Dye that binds to protein?

Question

why is it necessary to use a dye that binds to protein (the Biuret reagent) rather than just measuring the protein itself?

Answer 1

One can measure the concentration of the protein directly by absorbance at 280nm. The only disadvantages of that are that it depends

a) on the amino acid composition of the protein, which can vary greatly from protein to protein. The component of protein which absorbs at 280nm is the aromatic residues, of which the strongest contributor tryptophan, as well as some contribution from tyrosine and phenylalanine. So, if your protein contains more or less tryptophan than average, your measurements will be skewed

b)one needs a spectrophotometer which is capable of generating light in the UV range, which requires a UV lamp. In addition you need quarts cuvettes versus regular plastic cuvettes

I beleive that in the biuret and other colour development protein quatitation methods, the reagents react with the protein in a less residue specific manner.