Bicinchoninic acid assay

The bicinchoninic acid assay (also known as the BCA assay or Smith assay) is a biochemical assay for determining the total level of protein in a solution, 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.

Mechanism
A stock BCA solution contains the following ingredients in a highly alkaline solution with a pH 11.25:
 * Bicinchoninic acid
 * Sodium carbonate
 * Sodium bicarbonate
 * Sodium tartrate
 * Cupric sulfate pentahydrate

The BCA assay relies on two reactions. First, the peptide bonds in protein reduce Cu2+ ions from the cupric sulfate to Cu1+. 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 Cu1+ ion, forming a purple-colored product that strongly absorbs light at a wavelength of 562 nm.

The amount of protein present in a solution can be quantified by measuring the absorption spectra and comparing with protein solutions with known concentrations.