Coomassie R-250 and G-250 dyes are two chemical forms of disulfonated triphenylmethane compounds and are often used as base dyes in protein gel electrophoresis detection and Bradford method protein quantification reagents. The R-250 (red) form lacks two methyl groups present in the G-250 (green) form, which is also known as colloidal Coomassie dye.

Typically, Coomassie gel dyes and protein quantification reagents are formulated as highly acidic solutions of 25% to 50% methanol. Under acidic conditions, dyes bind to proteins mainly through basic amino acids (mainly arginine, lysine, and histidine), and the number of Coomassie dye ligands bound to each protein molecule is approximately the same as that of the protein. Proportional to the amount of positive charge carried. Protein binding causes the dye to change from reddish-brown to bright blue (absorbance maximum at 595 nm).

Features include:
•Easy to detect—forms brightly colored complexes with proteins
• High sensitivity—can be measured in gels Protein in matrix as low as 0.5 µg/cm2
• Reversible staining—The Coomassie Brilliant Blue dye anion formed in acidic staining media binds to the protonated amino groups of the protein through electrostatic interactions; The complex is reversible under appropriate conditions
• Distinguishes bound from unbound dye—at 555 nm when dissolved in 0.01 M citrate buffer, pH 3.0 It has maximum absorbance at 549 nm; the protein-dye complex is characterized by a peak that is slightly wider than that of the free dye, and the maximum absorbance is at 549 nm