Biuret test

In the presence of peptides, a copper(II) ion forms mauve-colored coordination complexes in an alkaline solution.

Due to its insensitivity and little interference by free amino acids, this assay is most useful for whole tissue samples and other sources with high protein concentration.

Peptides with the correct length of at least 3 amino acids are necessary for a significant, measurable colour shift with these reagents.

In these tests, the Cu+ formed during the biuret reaction reacts further with other reagents, leading to a deeper color.

In the Lowry protein assay, Cu+ is oxidized back to Cu2+ by MoVI in the Folin–Ciocalteu reagent, which forms molybdenum blue (MoIV).

[10] Molybdenum blue can in turn bind certain organic dyes such as malachite green and Auramine O, resulting in further amplification of the signal.