QUALITATIVE TESTS FOR PROTEINS

• Amino acids

  The basic unit of proteins. Each amino acid has a carboxyl group, an amino group, an α carbon, and an R group that gives it unique properties
• Classification of amino acids based on R group

  • Basic
  • Acidic
  • Polar
• Protein structures

  • Primary - amino acid sequence
  • Secondary - folding in specific areas
  • Tertiary - 3D folding of entire polypeptide
  • Quaternary - arrangement of multiple polypeptide chains
• Ninhydrin test


  General test for amines or α-amino acids. Positive result is dark purple color, except proline which yields yellow
• Biuret test

  Test for peptide bonds, with a minimum of two peptide bonds. Positive result is violet color
• Millon's test

  Test for phenol groups and derivatives, e.g. tyrosine. Positive result is red color
• Hopkins-Cole test

  Test for indole groups, e.g. tryptophan. Positive result is violet-colored junction between sample and sulfuric acid layer
• Fohl's test

  Test for specific sulfur-containing amino acids, e.g. cysteine and cystine. Positive result is a black precipitate

  
  

  Also called reduced sulfur test
• Xanthoproteic test

  Test for aromatic amino acids, e.g. tyrosine and tryptophan. Positive result is yellow to orange-yellow color
• Sakaguchi test

  Test for guanidine or guanidino groups, e.g. arginine. Positive result is a red color
• Pauly test

  Test for imidazole and phenol groups, e.g. histidine and tyrosine. Positive result is a red color
• Protein solubility

  • Affected by pH, salt concentration, temperature
  • Native conformation - functional 3D structure
  • Denatured - lost 3D structure and function
• Protein precipitation methods


  • Heat
  • Heavy metal salts
  • Strong acids and bases
  • Alkaloidal reagents
  • Organic solvents