High-performance, biobased materials can potentially be manufactured from polymerized alpha-amino acids (alpha-polypeptides). This paper reports on the synthesis, structure, and properties of both polyalanine enantiomers (PLAla and PDAla). The molecular structure of the polypeptide chains, their molecular weight, and polydispersity were investigated by H-1 NMR, MALDI-TOF, and size,exclusion chromatography. The secondary structure and crystalline order were probed via Fourier transform infrared spectroscopy, circular dichroism, and (synchrotron) wide-angle X-ray diffraction. The phase behavior and thermal stability were assessed by differential scanning calorimetry and thermogravimetric analysis. The kinetically trapped PAla chain conformation in the solid state, after synthesis or solvent treatments, is the alpha-helical shape. Upon heating, crystals from the alpha-helices convert into more stable crystals from beta-sheets at a temperature higher than 210 degrees C. This temperature is close to where polymer degradation sets in. The beta-sheet crystals combine melting with thermal degradation at temperatures above 330 degrees C. In the presence of superheated water, the conversion from alpha-helices to beta-sheets happens at lower temperatures, allowing for a conversion without degradation.