The pathogenesis of Alzheimer's disease is characterized by the aggregation and fibrillation of the 40-residue Abeta(1-40) and 42-residue Abeta(1-42) peptides into amyloid plaques. The structural changes associated with the conversion of monomeric Abeta peptide building blocks into multimeric fibrillar beta-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 Abeta peptide. To explore this effect at greater resolution, we carefully compared the H-1, N-15, and C-13 NMR chemical shifts of four Abeta peptides that had the Met35 reduced or oxidized (Abeta(1- 40)Met35(red), Abeta(1-40)Met35(ox), Abeta(1-42)Met35(red), and Abeta(1-42)Met35(ox)). With the use of a special disaggregation protocol, the highly aggregation prone Abeta pepticles could be studied at higher, millimolar concentrations (as required by NMR) in aqueous solution at neutral pH, remaining largely monomeric at 5 degreesC as determined by sedimentation equilibrium studies. The NOE, amide-NH temperature coefficients, and chemical shift indices of the (1)Halpha, (13)Calpha, and (13)Cbeta established that the four peptides are largely random, extended chain structures, with the Met35(ox) reducing the propensity for beta-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 degreesC 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 Abeta(1-40)Met35(ox), Abeta(1-40)Met35(ox), Abeta(1-42)Met35(red), and Abeta(1-42)Met35(ox) peptides may associate differently, through specific, sharp changes in structure during the initial stages of aggregation.