Dyeing crystals to dyeing tissues: Congo red in anisotropic media

In the past, we have studied the process of dyeing crystals through measurements of linear optical anisotropies (e.g., linear dichroism and linear birefringence). Techniques for analyzing the optical properties of dyed crystals are readily translated to stained crystalline tissues, countless examples of which have been described by chemical histologists. Moreover, questions pertaining to mechanisms of non-covalent association are comparable whether the structured host is a single crystal or crystalline tissue. Here, the azo dye, Congo red, in two types of anisotropic media, sucrose single crystals and fibrous, proteinaceous amyloid plaques, is described. Optical micrographs of amyloid from the brains of deceased Alzheimer's Disease patients made with a newly developed imaging system reveal previously unrecognized features. As formation of ordered amyloid plaques from their relatively small peptides may well be considered a pathological biocrystallization process, a clear understanding of the deposition mechanism may lead to strategies for crystallization inhibition.