The aggregation of the amyloid (A beta) peptide of 39-43 amino acids into plaques is observed in the brain of Alzheimer's disease (AD) patients, but the mechanisms underlying the neurotoxicity of A beta oligomers are still elusive. One suggested initial mechanism is related to the implications of amyloid membrane interactions, but characterization of these assemblies is challenging by experimental means. In this study, we have explored the stability of a trimer of A beta 11-40 in parallel and antiparallel beta-sheet structures for the wild-type sequence and its F20W mutant in a dipalmitoylphosphatidylcholine membrane using atomistic replica exchange molecular dynamic simulations. We show that both the U-shape organization and the assembly of beta-hairpins are maintained in the membrane and are resistant to the mutation F20W. In contrast the models are destabilized by the F19P mutation. Overall, our results indicate that these two assemblies represent minimal seeds or nuclei for the formation of either amyloid fibrils, a variety of beta-barrel pores, or various aggregates for many A beta sequences in a membrane-mimicking environment.