Bradford Protein Assay

The dye binds primarily to basic and aromatic amino acid residues in proteins. This interaction shifts the dye from a reddish form toward a blue form, increasing absorbance near 595 nm. Because the color response depends on protein-dye binding, the measured signal reflects both the amount of protein present and the amino acid composition of that protein.

Proteins with different proportions of basic and aromatic amino acids may bind Coomassie Brilliant Blue differently. Consequently, equal protein masses can produce different color responses, so the assay provides an estimate rather than a composition-independent measurement. This limitation matters when comparing samples containing different proteins or interpreting concentrations calculated from a standard curve.

A standard curve links absorbance readings to known protein concentrations. Researchers measure standards, plot their responses, and compare the absorbance of unknown samples with that relationship to estimate concentration. The curve provides the reference needed to convert a color signal into a protein measurement and supports consistent comparison among samples analyzed in the same workflow.

The workflow uses protein standards with known concentrations and biological samples whose concentrations are unknown. After allowing the dye to react with the standards and samples, researchers measure absorbance near 595 nm and compare sample readings with the standard curve. The resulting estimates can then guide sample normalization or evaluation of a purification workflow.

This assay is useful for routine protein quantification in biology and biochemistry. Researchers can use concentration estimates to normalize samples before an experiment, compare protein levels across preparations, or monitor changes during purification. Its value is greatest when samples are handled consistently and the limitations caused by protein composition or interfering substances are considered.

Interfering substances in a biological sample can alter the dye response and make the estimated concentration less reliable. Protein composition can also change the signal because the dye binds to particular amino acid residues. Therefore, researchers should interpret results in the context of sample composition and chemistry, especially when comparing unlike samples or tracking purification.