Neurotoxic assemblies of the amyloid b-protein (Ab) have been linked strongly to the pathogenesis of Alzheimer's disease (AD). Here, we sought to monitor the earliest step in Ab assembly, the creation of a folding nucleus, from which oligomeric and fibrillar assemblies emanate. To do so, limited proteolysis/mass spectrometry was used to identify protease-resistant segments within monomeric Ab(1-40) and Ab(1-42). The results revealed a 10-residue, protease-resistant segment, Ala21-Ala30, in both peptides. Remarkably, the homologous decapeptide, Ab(21-30), displayed identical protease resistance, making it amenable to detailed structural study using solution-state NMR. Structure calculations revealed a turn formed by residues Val24-Lys28. Three factors contribute to the stability of the turn, the intrinsic propensities of the Val-Gly-Ser-Asn and Gly-Ser-Asn-Lys sequences to form a b-turn, long-range Coulombic interactions between Lys28 and either Glu22 or Asp23, and hydrophobic interaction between the isopropyl and butyl side chains of Val24 and Lys28, respectively. We postulate that turn formation within the Val24-Lys28 region of Ab nucleates the intramolecular folding of Ab monomer, and from this step, subsequent assembly proceeds. This model provides a mechanistic basis for the pathologic effects of amino acid substitutions at Glu22 and Asp23 that are linked to familial forms of AD or cerebral amyloid angiopathy. Our studies also revealed that common C-terminal peptide segments within Ab(1-40) and Ab(1-42) have distinct structures, an observation of relevance for understanding the strong disease association of increased Ab(1-42) production. Our results suggest that therapeutic approaches targeting the Val24-Lys28 turn or the Ab(1-42)-specific C-terminal fold may hold promise.