Determination of Peptide Amide Configuration in a Model Amyloid Fibril by Solid-State NMR

The β-amyloid proteins (Aβ), especially the variant Aβ1−42, are the primary constituent of the amyloid plaques which are characteristic of Alzheimer's disease (AD). The C-terminus of Aβ has been identified as crucial for rapid amyloid formation. A 9-residue C-terminal fragment of Aβ (designated β34−42, with sequence LMVGGVVIA) forms a structured aggregate which is classified as an amyloid fibril based primarily on its morphological and solubility properties and the classic cross-β fiber diffraction pattern it displays. We have adopted this aggregate as a model of the Aβ-dominated amyloid plaques associated with AD. Previous results from a series of internuclear 13 C− 13 C distance measurements along the peptide backbone indicate that the β34−42 backbone conformation differs considerably from that of the classic β-sheet, possibly including an unusual cis amide bond in the peptide's central glycyl−glycyl region. Here we report evidence which demonstrates the presence of a trans configuration at the peptide linkage between these residues. The evidence derives from solid-state NMR static echo and n = 2 rotational resonance experiments which probe the orientation of the 13 C carbonyl chemical shift anisotropy (CSA) tensor of the peptide bond with respect to the dipolar tensor connecting the carbonyl carbon to the 13 C-labeled α-carbon on the succeeding residue.