The temperature-dependent conformations of alanine-rich polypeptides are investi- gated using generalized ensemble Monte Carlo simulations. Pure polyalanines form α helices at low temperature, but exhibit an intermediate β-sheet structure below the coil transition. For the substituted peptide WA13 , the simulation predicts the β conformation to be more stable than helices already at low temperatures, and the β motif is further favored by entropy. Measure- ments of the electric dipole of this peptide do not provide evidence for helical structures even at room temperature. These experimental observations are thus compatible with our suggestion of β conformations, even though random-coil structures cannot be ruled out. Finally, we show how to stabilize α helices by an intense electric field, possibly leading to electrofreezing behavior.