Elementary plant Biochemistry and Biotechnology (2+1)

a) Reactions due to amino group

Reaction with formaldehyde (Formal titration)

• Amino acid exists as zwitterion in aqueous medium. If an amino acid solution is treated with excess of neutralized formaldehyde solution, the amino group combines with formaldehyde forming dimethylol amino acid which is an amino acid formaldehyde complex.
• Hence the amino group is protected and the proton released is titrated against alkali.
  
• This method is used to find out the amount of total free amino acids in plant samples

Reaction with nitrous acid
• Nitrous acid reacts with the amino group of amino acids to form the corresponding hydroxyacids and liberate nitrogen gas

Reaction with ninhydrin
• Ninhydrin is a strong oxidizing agent.
  
• When a solution of amino acid is boiled with ninhydrin, the amino acid is oxidatively deaminated to produce ammonia and a ketoacid.
  
• The keto acid is decarboxylated to produce an aldehyde with one carbon atom less than the parent amino acid.
• The net reaction is that ninhydrin oxidatively deaminates and decarboxylates α-amino acids to CO2, NH3 and an aldehyde.
  
• The reduced ninhydrin then reacts with the liberated ammonia and another molecule of intact ninhydrin to produce a purple coloured compound known as Ruhemann's purple.
• This ninhydrin reaction is employed in the quantitative determination of amino acids.
• Proteins and peptides that have free amino group(s) (in the side chain) will also react and give colour with ninhydrin.