Water Responsive Protein Block Copolymers
Water-responsive (WR) biomaterials, which convert the chemical potential of water into mechanical work, offer immense potential to empower soft robotic actuators and energy-harvesting technologies. While prior studies have elucidated structure-function relationships in minimalistic di- or tri-peptide semi-crystalline systems, the design principles governing WR performance in multi-domain protein systems remain poorly understood. Herein, we designed, expressed, and purified two asymmetric diblock protein block copolymers (BCPs): CE and CL44AE, that differ by a single amino acid substitution, and subjected them to rigorous molecular and supramolecular level characterization. Upon desolvation, both BCPs gained secondary structural content, though CE preferentially gains ß-sheet character, whereas CL44AE develops latent a-helical content. At the mesoscale, CE assembles into small, densely-packed micelles that form smooth, minimally hydrated films, whereas CL44AE generates larger, heterogeneous micelles that yield highly porous films enriched in highly mobile water. These differences in molecular and supramolecular organization drive key variations in water uptake and release kinetics, critical bound-to-mobile water ratios, and mechanical stiffness of bilayer actuators, culminating in CL44AE generating the highest WR actuation energy in the BCP series to date (2,043 kJ/m3). Notably, these same self-assembly and WR behaviors are virtually absent from the individual monoblocks. Together, these results demonstrate how a single-point mutation can reprogram protein folding, self-assembly, and hydration pathways, enabling dramatic amplification of WR performance and establishing a powerful strategy for the rational design of high-energy-density protein-based actuators.