The color generated in the biuret reaction
(Chapter 2, page 33) is said to arise from reduction of CU(II) to Cu(I), whereas this color is generally considered to result from complexation of CU(II) by peptide nitrogens.
Formation of this complex between protein and cupric ions shifts the color of solutions from blue to purple and commonly has been termed the "biuret reaction" after one of the first compounds that was recognized to yield this color reaction.
Although the biuret reaction has been considered in the clinical laboratory to be relatively specific for measurement of proteins and peptides, there are studies extending back to the initial discovery of the reaction that describe reactions with a variety of other compounds, such as biuret, for which the reaction is named (1), amino acids and amino acid amides (30, 31), dipeptides (32), and a variety of other compounds (33).
The kinetic biuret method on the LX-20 was run with the cerebrospinal fluid application; it measures the absorbance change at 545 run in a biuret reaction between 4.0 and 7.7 s.