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

Pyroglutamate-modified amyloid-beta (pEA beta) has been described as a relevant A beta species in Alzheimer's-disease-affected brains, with pEAb (3-42) as a dominant isoform. A beta (1-40) and A beta (1-42) have been well characterized under various solution conditions, including aqueous solutions containing trifluoroethanol (TFE). To characterize structural properties of pEA beta (3-42) possibly underlying its drastically increased aggregation propensity compared to A beta (1-42), we started our studies in various TFE-water mixtures and found striking differences between the two A beta species. Soluble pEA beta (3-42) has an increased tendency to form b-sheet-rich structures compared to A beta (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 beta (3-42) in contrast to A beta (1-42). NMR spectroscopy was performed for backbone and side-chain chemical-shift assignments of monomeric pEA beta (3-42) in 40% TFE solution. Although the difference between pEA beta (3-42) and Ab (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 pEAb (3-42) contains two a-helical regions connected by a flexible linker, whereas the N-terminus remains unstructured. We found that these a-helices act as a transient intermediate to beta-sheet and fibril formation of pEA beta (3-42).