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 w ere investigated. The absorption, resonance Raman and NMR spectra give evidence of the existence of different protonated dye molecules in ac idic solutions. There exist a red form with an azoic structure and at least three blue forms with quinoid structures. The position of the eq uilibrium between the azoic and quinoid structures depends on the solv ent and the pH and is related to the capability of the solvent to form three and two dimensional solvation structures, respectively. Spectra l variations similar to those observed for Congo red dissolved in orga nic solvents accompany binding of the dye to amyloid-like proteins. Th is observation leads to the suggestion that the dye molecules must exp erience similar changes in environment in both cases.