We employ constant-temperature and replica exchange molecular dynamics to survey the free energy landscape of the ccbeta peptide using a united-atom potential and an implicit solvent representation. Starting from the experimental coiled-coil structure we observe alpha to beta conversion on increasing the temperature, in agreement with experiment. Various beta-sheet trimers are identified as free energy minima, including one that closely resembles the amyloid beta-sheet model previously proposed from experimental data. We characterize two alternative pathways leading to beta-sheets. The first proceeds via direct alpha to beta conversion without dissociation of the trimer, and the second can be classified as a dissociation/reassociation pathway.