The Lowry colorimetric method is one of the most sensitive methods for protein determination. The color development principle of this method is the same as that of the bis-urea method, except that a second reagent, the Folin-phenol reagent, is added to increase the amount of color development, thus improving the sensitivity of protein detection. This color reaction produces a deep orchid color because, under alkaline conditions, the peptide bonds in proteins bind to copper to form a complex. phosphomolybdate-phosphotungstate in the Folin-phenol reagent is reduced by tyrosine and phenylalanine residues in the proteins to produce a deep orchid color (a mixture of molybdenum and tungsten orchids). Under certain conditions, the depth of the orchid color is proportional to the amount of protein.

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