Conformation-Determined Segmental Dynamics in α-Poly- l -Lysine Hydrobromide: The Impact of Thermally Induced β-Sheet Formation on the Glass Transition Temperature

Abstract

Solid-state polypeptides undergo continuous conformational reorganization when subjected to various environmental stimuli, reshaping their macroscopic physical behavior. Here, we investigate this phenomenon specifically under thermal treatment using α-poly-l-lysine hydrobromide (α-PLL), an ionic polypeptide containing Br– counterions, as a model system. By combining thermogravimetric analysis (TGA), Fourier-transform infrared (FT-IR) spectroscopy, wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), and broadband dielectric spectroscopy (BDS), we show that freeze-dried α-PLL initially contains mixed α-helical, β-sheet, and β-turn/antiparallel β-sheet conformations. After removal of loosely bound water, thermal treatment drives a transformation toward β-sheet-rich structures. This process is cooperative and thermally activated, with an apparent activation energy of approximately 64 kJ mol–1, indicating that β-sheet formation requires collective rearrangement of hydrogen-bonded polypeptide chains. The structural transformation is not limited to local changes in secondary structure. WAXS reveals that α-PLL exhibits liquid-crystalline-like two-dimensional hexagonal packing, which is reorganized in a molecular-weight-dependent manner as β-sheet-rich architectures develop with increasing temperature. Most importantly, this conformational reorganization produces two unexpected changes in the physical response of α-PLL. First, the formation of β-sheet-rich structures is accompanied by a decrease in glass transition temperature (Tg), showing that secondary-structure ordering does not necessarily rigidify the polypeptide matrix. Second, the frequency-dependent conductivity, σ′(f), systematically decreases over a broad frequency range upon annealing, although the β-sheet-rich state exhibits stronger hydrogen bonds. This suggests that charge transport in α-PLL is promoted not by the stronger, more static interchain hydrogen-bonding network of β-sheets, but by the α-helical state, which may provide a more favorable electrostatic environment for Br– migration. These findings reveal a direct link between secondary-structure transformation, glass-transition dynamics, mesoscale packing, and ion transport, establishing α-PLL as a model solid-state polypeptide electrolyte in which conformational reorganization governs both molecular mobility and electrical response.