Finding the exact quantity of proteins in a solution is very often necessary in the biochemical practice and to analyse clinical samples as well as in research. There are many ways to measure protein concentration. In chromogenic methods, the absorbance of a coloured product formed by the protein and an organic molecule is measured. Protein concentration can also be determined from the protein's own (intrinsic) UV absorbance. However, these methods may give different results for different proteins of the same concentration. Also, different methods can yield somewhat different results for the same protein. Also, dilution factor may be a important parameter.
Molecules with two or more peptide bonds react with Cu2+ ions in alkaline solution and form a purple complex. Nitrogen atoms of the peptide bonds form a coordination bond with the metal ion. The quantity of the complexes formed is proportional to the number of peptide bonds.
In practice, the determination of protein concentration is done using a calibration curve created using samples of known concentration. The protein treated with biuret reagent is measured at 540 nm after the purple product is formed.
The advantages of the method include that only few materials (e.g. Tris and amino acid buffers) interfere with it, it can be done in a short time and does not depend on the amino acid composition of the protein. Its disadvantages are its low sensitivity and that it requires at least 1 mg of protein.
Lowry (Folin) protein assay
This is sensitive technique where a coloured product is formed similarly to the biuret reaction, but a reagent strengthen the colour, which is kown as Folin–Ciocalteu reagent (a mixture of phosphotungstic acid and phosphomolybdic acid in the Folin–Ciocalteu reaction). The reaction mechanism is not well understood, but involves reduction of the Folin–Ciocalteu reagent and oxidation of aromatic residues (mainly tryptophan, also tyrosine). The strong blue colour is created by two reactions: (1) formation of the coordination bond between peptide bond nitrogens and a copper ion and (2) reduction of the Folin-Ciocalteu reagent by tyrosine (phosphomolybdic and phosphotungstic acid of the reagent react with phenol). The measurement is carried out at 750 nm.
As in the biuret reaction, a calibration curve is created (for example using BSA, bovine serum albumin), and the concentration of the unknown protein is determined from the curve.
The advantages of the method include that it is quite sensitive and is able to detect even 1 µg of protein. Its disadvantages are that it takes rather long to carry out, is disturbed by various materials (including ammonium sulphate, glycine and mercaptans) and that the incubation time is critical. As different proteins contain different amounts of tyrosine, the amount of the coloured product will also be different. As a consequence, this method is more suited to compare the concentration of solutions of the same protein than to absolute measurement.