Test for protein

• what do you use to test for proteins?

  biuret reagent (sodium hydroxide and copper(II) sulphate)
• whats the method to test for proteins?

  1. add a few drops of the biuret solution to the sample
  2. the sodium hydroxide and copper sulphate react to make blue copper hydroxide
  3. if a protein is present, the copper hydroxide interacts with the peptide bonds in the protein to make biuret, which is purple
  4. so the colour change for positive biuret test is blue-purple
• why does the concentration of the protein matter when partaking in the biuret test?

  • at low protein concentration, the colour change is difficult to detect by eye
  • the more concentrated the protein, the darker the purple colour, so the test is qualitative
  • it could be used as a semi-quantitative test, comparing the intensity of purple in 2 identically treated solutions
• what can you do to make the test for proteins semi-quantitative?

  • you can measure the absorbance of the purple biuret in a calorimeter, using a yellow (580 nm) filter, giving a numerical estimate of the relative concentration of proteins present in a sample
  • this is semi-quantitative as an actual protein concentration isn't measured
• what can you do to make the test for proteins quantitative?

  to measure the actual concentration, a biosensor is needed