MODULE 6: AMINO ACIDS

• Amino acids


  Monomers of proteins
• Amino acids


  • They are the building blocks of proteins
  • The different properties result from variations in the structures of different R groups known as the side chain
  • Each amino acid has at least one amine and one carboxylic acid functional group
• Determine the solubility of some given amino acids

  1. Solubility Test

  2. Solvents: Hot Water, NaOH, HCl, Chloroform
• Ninhydrin Test


  • Ninhydrin (triketohydridene hydrate) is a chemical used to detect primary amino group
  • POSITIVE RESULTS: Amino acids: Blue to purple solution, Proline: Yellow solution
• Xanthoproteic Test


  • Nitration of aromatic amino acid
  • POSITIVE RESULT: Tyr & Trp: Yellow to orange solution,
  • NEGATIVE RESULT: Other amino acids: no color change
• Histidine Test


  • Bromination of histidine in acid solution
  • POSITIVE RESULT: His: Blue to violet solution
  • REAGENTS: Bromine water, Acetic acid, 5% ammonium carbonate solution
• Sakaguchi's Test


  • Guanidinium group of arginine reacts with alpha naphthol upon treatment with hypochlorite under alkaline condition
  • POSITIVE RESULT: Arg: Red solution
• Pauly's diazo Test


  • Diazotization of single cyclic side groups such Tyr and His
  • POSITIVE RESULT: His & Tyr: Scarlet red solution
• Millon's Test


  • Nitration of hydroxybenzene ring
  • POSITIVE RESULT: Tyr: Red precipitate
• Lead sulfide Test


  • Acquisition of sulfur from sulfur-containing amino acid to form lead sulfide precipitate
  • POSITIVE RESULT: Cys: Black precipitate
  • S-amino acid + 2 NaOH → Na2S + (CH3COO)2Pb → PbS + 2CH3COONa
• Adamkiewicz Hopkins' Test


  • Condensation of indole ring with aldehyde under acidic pH
  • POSITIVE RESULT: Trp: Purple ring at the middle of the solution
• Follin's McCarthy Sullivan Test

  • The test was developed when it was discovered that tryptophan and histidine both produce a red color with nitroprusside under similar conditions. However, by changing the pH, tryptophan will be undetectable. Similarly, it has been stated that the addition of glycine eliminates the detection of histidine. The red color is obtained after the addition of sodium nitroprusside to an alkaline solution of methionine followed by the acidification of the reaction.
  • POSITIVE RESULT: Met: Red colored solution
• Isatin Test


  • The detection of imino acid proline for a long time had been unsatisfactory as methods like the ninhydrin test can only detect proline after isolation from other amino acids. A reaction between proline and isatin can be used to qualitatively and quantitatively determine the amount of proline.
  • POSITIVE RESULT: Pro: Blue spot