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.
In the BCA test, Cu forms a deep purple complex with bicinchoninic acid (BCA), which absorbs around 562 nm, producing the signature mauve 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.
In the Lowry protein assay Cu is oxidized back to Cu by Mo in the Folin–Ciocalteu 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.