The accumulation of amyloid-beta (A beta) aggregates in brain contributes to the onset of Alzheimer's disease (AD). Recent structural analysis for the tissue obtained from AD patients revealed that A beta aggregates have a single structure with three-fold symmetry. To explain why this structure possesses significant stability, we here investigated the specific interactions between A beta peptides in the aggregate, using ab initio fragment molecular orbital calculations. The results indicate that the interactions between the A beta peptides of the stacked A beta pair are stronger than those between the A beta peptides of the trimer with threefold symmetry and that the charged amino-acids are important.(C) 2017 Elsevier B.V. All rights reserved.