Pyroglutamate-Modified Amyloid-ß(3-42) Shows α-Helical Intermediates before Amyloid Formation.

Pyroglutamate-modified amyloid-ß (pEAß) has been described as a relevant Aß species in Alzheimer's-disease-affected brains, with pEAß (3-42) as a dominant isoform. Aß (1-40) and Aß (1-42) have been well characterized under various solution conditions, including aqueous solutions containing trifluoroethanol (TFE). To characterize structural properties of pEAß (3-42) possibly underlying its drastically increased aggregation propensity compared to Aß (1-42), we started our studies in various TFE-water mixtures and found striking differences between the two Aß species. Soluble pEAß (3-42) has an increased tendency to form ß-sheet-rich structures compared to Aß (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 pEAß (3-42) in contrast to Aß (1-42). NMR spectroscopy was performed for backbone and side-chain chemical-shift assignments of monomeric pEAß (3-42) in 40% TFE solution. Although the difference between pEAß (3-42) and Aß (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 pEAß (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 ß-sheet and fibril formation of pEAß (3-42).