Poly(alanine): Structure and Stability of the D and L-Enantiomers

High-performance, biobased materials can potentially be manufactured from polymerized α-amino acids (α-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 (1)H 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 α-helical shape. Upon heating, crystals from the α-helices convert into more stable crystals from β-sheets at a temperature higher than 210 °C. This temperature is close to where polymer degradation sets in. The β-sheet crystals combine melting with thermal degradation at temperatures above 330 °C. In the presence of superheated water, the conversion from α-helices to β-sheets happens at lower temperatures, allowing for a conversion without degradation.