  The 16-22 amino acid fragment of the beta-amyloid peptide associated with the
Alzheimer's disease, Abeta, is capable of forming amyloid fibrils. Here we
study the aggregation mechanism of Abeta(16-22) peptides by unbiased
thermodynamic simulations at the atomic level for systems of one, three and six
Abeta(16-22) peptides. We find that the isolated Abeta(16-22) peptide is mainly
a random coil in the sense that both the alpha-helix and beta-strand contents
are low, whereas the three- and six-chain systems form aggregated structures
with a high beta-sheet content. Furthermore, in agreement with experiments on
Abeta(16-22) fibrils, we find that large parallel beta-sheets are unlikely to
form. For the six-chain system, the aggregated structures can have many
different shapes, but certain particularly stable shapes can be identified.
