E. Pigorsch et al., SPECTROSCOPIC STUDY OF PH AND SOLVENT EFFECTS ON THE STRUCTURE OF CONGO RED AND ITS BINDING MECHANISM TO AMYLOID-LIKE PROTEINS, Spectrochimica acta. Part A: Molecular spectroscopy, 50(12), 1994, pp. 2145-2152
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.