Site-Specific Identification of Non-β-Strand Conformations in Alzheimer's β-Amyloid Fibrils by Solid-State NMR

ABSTRACT The most well-established structural feature of amyloid fibrils is the cross- β motif, an extended β -sheet structure formed by β -strands oriented perpendicular to the long fibril axis. Direct experimental identification of non- β -strand conformations in amyloid fibrils has not been reported previously. Here we report the results of solid-state NMR measurements on amyloid fibrils formed by the 40-residue β -amyloid peptide associated with Alzheimer's disease (A β 1-40 ), prepared synthetically with pairs of 13 C labels at consecutive backbone carbonyl sites. The measurements probe the peptide backbone conformation in residues 24-30, a segment where a non- β -strand conformation has been suggested by earlier sequence analysis, cross-linking experiments, and molecular modeling. Data obtained with the fpRFDR-CT, DQCSA, and 2D MAS exchange solid-state NMR techniques, which provide independent constraints on the ϕ and ψ backbone torsion angles between the labeled carbonyl sites, indicate non- β -strand conformations at G25, S26, and G29. These results represent the first site-specific identification and characterization of non- β -strand peptide conformations in an amyloid fibril.