The self-assembly of the amyloid-beta (A beta) peptide of 39-43 amino acids into senile plaques is one hallmark of Alzheimer's disease (AD) pathology. While A2 V carriers remain healthy in the heterozygous state, they suffer from early onset AD in the homozygous state. As a first toward understanding the impact of A2 V on A beta at its earlier stage, we characterized the equilibrium ensemble of the A beta 1-28 wild type and A beta 1-28 A2 V monomers by means of extensive atomistic replica exchange molecular dynamics simulations. While global conformational properties such as the radius of gyration and the average secondary structure content of the whole peptides are very similar, the population of beta-hairpins is increased by a factor of 4 in A2 V, and this may explain the enhanced A beta 1-40 A2 V aggregation kinetics with respect to A beta 1-40 wild type. Both peptides display a non-negligible population of extended metastable conformations differing however in their atomic details that represent ideal seeds for polymerization. Remarkably, upon A2 V mutation, the intrinsic disorder of A beta 1-28 monomer is reduced by a factor of 2, and the free energy landscape is completely different. This difference in the conformational ensembles of the two peptides may explain in part why the mixture of the A beta 40 WT and A2 V peptides protects against AD.