Structure of Spherulites in Insulin, β-Lactoglobulin, and Amyloid β

Under denaturing conditions such as low pH and elevated temperatures, proteins in vitro can misfold and aggregate to form long rigid rods called amyloid fibrils; further self-assembly can lead to larger structures termed spherulites. Both of these aggregates resemble amyloid tangles and plaques associated with Alzheimer's disease in vivo. The ability to form such aggregates in a multitude of different proteins suggests that it is a generic ability in their mechanism to form. Little is known about the structure of these large spherulites ranging from 5 to 100 microns and whether they can reproducibly form in amyloid beta (1-40) (A beta 40), a 40-amino acid residue peptide, which is one of the major components of Alzheimer's amyloid deposits. Here, we show that spherulites can readily form in A beta 40 under certain monomerization and denaturing conditions. Using polarized and nonpolarized Raman spectroscopy, we analyzed the secondary structure of spherulites formed from three different proteins: insulin, beta-lactoglobulin (BLG), and A beta 40. Visually, these spherulites have a characteristic Maltese Cross structure under crossed polarizers through an optical microscope. However, our results indicate that insulin and A beta 40 spherulites have similar core structures consisting mostly of random coils with radiating fibrils, whereas BLG mostly contains beta-sheets and fibrils that are likely to be spiraling from the core to the edge.