We characterized the dimer of the amyloid-beta wild-type (WT) peptide, AA of 42 residues and its disulfide bond -locked double mutant (S8C) by replica exchange molecular dynamics simulations. A beta dimers are known to be the smallest toxic species in Alzheimer's disease, and the S8C mutant has been shown experimentally to form an exclusive homogeneous and neurotoxic dimer. Our 50 mu s all-atom simulations reveal similar secondary structures and collision cross-sections but very different intramolecular and intermolecular conformations upon double S8C mutation. Both dimers are very dynamic with hundreds of free-energy minima that differ from the U-shape and S-shape conformations of the peptides in the fibrils. The only common structural feature, shared by both species with a probability of 4% in WT and 12% in S8C S8C, is a three-stranded beta-sheet spanning the 17-23, 29-36, and 39-41 residues, which does not exist in the A beta 40 WT dimers.