Amyloids, Congo red and the apple-green effect

Abstract

This paper attempts to find evidence of the previously proposed opinion that amyloids complex with Congo red molecules which preserve their supramolecular organization. As evidence of the overpowering tendency of Congo red molecules to self-assemble, we present an increasing acidity of molecules that follows increasing concentration of the dye, and a highly notable nonlinear increase in absorbance in the UV band (300–400 nm). This effect is analyzed in a model where the amyloid fibril is simulated by polyvinyl alcohol, providing a scaffold to stabilize a long Congo red micelle. Enormous absorbance in the UV band, coupled with the increasing association capabilities of individual Congo red molecules may cause the absorbance to extend even into the visible band. In addition, the UV and visual absorbance bands shift significantly, depending on conditions, and may either approach or recede from each other, leading to spectral changes which may be observed under polarized light. This commonly observed spectral variability appears to be associated with the strong capacity for electron delocalization in supramolecular Congo red complexed with amyloids.