Structural Evolution of a Stimulus-Responsive Diblock Polypeptide Micelle by Temperature Tunable Compaction of its Core

With a perfectly defined primary structure, both in terms of monomer sequence and chain length, recombinant polypeptides obtained by protein engineering techniques allow the investigation of structure property relationships at a level of detail that is difficult to achieve with traditional synthetic polymers because of the precision with which their sequence can be defined. In the present work, we have studied the behavior and temperature-triggered self-assembly of a series of diblock recombinant elastin-like polypeptides (ELPs) with the goal of elucidating the mechanism of their self-assembly into micelles. Aqueous solutions of diblock ELPs were studied below and above their critical micellar temperature (CMT) by multiangle light scattering and small-angle neutron scattering techniques. Below the CMT, the radius of gyration of soluble ELP chains follows a power law as a function of molecular weight with an exponent value close to 0.5 that is characteristic of Gaussian coil conformations. As the temperature reaches the CMT, attractive interactions between the more hydrophobic block of diblock ELP chains leads to the self-assembly of monodisperse spherical micelles at thermodynamic equilibrium. Above the CMT, micelles expel water molecules from their core whose densification is evidenced by the monotonic increase in the light and neutron scattering intensities as a function of temperature. The behaviors of these different diblock ELPs in solution and as self-assembled nanoparticles above the CMT following universal experimental scaling laws make them analogous to synthetic amphiphilic diblock copolymers (star-like vs crew-cut micelle models). These studies also shed light on the important role of water in the thermal behavior of these thermally responsive self-assembling diblock polypeptides and suggest a new design parameter thermally triggered desolvation and densification of the core of micelles that can be fine-tuned at the sequence level to control the density of self-assembled polymer nanoparticles.