Introduction

Illustration 38. Identify the end product (C)

Amino acids and Proteins:

Amino acids are derivatives of carboxylic acids in which a hydrogen atom at the is replaced by an amino group. Amino acids are obtained by hydrolysis of proteins. The amino acids can be represented by

About twenty five amino acids are obtained from proteins, of which about ten are essential, i.e. a deficiency in any one prevents growth in young animals and may even cause death. Simplest amino acids are

Amino acids are amphoteric.

Amino acids are linked together by peptide linkages.

Peptides are condensation products of two or more amino acids through their amino and carboxylic groups involving elimination of water molecules. Amino acid polymers with relative molecular masses 10,000 or less are called polypeptides. Those with larger molecular masses are called proteins e.g. egg albumen 40,000.

Illustration 39. Explain why does glycine exist as a Zwitter ion but anthranilic acid does not.

Solution: The ⎯COOH group of glycine releases H+ ion which is accepted by ⎯NH₂ group. Thus glycine exists as a zwitter ion.

However, the electron withdrawing nature of ⎯COOH group reduces electron density on N in anthranilic acid. Thus nitrogen atom of amino group fails to accepts the proton released by the ⎯COOH group. That is why anthranilic acid does not exist as zwitter ion.

Test for Proteins:

Presence of proteins can be shown by warming the suspected protein with dilute NaOH solution and CuSO₄ solution. A violet colouration confirms presence of proteins.

Methods of preparation of amino acids:

(i) Amination of

(ii) Gabriel phthalimide synthesis:

acid or ester combines with potassium phthalimide. Product on hydrolysis gives amino acid

(iii) Knoop synthesis:

keto acid on treatment with NH₃ forms an imine which on catalytic reduction gives amino acid.

(iv) Strecker synthesis:

Aldehydes react with HCN and ammonia or NH₄CN and the product on hydrolysis yields acids.

Some of the reactions given by amino acids are as follows:

Action of heat:

amino acids lose two molecules of water and form cyclic amides.

ß amino acids lose a molecule of ammonia per molecule of amino acid to yield unsaturated acids.