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 β (1-40) (Aβ40), 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 Aβ40 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, β-lactoglobulin (BLG), and Aβ40. Visually, these spherulites have a characteristic“Maltese Cross” structure under crossed polarizers throughan optical microscope. However, our results indicate that insulinand Aβ40 spherulites have similar core structures consistingmostly of random coils with radiating fibrils, whereas BLG mostlycontains β-sheets and fibrils that are likely to be spiralingfrom the core to the edge.
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