The BCA Protein Assay combines the well-known reduction of Cu2+ to Cu1+ by protein in an alkaline medium with the highly sensitive and selective colorimetric detection of the cuprous cation (Cu1+) by bicinchoninic acid. Two molecules of BCA chelate with one cuprous ion forming purple-coloured product. The BCA/copper complex is water-soluble and exhibits a strong absorbance at 562nm. Protein concentration can be determined based on absorbance because of the linear relation between them.