The bicinchoninic acid assay (BCA assay), also known as the Smith assay, after its inventor, Paul K. Smith at the Pierce Chemical Company,[1] is a biochemical assay for determining the total level 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.
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A stock BCA solution contains the following ingredients in a highly alkaline solution with a pH 11.25:
The BCA assay primarily relies on two reactions.
Firstly, the peptide bonds in protein reduce Cu2+ ions from the cupric 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 product that strongly absorbs light at a wavelength of 562 nm.
The bicinchoninic acid Cu+ complex is aided in protein samples by the presence of cysteine, cystine, tyrosine, and tryptophan side chains. At higher temperatures (37 to 60 °C), peptide bonds assist in the formation of the reaction product. Incubating the BCA assay at higher temperatures is recommended as a way to increase assay sensitivity while minimizing the variances caused by unequal amino acid composition.[2]
The amount of protein present in a solution can be quantified by measuring the absorption spectra and comparing with protein solutions with known concentrations.