We searched stable conformations of amyloid-beta (A beta) dimers composed of A beta(1-42) or A beta(1-43) protein in water by replica-exchange molecular dynamics simulations and found that Thr43 of the C-terminal of A beta(1-43) is hydrogen bonded to Arg5 of the same monomer in the A beta(1-43) dimer, resulting in its ring-shaped conformation, while A beta(1-42) has no such hydrogen-bond. This conformation is expected to aggregate more easily into a compact conformation of A beta fibrils. We also investigated the binding affinity and the specific interactions between A beta monomers by ab initio fragment molecular orbital calculations to elucidate which A beta residues contribute to the dimerization. (C) 2014 Elsevier B.V. All rights reserved.