Organic Chemistry

Module 5. Amino acids and peptides

Lesson 16
IMPORTANT REACTIONS OF AMINO ACIDS

16.1 Introduction

1. Reaction with formaldehyde

Fig. 16.1

• Zwitter ion structure is eliminated- carboxylic acid group sets free- can be quantitatively titrated against standard alkali solution using phenolphthalein as an indicator -used for quantitative determination

2. Peptide bond formation

• -COOH group of α-carbon of one amino acid reacts with –NH₂ group of α-carbon of another amino acid- formation of peptide bond

Fig. 16.2 Dipeptide

• One more –NH₂ group of amino acid react with free –COOH group of dipeptide to give tripeptide- the reaction can go on indefinitely hundreds of amino acids
• When 100 or more amino acids are condensed through peptide linkage resulting a polypeptide having molecular weight/mass more than 10,000 is called protein. So α-amino acids are the building blocks of proteins.

Fig. 16.3 Polypeptide=Protein

3. Reaction with ninhydrin

• Often used for qualitative and quantitative analysis of free amino acids
• When an excess amount of ninhydrin reacts with amino acid

• For each mole of amino acid reacted with ninhydrin, one mole each of ammonia, aldehyde, carbon dioxide and hydrindantin are formed as intermediates.

Fig. 16.4 Reaction with ninhydrin

• The liberated ammonia subsequently reacts with one mole of ninhydrin and one mole of hydrindantin, forming a purple product

• Known as Ruhemann’s purple
• Has maximum absorbance at 570 nm

• Proline and hydroxyproline give a yellow product that has a maximum absorbance at 440 nm
• These colour reactions are the basis for colorimetric determination of amino acids

Fig. 16.5 Estimation of amino acids