Haynes:Bradford
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Bradford Assay
by Karmella Haynes, 2012
Principle: The dye in the Bradford reagent turns from brown to blue in the presence of protein. The color change is proportional to the protein concentration. See http://en.wikipedia.org/wiki/Bradford_protein_assay
MATERIALS
- Transparent flat-bottom 96-well plate (e.g. Greiner Bio-One 655101)
- Bradford reagent (e.g., Sigma B6916-500ML)
- Bovine Serum Albumin (BSA) (e.g., New England Biolabs 10 mg/mL B9001S)
EQUIPMENT
- BioTek Synergy H1 Plate Reader (or similar)
PROCEDURE
- Label enough 1.5 mL eppendorf tubes for one blank (1) , five standard samples (2-6), and all of your unknown samples (7-...n).
- Add 500 μL Bradford Reagent to each tube. You will add protein to these later, and ignore the negligible change caused by the additional volume.
- Dilute the stock BSA in a new tube to make 50 μL of 1 μg/μL BSA. Example: if the stock BSA is 10 mg/mL, add 5 μL of BSA to 45 μL dH2O in a fresh tube.
- Add a BSA standard protein solution to tubes 1-6. See Table 1.
- Add 5.0 μL of unknown to each remaining tube. Keep track of your samples with good labeling!
- Close all caps and invert the tubes to thoroughly mix the samples.
- Transfer 200 μL of the blank (tube one) into the first well in a clear 96-well flat-bottom plate.
- Do the same for the other samples, using new wells.
- Use a plate reader to record absorbance at 590 nm (OD 590). If using the BioTek Synergy H1 Software, set up a new protocol and under Procedure > Action > Read use the following settings
- Detection Method = Absorbance
- Read Type = Endpoint
- Wavelength (1) = 590 nm (type-in the value manually)
Table 1. Standard sample set-up
Reagent | Tube 1 (0 μg BSA) |
Tube 2 (1 μg BSA) |
Tube 3 (2 μg BSA) |
Tube 4 (4 μg BSA) |
Tube 5 8 μg BSA) |
Tube 5 (16 μg BSA) |
Bradford Reagent | 500 μL | 500 μL | 500 μL | 500 μL | 500 μL | 500 μL |
BSA diluted to 1 μg/μL | 0 μL | 1.0 μL | 2.0 μL | 4.0 μL | 8.0 μL | 16.0 μL |
What to do with your data: calculate unknown protein concentration(s)
- Subtract the blank OD 590 value from all other values.
- Plot a standard curve (using Excel) with BSA concentration (x-axis) vs. Absorbance at 590 nm (y-axis). See this example.
- Add a line of best fit (not a curve) and display the equation.
- Solve the equation for x. Substitute y with the background-subtracted OD 590 for the unknowns.
Protein concentration of the unknown = x μg/ 5.0 μL.