Giant amyloid spherulites reveal their true colours

Abstract

The prevalence of degenerative conditions such as Alzheimer's and Parkinson's disease amongst an increasingly elderly population has led to substantial research efforts into understanding the properties and structures of amyloid protein aggregates. One such aggregate, the amyloid spherulite, consists of a central core surrounded by long fibres of aggregated protein (fibrils) which grow radially outwards. Spherulites (5–50 μm in diameter) exhibit four white lobes when observed using crossed polarised microscopy, due to their birefringence. Here we report the growth of giant amyloid spherulites (GAS, diameters 0.4–1 mm) that produce colourful patterns when placed between crossed polarisers. A ray tracing model was developed that accounts for these patterns by calculating the effects of birefringence on light passing through the GAS. This new model links for the first time the optical properties of spherulites to the density and orientation of the fibrils, providing a route to understanding the formation of these important protein aggregates.