Direct determination of helix structures involved in the screw‐sense reversal of poly(β‐phenylpropyl l‐aspartate) by synchrotron X‐ray diffraction

The helix‐sense reversal of poly(β‐phenylpropyl l‐aspartate) (3PLA) in the solid state was studied by synchrotron wide‐angle X‐ray diffraction and small‐angle X‐ray scattering. The direct determination of the characteristic helical pitch before and after the transition revealed that the transition takes place reversibly between the two α‐helices having opposite screw‐sense during the heating and cooling cycle. While the hexagonal packing remains unaltered, the helix‐sense inversion causes discontinuous changes in the molecular arrangement and, by extension, the crystalline dimension. In this study, another transition was detected at a higher temperature from the left‐handed α‐helix to the π‐helix, the molecular chirality being unaffected. Helix transition of poly(β‐phenylpropyl l‐aspartate) in the solid state was studied by synchrotron WAXD/SAXS. The unit heights characterizing the helix structures were directly observed. A left‐handed α‐helix was formed after helix‐sense inversion.