Solution NMR Studies of the Aβ(1-40) and Aβ(1-42) Peptides Establish that the Met35 Oxidation State Affects the Mechanism of Amyloid Formation

The pathogenesis of Alzheimer's disease is characterized by the aggregation and fibrillation of the 40-residue Aβ(1-40) and 42-residue Aβ(1-42) peptides into amyloid plaques. The structural changes associated with the conversion of monomeric Aβ peptide building blocks into multimeric fibrillar β-strand aggregates remain unknown. Recently, we established that oxidation of the methionine-35 side chain to the sulfoxide (Met35^red → Met35^ox) significantly impedes the rate of aggregation and fibrillation of the Aβ peptide. To explore this effect at greater resolution, we carefully compared the ¹H, ¹⁵N, and ¹³C NMR chemical shifts of four Aβ peptides that had the Met35 reduced or oxidized (Aβ(1-40)Met35^red, Aβ(1-40)Met35^ox, Aβ(1-42)Met35^red, and Aβ(1-42)Met35^ox). With the use of a special disaggregation protocol, the highly aggregation prone Aβ peptides could be studied at higher, millimolar concentrations (as required by NMR) in aqueous solution at neutral pH, remaining largely monomeric at 5 °C as determined by sedimentation equilibrium studies. The NOE, amide-NH temperature coefficients, and chemical shift indices of the ¹Hα, ¹³Cα, and ¹³Cβ established that the four peptides are largely random, extended chain structures, with the Met35^ox reducing the propensity for β-strand structure at two hydrophobic regions (Leu17-Ala21 and Ile31-Val36), and turn- or bendlike structures at Asp7-Glu11 and Phe20-Ser26. Additional NMR studies monitoring changes that occur during aging at 37 °C established that, along with a gradual loss of signal/noise, the Met35 ^ox significantly hindered upfield chemical shift movements of the 2H NMR signals for the His6, His13, and His14 side chains. Taken together, the present NMR studies demonstrate that the Met35^red → Met35 ^ox conversion prevents aggregation by reducing both hydrophobic and electrostatic association and that the Aβ(1-40)Met35^red, Aβ(1-40)Met35^ox, Aβ(1-42)Met35^red, and Aβ(1-42)Met35^ox peptides may associate differently, through specific, sharp changes in structure during the initial stages of aggregation.