Optical properties of amyloid stained by Congo red: History and mechanisms |
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Authors: | Alexander J Howie Douglas B Brewer |
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Institution: | 1. Laboratory of Physics of Living Matter, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland;2. Laboratory of Molecular and Chemical Biology of Neurodegeneration, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland;3. Laboratory of Food and Soft Materials Science, Institute of Food, Nutrition and Health, Eidgenössische Technische Hochschule Zürich, Schmelzbergstrasse 9, LFO E23, 8092 Zürich, Switzerland |
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Abstract: | Amyloid stained by Congo red has striking optical properties that generally have been poorly described and inadequately explained, although they can be understood from principles of physical optics. Molecules of Congo red are orientated on amyloid fibrils, and so the dye becomes dichroic and birefringent. The birefringence varies with wavelength in accordance with a fundamental property of all light-transmitting materials called anomalous dispersion of the refractive index around an absorption peak. The combination of this and absorption of light, with modification by any additional birefringence in the optical system, explains the various colours that can be seen in Congo red-stained amyloid between crossed polariser and analyser, and also when the polariser and analyser are progressively uncrossed. These are called anomalous colours. |
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