Formation of intracellular plaques and small oligomeric species of amyloid beta (A beta) peptides inside neurons is a hallmark of Alzheimer's disease. The most abundant A beta species in the brain are A beta 1-40 and A beta 1-42, which are composed, respectively, of 40 and 42 residues. A beta 1-42 differs from A beta 1-40 only in two residues, Ile41 and Ala42, yet it shows remarkably faster aggregation and greater neurotoxicity than A beta 1-40. Thus, it is crucial to understand the relative contributions of Ile41 and Ala42 to these distinct behaviors. To achieve this, secondary structures of the A beta 1-41 monomer, which contribute to aggregation propensity, were studied by all-atom molecular dynamics simulation in an implicit solvent and compared to those of A beta 1-40 and A beta 1-42. We find that the secondary structure populations of A beta 1-41 are much closer to those of A beta 1-40 than to those of A beta 1-42, suggesting that A beta 1-41 and A beta 1-40 are likely to have Similar aggregation properties. This prediction was confirmed through a thioflavin-T aggregation assay. Thus, our finding indicates that the hydrophobic residue at position 42 is the major contributor to the increased fibril formation rates and consequently neurotoxicity of A beta peptides.