Constraints on Supramolecular Structure in Amyloid Fibrils from Two-Dimensional Solid-State NMR Spectroscopy with Uniform Isotopic Labeling

Abstract

We show that strong constraints on supramolecular structure in amyloid fibrils can be obtained from solid-state nuclear magnetic resonance measurements on samples with uniformly ¹³C-labeled segments. The measurements exploit two-dimensional (2D) ¹³C−¹³C exchange spectroscopy in conjunction with high-speed magic angle spinning, with proton-mediated exchange of ¹³C nuclear spin magnetization as recently demonstrated by Baldus and co-workers (J. Am. Chem. Soc.2002, 124, 9704−9705). Proton-mediated 2D exchange spectra of fibrils formed by residues 16−22 of the 40-residue Alzheimer's β-amyloid peptide show strong nonsequential, intermolecular cross-peaks between α-carbons that dictate an antiparallel β-sheet structure in which residue 16+k aligns with residue 22−k. The strong α/α cross-peaks are absent from conventional, direct 2D exchange spectra. Proton-mediated 2D exchange spectra of fibrils formed by residues 11−25 indicate an antiparallel β-sheet structure with a pH-dependent intermolecular alignment. In contrast, proton-mediated 2D exchange spectra of fibrils formed by the full-length β-amyloid peptide are consistent with a parallel β-sheet structure. These data show that the supramolecular structure of amyloid fibrils is not determined by the amino acid sequence at the level of 7-residue or 15-residue segments. The proton-mediated 2D exchange spectra additionally demonstrate that the intermolecular alignment in the β-sheets of these amyloid fibrils is highly ordered, with no detectable evidence for “misalignment” defects.