Congo red dichroism with dispersed amyloid fibrils, an extrinsic cotton effect.

The spectral absorption, optical rotatory dispresion, and circular dichroism associated with interaction of Congo red dye with partly purified suspensions of amyloid fibril fragments were examined. A set of phenomena consistent with a Cotton effect was found. A nearly identical set of phenomena was obtained with poly-L-lysine in its alpha-helical form. The dichroism seen when Congo red binds to amyloid substance in tissue sections can also be interpreted as a Cotton effect. This suggests that some special conformation, presumably in protein, is present in a major constituent of amyloid. This conformation is not present in gamma globulin, Bence-Jones protein, albumin, fibrinogen, or other proteins tested so far. These and other optical properties of amyloid substance can be used to compare amyloid deposits in different human cases and in different species. Extension of the use of polarization microscopy with other dyes that bind to other substances in tissue sections should permit more exquisite probing of the conformation of important macromolecules in situ in cells and tissues than has hitherto been possible.