Beta-Sheet Co-Assembly Interactions Guide Organization of Charged Peptide-Polydiacetylene Conjugates

Thermochromic polymers, such as polydiacetylene (PDA), have shown promise as molecular sensors due to their environmental stimuli-dependent chromogenic properties. These polymer materials have potential for applications such as food quality indicators, chemical detectors, and temperature sensors. The color-changing responses arise from structural changes and are dependent on polymer sidechain characteristics; however, the effect of sidechain chemistries on thermochromic responses remains poorly understood. Previously, Lim et al. demonstrated that amphiphilic peptide-PDA co-assemblies form 1D nanostructures that exhibit thermochromic properties unique from self-assemblies of the positive and negative peptide-PDA monomers. However, the structure and order of these materials has not been extensively studied. Here, we expand upon prior work by using 2D solid-state NMR to characterize the self-assembled and co-assembled structures of charged peptide-PDA materials. We find that selectively ¹³C- and ¹⁵N-labeled self-assemblies of KK₂KGV₅-PDA form anti-parallel β-sheet structures, while DD₂DG₄V-PDA forms semi-ordered random coil structures, consistent with previous CD and FTIR results. Furthermore, we demonstrate that a 1:1 mixture of KK₂KGV₅-PDA and DD₂DG₄V-PDA exhibits cooperative co-assembly interactions between oppositely charged peptide sidechains that suppress self-assembly. The resulting ordered parallel β-sheets are structurally distinct from the peptide-PDA monomers. Our results confirm that elements of peptide co-assembly design can indeed be applied to polymer sidechain engineering, enabling the study of sidechain chemistries using the language of protein design. This expands future possibilities for synthesis of tunable, bioinspired polymer sensors.