Distinct Local and Global Dynamics of α-Helices and β-Sheets in Poly(γ-benzyl-l-glutamate) Peptides

A series of poly(γ-benzyl-l-glutamate) (PBLG) peptides are synthesized, with a broad range of molar masses and different end-groups. By combining static (wide-angle X-ray scattering and ¹³C solid-state nuclear magnetic resonance (NMR)) with dynamic probes (¹³C solid-state NMR, dielectric spectroscopy (DS) as a function of temperature and pressure, and rheology), we could identify distinct local and global dynamics associated with α-helices and β-sheets. The local dynamics reflect segmental relaxations of amorphous segments interrupting the α-helices/β-sheets and at their chain-ends. Two glass temperatures (T_gs) were identified in oligopeptides exhibiting both secondary structures. This is the first report for β-sheet-associated T_g in completely nonhydrated polypeptides. At longer timescales, the relaxation of the α-helical and β-sheet macrodipoles was also evident in DS. Peptides with different secondary structures have distinct viscoelastic signatures. Overall, polypeptide chain length and end-group chemistry can be employed to engineer α-helices and/or β-sheets, enabling deliberate control over the structural, dynamical, and viscoelastic properties.