A mathematical model for the description of the Coomassie brilliant blue protein assay

The Coomassie brilliant blue dye-binding method for protein assay has become important relatively recently. The basis of the assay method is the binding of dye to protein, with production of a dye-protein complex which absorbs light intensely at 620 nm, but the mechanism of the binding process is not well understood. In this paper, two mathematical models for the binding process are developed, one involving the binding of both protonated (green) and deprotonated (blue) forms of the dye. The second model allows only binding of the blue species to proteins. These models are tested for their ability to estimate number of dye-binding sites (n) and binding constants (Kd) from protein assay data. The models are also tested for their ability to reproduce the experimental assay curve using either known values or reasonable estimates of the equation parameters. The models are shown to be approximately equal in ability to reproduce experimental data related to the protein assay, which somewhat favors the simpler of the two models. In this paper, a method for estimating n and Kd from standard curve-fitting procedures is established. Hitherto, binding constants were only available from assay data taken under conditions of very large molar protein/dye ratios. The possibility of protonated forms of the dye binding to proteins was not ruled out by this study, but for many purposes the use of the simple dye-binding model, in which only the deprotonated dye species binds, is sufficient.