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…
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