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

Beta-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 beta-peptide abbreviated as beta-(1-28)]. The beta-(1-28) peptide produces insoluble beta-pleated sheet structures in vitro, similar to the beta-pleated sheet structures of beta-peptide in amyloid deposits in vivo. For peptide solutions in the millimolar range, in aqueous solution at pH 1-4 the beta-(1-28) peptide adopts a monomeric random coil structure, and at pH 4-7 the peptide rapidly precipitates from solution as an oligomeric beta-sheet structure, analogous to amyloid deposition in vivo. The NMR work shown here demonstrates that the beta-(1-28) peptide can adopt a monomeric alpha-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 alpha H protons, and (3) temperature coefficients of the NH chemical shifts. The results show that at pH 1.0 and 10 degrees C the beta-(1-28) peptide adopts an alpha-helical structure that spans the entire primary sequence. With increasing temperature and pH, the alpha-helix unfolds to produce two alpha-helical segments from Ala2 to Asp7 and Tyr10 to Asn27. Further increases in temperature to 35 degrees C cause the Ala2-Asp7 section to become random coil, while the His13-Phe20 section stays alpha-helical. A mechanism involving unfavorable interactions between charged groups and the alpha-helix macrodipole is proposed for the alpha-helix----beta-sheet conversion observed at midrange pH.