RESUMO
A popular working hypothesis of Alzheimer's disease causation is amyloid ß-protein oligomers are the key neuropathogenetic agents. Rigorously elucidating the role of oligomers requires the production of stable oligomers of each size. We previously used zero-length photochemical cross-linking to allow stabilization, isolation, and determination of structure-activity relationships of pure populations of Aß40 dimers, trimers, and tetramers. We also attempted to study Aß42 but found that Aß42 oligomers subjected to the same procedures were not completely stable. On the basis of the fact that Tyr is a critical residue in cross-linking chemistry, we reasoned that the chemical accessibility of Tyr10 in Aß42 must differ from that in Aß40. We thus chemically synthesized four singly substituted Tyr variants that placed the Tyr in different positions across the Aß42 sequence. We then studied the stability of the resulting cross-linked oligomers as well as procedures for fractionating the oligomers to obtain pure populations of different sizes. We found that [Phe(10),Tyr(42)]Aß42 produced stable oligomers yielding highly pure populations of dimers through heptamers. This provides the means to establish formal structure-activity relationships of these important Aß42 assemblies. In addition, we were able to analyze the dissociation patterns of non-cross-linked oligomers to produce a model for oligomer formation. This work is relevant to the determination of structure-activity relationships that have the potential to provide mechanistic insights into disease pathogenesis.