Conformation of Pyroglutamated Amyloid β (3-40) and (11-40) Fibrils - Extended or Hairpin?

Amyloid β (Aβ) is a hallmark protein of Alzheimer's disease. One physiologically important Aβ variant is formed by initial N-terminal truncation at a glutamic acid position (either E 3 or E 11 ), which is subsequently cyclized to a pyroglutamate (either pE 3 or pE 11 ). Both forms have been found in high concentrations in the core of amyloid plaques and are likely of high importance in the pathology of Alzheimer's disease. However, the molecular structure of the fibrils of these variants is not entirely clear. Solid-state NMR spectroscopy studies have reported a molecular contact between Gly 25 and Ile 31 , which would disagree with the conventional hairpin model of wildtype (WT-)Aβ 1-40 fibrils, most often described in the literature. We investigated the conformation of the monomeric unit of pE 3 -Aβ 3-40 and pE 11 -Aβ 11-40 (and for comparison also wildtype (WT)-Aβ 1-40 ) fibrils to find out whether the hairpin or a newly suggested extended structure dominates the structure of the Aβ monomers in these fibrils. To this end, solid-state NMR spectroscopy was applied probing the inter-residual contacts between Phe 19 /Leu 34 , Ala 21 /Leu 34 , and especially Gly 25 /Ile 31 using suitable isotopic labeling schemes. In the second part, the flexible turn of the Aβ 40 peptides was replaced by a (3-(3-aminomethyl)phenylazo)phenylacetic acid (AMPP)-based photoswitch, which can predefine the peptide conformation to either an extended ( trans ) or hairpin ( cis ) conformation. This enables simultaneous spectroscopic assessment of the conformation of the AMPP-photoswitch, allowing in situ structural investigations during fibrillation in contrast to structural techniques such as NMR spectroscopy or cryo-EM, which can only be applied to stable conformers. Both methods confirm an extended structure for the peptidic monomers in fibrils of all investigated Aβ variants. Especially the Gly 25 /Ile 31 contact is a decisive indicator for the extended structure along with the characteristic absorption spectra of trans -AMPP-Aβ.