NMR studies of amyloid β-peptides: proton assignments, secondary structure, and mechanism of an α-helix-β-sheet conversion for a homologous, 28-residue, N-terminal fragment

β-Peptide is a major component of amyloid deposits in Alzheimer's disease. We report here a proton nuclear magnetic resonance (NMR) spectroscopic investigation of a synthetic peptide that is homologous to residues 1-28 of β-peptide [abbreviated as β-(l-28)]. The j3-(l'28) peptide produces insoluble β-pleated sheet structures in vitro, similar to the β-pleated sheet structures of β-peptide in amyloid deposits in vivo. For peptide solutions in the millimolar range, in aqueous solution at pH 1-4 the β-(l-28) peptide adopts a monomeric random coil structure, and at pH 4-7 the peptide rapidly precipitates from solution as an oligomeric β-sheet structure, analogous to amyloid deposition in vivo. The NMR work shown here demonstrates that the α-(l-28) peptide can adopt a monomeric α-helical conformation in aqueous trifluoroethanol solution at pH 1-4. Assignment of the complete proton NMR spectrum and the determination of the secondary structure were arrived at from interpretation of two-dimensional (2D) NMR data, primarily (1) nuclear Overhauser enhancement (NOE), (2) vicinal coupling constants between the amide (NH) and αH protons, and (3) temperature coefficients of the NH chemical shifts. The results show that at pH 1.0 and 10 °C the β-(l'28) peptide adopts an α-helical structure that spans the entire primary sequence. With increasing temperature and pH, the a-helix unfolds to produce two a-helical segments from Ala2 to Asp7 and TyrlO to Asn27. Further increases in temperature to 35 °C cause the Ala2-Asp7 section to become random coil, while the Hisl3-Phe20 section stays α-helical. A mechanism involving unfavorable interactions between charged groups and the a-helix macrodipole is proposed for the α-helix-β-sheet conversion observed at midrange pH.