<P> The reagent is commonly used in the biuret protein assay, a colorimetric test used to determine protein concentration by UV / VIS spectroscopy at wavelength 550 nm . </P> <P> Two major modifications of the biuret test are commonly applied in modern colorimetric analysis of peptides: the bicinchoninic acid (BCA) assay and the Lowry assay . In these tests, the Cu formed during the biuret reaction reacts further with other reagents, leading to a deeper color . </P> <P> In the BCA test, Cu forms a deep purple complex with bicinchoninic acid (BCA), which absorbs around 562 nm, producing the signature violet color . The water - soluble BCA / copper complex absorbs much more strongly than the peptide / copper complex, increasing the sensitivity of the biuret test by a factor of around 100: the BCA assay allows to detect proteins in the range of 0.0005 to 2 mg / mL). Additionally, the BCA protein assay gives the important benefit of compatibility with substances such as up to 5% surfactants in protein samples . </P> <P> In the Lowry protein assay Cu is oxidized back to Cu by Mo in Folin - Ciocalteu's reagent, which forms molybdenum blue (Mo). Tyrosine residues in the protein also form molybdenum blue under these circumstances . In this way, proteins can be detected in concentrations between 0.005 and 2 mg / mL . Molybdenum blue in turn can bind certain organic dyes such as malachite green and Auramine O, resulting in further amplification of the signal . </P>

How can you tell by using this test that a substance contains protein