Spectroscopic study of pH and solvent effects on the structure of Congo red and its binding mechanism to amyloid-like proteins

The pH and solvent effects on the structure of the azo dye Congo red were investigated. The absorption, resonance Raman and NMR spectra give evidence of the existence of different protonated dye molecules in acidic solutions. There exist a red form with an azoic structure and at least three blue forms with quinoid structures. The position of the equilibrium between the azoic and quinoid structures depends on the solvent and the pH and is related to the capability of the solvent to form three and two dimensional solvation structures, respectively.

Spectral variations similar to those observed for Congo red dissolved in organic solvents accompany binding of the dye to amyloid-like proteins. This observation leads to the suggestion that the dye molecules must experience similar changes in environment in both cases.