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.