Secondary structure and interfacial aggregation of amyloid-β(1-40) on sodium dodecyl sulfate micelles

Alzheimer's disease ( AD) is characterized by the presence of large numbers of fibrillar amyloid deposits in the form of senile plaques in the brain. The fibrils in senile plaques are composed of 40- and 42-residue amyloid-beta (A beta) peptides. Several lines of evidence indicate that fibrillar A, and especially soluble A beta aggregates are important in the pathogenesis of AD, and many laboratories have investigated soluble A beta aggregates generated from monomeric A beta in vitro. Of these in vitro aggregates, the best characterized are called protofibrils. They are composed of globules and short rods, show primarily, beta-structure by circular dichroism (CD), enhance the fluorescence of bound thioflavin T, and readily seed the growth of long fibrils. However, one difficulty in correlating soluble A beta aggregates formed in vitro with those in vivo is the high probability that cellular interfaces affect the aggregation rates and even the aggregate structures. Reports that focus on the features of interfaces that are important in A beta aggregation have found that amphiphilic interactions and micellar-like A beta structures may play a role. We previously described the formation of A beta(1-40) aggregates at polar-nonpolar interfaces, including those generated at microdroplets formed in dilute hexafluoro-2-propanol (HFIP). Here we compared the A beta(1-40) aggregates produced on sodium dodecyl sulfate (SDS) micelles, which may be a better model of biological membranes with phospholipids that have anionic headgroups. At both HFIP and SDS interfaces, changes in peptide secondary structure were observed by CD immediately when A beta(1-40) was introduced. With HFIP, the change involved an increase in predominant, beta-structure content and in fluorescence with thioflavin T, while with SDS, a partial alpha-helical conformation was adopted that gave no fluorescence. However, in both systems, initial amorphous clustered aggregates progressed to soluble fibers rich in beta-structure over a roughly 2 day period. Fiber formation was much faster than in the absence of an interface, presumably because of the close intermolecular proximity of peptides at the interfaces. While these fibers resembled protofibrils, they failed to seed the aggregation of A beta(1-40) monomers effectively.