The monomeric conformational ensembles of A β 40 and A β 42 encode their differential amyloid aggregation propensity

A β 40 and A β 42 peptides differ by just two C-terminal residues, yet they display strikingly different aggregation and toxicity profiles. Whether this distinction is already encoded at the monomer level is still under debate. Here, we combine extensive all-atom simulations in explicit solvent, well-tempered metadynamics, and a tailored consensus cluster analysis to compare the monomeric ensembles of the two isoforms under identical conditions. Both peptides populate broad, coil-like conformational distributions; however, A β 42 shows systematically higher β -structure propensity, especially in the C-terminal region, and samples more extended conformations with higher hydrophobic exposure compared to A β 40. These results support a mechanistic link between sequence-encoded monomer conformational preferences and the differential amyloidogenicity of the two isoforms, highlighting monomer-level determinants of A β 42’s distinct aggregation behavior.