The bicinchoninic acid assay (BCA assay) is a biochemical assay for determining the total concentration of protein in a solution (0.5 μg/mL to 1.5 mg/mL), similar to Lowry protein assay, Bradford protein assay or biuret reagent.
The total protein concentration is exhibited by a color change of the sample solution from green to purple in proportion to protein concentration, which can then be measured using colorimetric techniques
BCA assay primarily relies on two reactions. First, the peptide bonds in protein reduce Cu2+ ions from the copper(II) sulfate to Cu+ (a temperature dependent reaction). The amount of Cu2+ reduced is proportional to the amount of protein present in the solution. Next, two molecules of bicinchoninic acid chelate with each Cu+ ion, forming a purple-colored complex that strongly absorbs light at a wavelength of 562 nm.