Structural changes in poly(L‐alanine)(PLA), a model compound related to tussah silk fibroin, induced by heat treatment have been studied by differential thermal analysis, x‐ray diffraction, and infrared spectroscopy. PLA heated below 300°C shows x‐ray patterns very similar to those of the α‐helix crystalline phase, in addition to the diffraction patterns due to the β structure. Samples heated at 368°C exhibit predominantly the diffraction patterns due to the β crystalline phase. From infrared spectra, PLA samples heated below 280°C are found to be composed of all three molecular conformations: β sheet, random coil, and α helix. The intensity of the infrared absorption at 650 cm−1 (amide V) assigned to the random coil conformation is decreased by heat treatment above 325°C. The content of the β‐sheet structure remains almost constant when the specimen is heated below 325°C, and increases abruptly on heating to around 335°C, while the random coil content is decreased abruptly by heat treatment above 335°C. The α‐helix content does not change, regardless of heat treatment. It is suggested therefore that the random coil conformation of PLA transforms directly into the β sheet on heat treatment above 335°C.
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Tsukada et al. (1987) studied this question.