Pyroglutamate-Modified Amyloid-β(3–42) Shows α-Helical Intermediates before Amyloid Formation

Abstract Pyroglutamate-modified amyloid- β (pEA β ) has been described as a relevant A β species in Alzheimer's-disease-affected brains, with pEA β (3–42) as a dominant isoform. A β (1–40) and A β (1–42) have been well characterized under various solution conditions, including aqueous solutions containing trifluoroethanol (TFE). To characterize structural properties of pEA β (3–42) possibly underlying its drastically increased aggregation propensity compared to A β (1–42), we started our studies in various TFE-water mixtures and found striking differences between the two A β species. Soluble pEA β (3–42) has an increased tendency to form β -sheet-rich structures compared to A β (1–42), as indicated by circular dichroism spectroscopy data. Kinetic assays monitored by thioflavin-T show drastically accelerated aggregation leading to large fibrils visualized by electron microscopy of pEA β (3–42) in contrast to A β (1–42). NMR spectroscopy was performed for backbone and side-chain chemical-shift assignments of monomeric pEA β (3–42) in 40% TFE solution. Although the difference between pEA β (3–42) and A β (1–42) is purely N-terminal, it has a significant impact on the chemical environment of >20% of the total amino acid residues, as revealed by their NMR chemical-shift differences. Freshly dissolved pEA β (3–42) contains two α -helical regions connected by a flexible linker, whereas the N-terminus remains unstructured. We found that these α -helices act as a transient intermediate to β -sheet and fibril formation of pEA β (3–42).