Bicinchoninic acid is a weak acid composed of two carboxylated quinoline

Quinoline

Quinoline is a heterocyclic aromatic organic compound. It has the formula C9H7N and is a colourless hygroscopic liquid with a strong odour. Aged samples, if exposed to light, become yellow and later brown...

 rings.

Bicinchoninic acid is most commonly employed by biochemists in the bicinchoninic acid assay

Bicinchoninic acid assay

The bicinchoninic acid assay , also known as the Smith assay, after its inventor, Paul K. Smith at the Pierce Chemical Company, 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...

, which is used to determine the total level of protein in a solution. In this assay, two molecules of bicinchoninic acid chelate a single Cu

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

¹⁺ ion, forming a purple water-soluble complex that strongly absorbs light at 562 nm.