On the Binding of Congo Red by Amyloid

Congo red in alkaline 80% ethyl alcohol solution saturated with NaCl stains amyloid selectively. The distribution of the dye is very similar to that observed in serial sections stained with Highman's Congo red method. In sections stained with Congo red in aqueous or alcoholic solutions without differentiation all tissue structures are intensely colored, but only amyloid is dichroic. The rapid removal of dye by alkaline solutions from structures other than amyloid suggests ionic or salt-type bonds between Congo red and these structures. In contrast, the binding of Congo red by amyloid from alkaline alcoholic solutions and the increase in intensity of staining upon addition of NaCl indicate a non-ionic type linkage between amyloid and dye. The effects of deamination, acetylation and various oxidation procedures on the binding of Congo red indicate interaction of the dye with hydroxyl groups of amyloid. According to data on the direct dyeing of cellulose in textile dyeing, Congo red is adsorbed by hydrogen bonding between hydroxyl groups of the polysaccharide chains and the amino groups of the dye. It seems probable that a similar mechanism is involved in the binding of Congo red by amyloid. It is suggested that the selective staining of amyloid with Congo red is due to steric configurations of amyloid or one of its components which favor non-ionic binding of the eye.