Helical peptide structure improves conductivity and stability of solid electrolytes

Polymers electrolytes with adding salt or tethered ions on the backbone have been studied for decades with random coil backbones. The role of a helical secondary structure on ionic conductivity has not been investigated for solvent-free systems. Cationic polypeptides with ammonium groups were prepared with either random coil or helical secondary structure using monomers with different chirality but the same polymer chemistry. The degree of polymerization was controlled from N= 50 – 1000, and longer helices lead to higher conductivity. The random coil peptide analogues show substantially lower conductivity and are consistent with prior works. This is attributed to the role of the macrodipole along the helix which grows with backbone length and substantially increases the dielectric constant. X-ray scattering a more intense ion-ion correlation peak with longer helical peptides and are more pronounced than for random coils. The helix is stable in the solid state based on circular dichroism and FTIR up to at least 200 °C. The hydrogen bonding of the helix also imparts thermal and electrochemical stability, while allowing for facile dissolution back to monomer in acid. Peptide electrolytes demonstrate the important role of secondary structure on conductivity and stability.