Block sequence affects thermosensitivity and nano-assembly: PEG-l-PA-dl-PA and PEG-dl-PA-l-PA block copolymers

We are reporting triblock copolymers of (ethylene glycol)44–(L-alanine)9–(DL-alanine)9 (PEG–L-PA–DL-PA) with α-helical L-PA localized between flexible PEG and DL-PA, and (ethylene glycol)44–(DL-alanine)9–(L-alanine)9 (PEG–DL-PA–L-PA) with gradient flexibility in water. Aqueous solutions of PEG–L-PA–DL-PA underwent only sol-to-gel transition, whereas those of PEG-DL-PA-L-PA underwent sol-to-gel-to-squeezed gel transitions as the temperature increased. The L-PAs of both polymers have an α-helical secondary structure in water at low temperature. However, the α-helical structure of the PEG-DL-PA-L-PA changed into a random coil structure as the temperature increased above 40 °C, whereas the PEG-L-PA-DL-PA kept the α-helical secondary structure over the same investigated temperature range of 4 °C to 50 °C. Cryo-transmission electron microscopy images and dynamic light scattering suggested that the PEG-L-PA-DL-PA develops spherical micelles, whereas the PEG-DL-PA-L-PA develops cylindrical bundles as well as spherical micelles in water. Even though both block copolymers have a similar composition of (ethylene glycol)44, (L-alanine)9, and (DL-alanine)9, they showed significantly different temperature-sensitivities as well as different nano-assemblies in water. This report suggests that the block sequence of a polymer is very important in developing a specific nano-structure as well as in controlling thermosensitivity of the polymer, thus providing useful molecular information in designing a biomaterial.