Thermochromic behavior of polydiacetylene nanomaterials driven by charged peptide amphiphiles

The stimuli-responsive chromogenic transitions in polydiacetylenes (PDA) result from the conformational changes that directly alter the electronic structure of the π-conjugated backbone. In this work, we studied the ability of electrostatic interactions to modulate the chromatic phases afforded by charged, amphiphilic peptide-PDA nanostructures and their coassembly under pH-neutral, aqueous environments. Two oppositely charged, diacetylene-bearing peptide sequences were used as model monomers in this study. Based on solid-state nuclear magnetic resonance (ssNMR) analyses, the positively charged, trilysine-containing sequence (K3GV) forms a β-sheet-like assembly with higher structural order than the disordered, negatively charged triaspartic acid-containing sequence (D3GV). The equimolar mixture of the two charged monomers results in a more ordered coassembly structure than D3GV, as evidenced by ssNMR, which still fulfills the geometric requirement to topochemically polymerize diacetylenes. At various temperatures tested, single component assemblies and coassembled peptide-PDAs all demonstrate a thermochromic response, with the hysteresis of the blue phase (planar conformation) spectral features being much larger than that of the red phase (twisted conformation). Resonance Raman spectroscopy of thin films revealed that the coexistence of blue and red phase chains is more stabilized in coassemblies that have electrostatic complementarity compared to their single component counterparts. Both the characterization of the solution and film samples at 20–80°C suggest that the red phase PDA with twisted conformation is more favored by the charge-coassembled material. In summary, this work shows the important role of the attractive electrostatic interactions arising from the peptidic side chains in controlling the adaptive properties of amphiphilic PDAs. Furthermore, this work sheds light on the nature of structural changes responsible for the thermally responsive chromatic transitions of a biomolecule-functionalized polymeric material and how this process can be directed by design-tunable electrostatic interactions.