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

Congo red is used as a histological dye to trace Alzheimer’s disease amyloid-protein deposits in cerebal tissue. Although the interaction of the dye with the protein is very specific, the binding mechanism is so far poorly understood. In order to understand the state of the dye molecule adsorbed on the protein it is necessery to get a comprehensive knowledge of the behaviour of the molecule in dependance of the surrounding medium. In the present work we studied the pH and solvent effects on the structure of congo red. The experiments were carried out with a 5·10-6 M congo red solution in a 0.1 M citric buffer. For the solutions in organic solvents 75% of the water content was replaced by the solvent.