proteins, only α-amino acids are formed. Amino acids may also contain other functional groups.

• Amino acids which can be synthesized in our body are known as non-essential amino acids while those can not be synthesized in our body are known as essential amino acids.
• They are usually colourless, water-soluble, high melting and crystalline solids.
• Except glycine, all other naturally occurring amino acids are optically active.
• Most naturally occurring amino acids have L-configuration. 

Since, the -NH₂ group is basic and -COOH group is acidic, in neutral solution, it exists in an internal ionic form called a zwitter ion where the proton of -COOH group is transferred to the -NH₂ group to form inner salt, also known as dipolar ion.

The zwitter ion is dipolar, charged but overall electrically neutral and contains both a positive annd negative charge. Therefore, amino acids are high melting crystalline solids moderately soluble in water and amphoteric in nature. Depending on the pH of the solution, the amino acid can donate or accpet proton. In the acidic medium, COO^- ion acts as the base and accepts a proton to form the cation(II) while in the basic medium, ⁺NH₃ ion loses a proton to form the anion(III). Thus, ⁺NH₃ group acts as the acid while COO^- group acts as the base. 
When an ionised form of amino acid is placed in an electric field, it will migrate towards the opposite electrodes. Depending on the pH of the medium, following three things may happen:

1. In acidic solution(low pH), the positive ion moves towards cathode.
2. In basic solution, the negative ion moves towards anode.
3. The zwitter ion does not move towards any of the electrodes.

Electrophoresis is a method used for the separation and analysis of amino acids. At isoelectric point, an amino acid has the least solubility in water. This method is based on pH control and electric charge. The amino acids differ in their isoelectric point.

Amino acids                 Isoelectric point
(i) Neutral                   pH lies between 5 - 6.3
(ii) Acidic                   pH lies between 3 - 5.4
(iii) Basic                   pH lies between 7.6 - 10.8

NOTE: 

1. Esterification of amino acid should be carried out in acidic medium due to the presence of structure(II).
2. Acylation should be carried out in basic medium due to the presence of structure(III).
3. p-Aminobenzoic acid and o-Aminobenzoic acid do not exist in the form of zwitter-ion due to lesser basic strength of aromatic amines.
4. p-Aminobenzenesulphonic acid can exist in the form of zwitter-ion.

Test for amino acids and proteins
There are two types of tests, viz:

1. Ninhydrin test: This test is given by all proteins and amino acids. When a protein is boiled with a dilute solution of ninhydrin, a blue-violet colour is produced.
2. Biuret test: On adding a dilute solution of copper sulphate to alkaline solution of protein, violet colour is developed. This test is due to the presence of the peptide (-CO-NH-) linkage.